The gear within the photo above consists of, from left to right: meccano hand-cranked gramophone with acoustic recorder (out of body) to chop 78 rpm disc data on wax or celluloid; 'Crosley' 1926-vintage two valve regenerative receiver (above) with 1925-vintage crystal radio under it. Stromberg-Carlson portable battery-valve broadcast band receiver c.1948 used as a BFO for my foremost transistor receiver. Small multimeter in front of S-C radio. National Panasonic 4-band 11-transistor (all germanium!) radio set receiving 160 metres (VK3AML on that occasion) with my Emmco headphones related to it. The curly wire above the transistor radio was our communal non-public telephone line to my mates David and Bruce Bowden, who lived around the nook in Pleasant Road. HMV portable wind-up gramophone sort C101, circa 1923, with carbon microphone to transmit 78 rpm discs down the cellphone line. Various broadcast-band and shortwave DX QSL playing cards and a single-sided disc on the noticeboard above (sung by a Madame Alma Gluck, from reminiscence!). In my left hand, I held the oldest disc report then in my assortment, London, circa 1903, of a brass band enjoying the 'Tancredi' overture. On the suitable, spare parts for our personal neighbourhood line, and an previous electrodynamic speaker. This was the only photo of my 'radio shack' taken within the 1960s. It was the 'sleep-out' behind 6 Torring Road, East Hawthorn, 5 miles East of Melbourne's GPO (Australia). The home was demolished in 1997. Three residence units now stand there.

Author's word, 28 April 2005: As this web page deals with my own modulated light experiments I hope readers will tolerate these reminiscences earlier than I reproduce my 1979 article on the subject from 'Amateur Radio' magazine:

WHY MODULATE Light?

Today's younger Australians can barely imagine the frustration of teenagers within the so-referred to as 'radical' 1960s the place digital communication was concerned. By trendy standards, radio was locked in legislative totalitarianism, dominated by the governmental communication monopoly of the Australian PMG's Department, and run on inflexible post-colonial British public service strains. Something as innocuous as an intercom line run along your back fence to adjacent buddies was unlawful - 'a financial menace to the monopoly of the general public telecommunications network' within the eyes of the PMG. I discovered this when i set up a neighbourhood cellphone network to other native youngsters' places early in 1967! Music transmission by way of newbie radio had been banned in Australia since 1939, so that the material legally conveyed on any 'wireless' system was limited, significantly for music-mad teenagers. CB radios, principally of the one channel hand-held variety, were imported in limited quantities and sold freely - however paradoxically they could not be legally licensed or used. You had to be fifteen years old to sit for the amateur radio license, then involving a trifecta of exams in full principle, rules, and Morse at 12 words per minute. For the theory, discursive essay solutions were required, theprimefans.com and there was no stage of theory examination beneath the single maximum standard. From a young person's perspective, you had to drag your self over a discipline of damaged glass to achieve a legal entry point. Even then, you had to wait on your sixteenth birthday to function ham radio legally in Australia.

If, like me, you were thirteen years previous in 1967 and had electronic experimenter associates close by, your aspirations to speak faced years of seemingly infinite frustration. There have been good reasons to research 'alternative' communication applied sciences - including modulated gentle.

Together with that we had the Vietnam War; the imminent threat of the obligatory army draft; a conservative government in energy since our start; and British-styled college uniforms unsuited to our hot summers. Australian teenagers of the 1960s had many legitimate reasons for discontent. The protest motion had its roots in lots of aspects of Australian society, not just the political issues for which any road march was a convenient excuse...

During the subsequent decade Australians noticed the advent of full citizenship for Aborigines (1967); the Vietnam Draft Resister's Union, and its pirate radio station '3DR' (1971); a radical Labor authorities coming to energy (1972); the primary Sunbury rock festival, Australia's 'Woodstock' (1972); novice beginner radio licensing (circa 1973); group broadcasting (1974); ethnic, multicultural broadcasting (1975); and the local legalisation of CB radio (1977). Finally, in the 1980s, personal carriers within the telecommunication market were allowed to compete with the PMG and its descendant organisation, Telstra. Communication monopolies have been damaged and the authorized restraints on private digital communication eased markedly. With the appearance of the Internet, how might it's otherwise at present?

I would argue that Australia was fairly unlike America in the 1960s, in that conservatism reigned by many of the 1960s right here, notably in our communication legal guidelines. Australia's 'liberation' - if you could call it that - largely got here after 1970...

The advent OF Educational Solid-STATE KITSETS

So, how did your average kid with greater than common curiosity get into electronics, and more notably into optical communication, forty years in the past?

Within the mid-1960s, when the cost of germanium transistors had fallen to an inexpensive level, electronic construction kits had been made accessible to stimulate the minds of future radio hams and experimenters. These kits were in contrast to the single-venture assembly outfits then accessible from Heathkit and Lafayette. Their accent was on training, offering a set of electronic elements to be organized in numerous configurations on an insulating 'breadboard' with spring clips and connecting wires. With assistance from a guidebook explaining the operation of every arrangement, more than twenty totally different circuits could be constructed, together with Morse code oscillators, simple radio receivers, excessive-gain audio amplifiers and low-power radio transmitters. At the moment, we also had the bonus of a local Melbourne journal for young experimenters, 'Transistor Kits' revealed by Colin Witchell - more just lately of 'Talking Electronics' journal fame - from a tiny store in Church Street, Brighton. For individuals who were fascinated, the mysteries of electronics held the key to a new world of technical prospects - and many of Colin's projects involved light detectors in some artistic approach...

An previous buddy from my main school days, Rowland Legg, acquired a Philips twenty-in-one package for the Christmas of 1965, and I used to be given the same Japanese 'Eleco' kit on the same day. My parents had apparently noted my early aptitude for constructing crystal radio receivers, so they'd decided to 'push the envelope'. The natural end result was that over the following 5 years Rowland and that i spent many afternoons collectively, unravelling the mysteries of digital amplification. Whatever time wasn't spent indoors with the kits or on different electronic projects, we spent up on our rooves erecting prolonged wire radio antennas, or attempting to get them up on progressively higher trees or supporting masts.

During these antenna-elevating sessions, Rowland Legg and I discovered that we may see each other's properties from vantage factors on our rooves. In an try to arrange a signalling system, we spent many evenings up ladders with kerosene lanterns, using a black card to cut off the sunshine and send messages to each other in Morse code. The standard Morse alphabet appeared fairly complicated, so Row devised considered one of his own, following a logical mathematical progression:

This Also proved to be too advanced to memorise and, with mild flashes, too tough to relate to any typed list! We looked for a more facile means of communication, with enough security not to draw consideration from the licensing authorities.

OUR FIRST OPTICAL COMMS - JUNE 1968

In 1967 I grew to become aware of the potential for transmitting speech over modulated light beams after i purchased a duplicate of an ancient, leather-based-sure book 'Science For All' (1884), containing William Ackroyd's account of Bell and Tainter's then-new 'photophone'. Another early affect was a guide revealed in 1921, 'The Boy Electrician', which gave constructional particulars of selenium gentle delicate cells, Tesla coils, audio transmission by multi-turn induction loops (which I constructed) and even the main points of a small X-ray machine (which, thank God, I didn't assemble).

Like many teenagers of the late 1960s, I experimented with modulated mild communication using amplifier-pushed torch globes or neon lamps for transmitting and CdS photoconductive cells or OCP71 germanium phototransistors for receiving. A web page from my bench notebook dated 25 May 1968 - a couple of weeks after my 14th birthday - reveals my earliest plans for an optical system:

On the next weekend, 1st June 1968, a high school good friend named Howard McCallum and that i arrange the deliberate modulated gentle system with an incandescent torch bulb for transmitting and a CdS LDR in series with a 9 Volt battery and headphones for receiving, collimating a light beam between the 2 with magnifying lenses. The results over a distance of four metres had been loud, but very distorted with frequency doubling effects - a result of our initial makes an attempt to transmit without DC bias on the filament lamp!

Soon afterwards, a Mr A G Murrell of Penola, South Australia, revealed the main points of his easy 'photophone' in the 'A Reader Built It' page of March 1969's 'Electronics Australia' (pps. 91-93). The mission's simplicity made it a sexy proposition for younger individuals, and plenty of Australian experimenters of my generation will remember it:

The geographical setting for our personal reconstruction of Murrell's gadget was less than salubrious. In fact, it was one of the crucial eccentric radio 'shacks' that I've ever encountered. At the underside of my good friend Rowland Legg's back yard, a large picket packing crate initially used for shipping a Volkswagen car to Australia was arrange by Row's father, Ern Legg, as a tiny electronics room - 'the tin shed' as we known as it. To maintain out the drafts, its partitions had been papered with out-dated advertising posters for Melbourne's weekly scandal-rag, 'The truth', obtained from our kindly native newsagent across the nook in Tooronga Road. It was the sort of newspaper that no person would admit to buying, although the newsagent assured us of its glorious local circulation. From each angle in Rowland's radio shed, headlines in an infinite typeface assaulted the eye: "SHOCK BIKIE Film" - "BLACK PANTIES Murder" - "CATHOLIC FATHER Wants Sex Surgery"! The piece de resistance amongst these posters was tactfully hidden behind a cupboard door, its wording being one thing like - "UNWED Mother TELLS Court: 'HE SHAGGED ME; THEN WE HAD INTERCOURSE' !!" - and I'm nonetheless questioning concerning the implications of that!

In these inglorious surroundings, and within the backroom of my dwelling proven within the photo at the top of this internet page, we spent many weekends constructing electronics initiatives of every conceivable sort. Together, we quickly had a version of Murrell's photophone transmitter built into the optics of a small ex-WW2 Aldis signalling lamp with a concave parabolic mirror of about 7.5 cm diameter. The Aldis housing was finally mounted (with yards of PVC tape!) on a scorching water overflow pipe protruding from the tiled roof at 1 Cole Street, Rowland's residence.

However, my very own preliminary optical comms assessments with the Murrell equipment had been finished around March 1970 from the the yard radio shack then occupied by Hughie Paton, VK3ZEP, at forty nine Havelock Road, throughout that home's again garden, down its again driveway and throughout Torring Road to the glassed-in front veranda of our family residence (to the left of the front door in the picture beneath) - a distance of about 50 metres. The system provided a hyperlink in a single direction solely. These exams were logged on open reel audio tape, so that I could check my audio quality without the necessity for an assistant. My mother often got here onto our veranda to hearken to the result and talk her response by waving through the windows. The tape nonetheless indicates the pretty good audio quality that one may get with a limited depth of modulation. It additionally demonstrates the very high hiss stage of the OCP71, the actual gadget used on that day being seen in the photograph on the fitting.

I used to be out in the open air on the back of the Havelock Road property, throughout Torring Road from my home, shown above, with the Aldis lamp's glass parabolic reflector optics on a telescope tripod. This had a three volt 300 mA torch globe at its focus. I later discovered that the excessive frequency response of the system was inversely proportional to the thermal inertia of the filament. The perfect high frequency response was obtained with lamps of decrease rated maximum current, a thin filament with thick lead-out wires to dissipate the heat shortly.

The transmitter gear, consisting of a microphone pre-amp, three watt transistorised audio power amp with transformer output, and two measurement D torch cell battery supplies (one for the amp and one as DC bias for the lamp) all sat in a carton between the legs of the tripod. On the receiving end, a 7.5 cm diameter magnifying lens focussed the image of the Aldis lamp onto an OCP71. I can't remember whether or not the OCP71 was instantly connected to the microphone input of the Sanyo 5" open reel tape recorder (photovoltaic configuration), or whether it was working into the road input by means of the usual two-transistor pre-amp (photoconductive arrangement). I seem to do not forget that each had been tried at different occasions, with a minimal distinction of results. An occasional transient buzz may be heard when frequent Australian blowflies occurred to fly through the beam, modulating the sunshine with their wing beats. Within the latter part of the extract, a mild shower of rain occurred, the droplets falling through the beam producing a sort of tender 'plip-plap' sound. The effect of waving one's hand by way of the beam was additionally demonstrated. Whenever the beam was interrupted the hiss level rose considerably - an effect for which, even now, I don't have any clarification, besides that it obviously had one thing to do with the impedance of the phototransistor source rising as the sunshine enter fell.

The audio tape of the check was made on a heat, fairly overcast Saturday afternoon. Within the background, Ghera Harris (1896 - 1991) and her architect daughter Berenice Harris (1925 - 2002), who owned the Havelock Road property, will be heard washing dishes after lunch and planting hop bushes in the backyard. The first World War veteran aero-engine fitter Reg Harris (1894 - 1979), Ghera's husband, was apparently planning a brand new ingredient for some home brew! The son of the family, Brian Harris (1936 - 1992), was briefly VK3ZFH within the late 1950s. Brian confirmed the author how to tune up a transmitter and browse an oscilloscope on the age of 4, in 1958. This set Chris on the lifelong 'downward' path into technical interests... One in all Brian's final jobs involved the design of a number of the communications gear for the Hubble area telescope. Suburban life may very well be interesting with neighbours just like the Harris family at 49 Havelock Road!

In this ten-minute mp3 extract from the original 45-minute tape, the writer on the age of fifty one in 2005 introduces his squeaky-voiced 16-yr-previous self in 1970. Computer nerds didn't exist then - WE have been RADIO nerds! Anyone fascinated can download the audio log file of this March 1970 test as an mp3 file here:

(Currently broken - coming quickly) March1970.mp3

One curious facet of the germanium phototransistor OCP71 was its extreme infra-pink sensitivity. It was attainable to transmit audio, as one can hear on the tape above, with the torch bulb operating at a voltage so low that it had no perceptible visual output. The germanium detector might resolve the modulated HEAT from the lamp filament. Its work operate as a photodetector was very low, which meant that it was inherently subject to giant amounts of thermal noise, a lot greater than silicon and really a lot greater than photomultipliers:

On 1 September 1970 we extended the range of the system to transmit audio from Rowland's electronics room ('the tin shed') at 1 Cole Street to my house at 6 Torring Road, East Hawthorn, where I stood atop a ladder subsequent to our backyard bungalow with an an optical unit fabricated from tin cans, like Murrell's within the photograph above. The audio link lined about 600 metres, and that i might clearly see the light focussed on the OCP71 in the receiver tube. In truth, I could intention the receiver by having the focussed spot disappear behind the square sensitive materials contained in the phototransistor. The hiss level from the OCP71 germanium phototransistor was gross, the 7.5 cm diameter of our optics was inadequate for the vary and the trebles had been restricted by the thermal inertia of the torch bulb, however this was our first actual 'gentle beam DX'. On the age of 16, the pleasure of hearing these outcomes prompted my fingers to shake, in order that my optical receiver's goal was erratic, however I managed to log the whole contact on an audio tape in two 'bursts', with a pause to telephone Rowland to tell him that all was being acquired, midway. Initially, Rowland and his good friend Neil Florence have been simply relaying the 7:30 pm information broadcast from the Melbourne broadcast station 3AK with its objects on the Vietnam War, and floods in New Zealand. As I slowly managed to align the receiver, the transmission regularly rose out of the noise - although not by a lot! Later, typical teenage music of the day may be heard - The Crystals singing 'After which He Kissed Me' (with Rowland attempting to interpolate a 1970-vintage type of 'karaoke') and the record of 'Lay Down' sung by Melanie Safka. When Rowland switched to his carbon microphone to announce 'Hello, Chris... this is being transmitted on the first day of Spring, the first of September 1970 - and if you can't hear this after all this hassle I'll should kill you!', I might simply have fallen off the ladder in amazement. We'd finally devised a technique of circumventing the unlawful usage of a radio transmitter - though to be life like, with this appalling signal-to-noise ratio the success was only marginal:

http://www.bluehaze.com.au/modlight/1Sept1970.mp3

I've hardly ever experienced more thrill from experimental work than I did on that evening 35 years ago, and that i need hardly add that immediately afterwards, adrenalin-charged, I ran non-cease to Rowland's to report our success! The three of us posed for a photo round that time, with me holding a pair of 1920s-vintage headphones which we regularly used. The image could possibly be titled 'hear evil, SEE evil, DO EVIL' - however mostly, our backgrounds have been just too 'Eastern Suburbs' and discreetly shy for any of that!

OPTICAL COMMS IN SOUTH AUSTRALIA - 1968 TO 1972

Elsewhere, other Australian experimenters have been making an attempt the potential of atmospheric optical communication extra critically. In Adelaide throughout 1968, my future collaborator Mike Groth (currently VK7MJ, then VK5ZMG) and a lab assistant buddy, Stewart Powell, built a pair of optical communication units within the suburb of Hammersmith utilizing torch globes and OAP12 germanium photodiodes. They'd a most range of about 1 km, and, as Mike puts it, "terrible fidelity". Mike's spare time for optical exams was restricted at the time by the demands of doing the third 12 months of a BSc - with a brand new wife.

However, in 1969 Mike Groth did his Honours in Adelaide, and one of many course initiatives in that yr involved modulated gentle. He explained the following events to me in a letter dated 28 February 1988:

"The undertaking involved the evaluation of modulated light as a technique of transmitting geomagnetic data over quick distances to avoid wire links to the remote sensors, which tended to introduce hum loops if not carefully balanced. This was a golden alternative to be taught the idea behind optical hyperlinks and search again by means of the literature at the University Library. It turned obvious that it can be no downside to transmit data over a few hundred metres on a transparent evening utilizing the brand new infra-red diodes as sources, however the reliability of the link was unknown, especially as it was for use at a subject station within the Adelaide hills, the place fogs and mists were widespread. I constructed an infra-pink hyperlink at 930 nm which measured the path loss over the winter and spring of 1969, and my estimates of the trail losses [in the 1987 'Amateur Radio' article 'Photophones Revisited'] had been based mostly on this information.

Optical communications fell into the background for the next sixteen years, as I spent 1970 in New Guinea instructing and moved to New Zealand in 1971 to do postgraduate work at the University of Otago in Dunedin. I returned to Australia at the top of 1979, but was rather inactive in experimentation till 1985, after i determined to jot down my experiences as a assessment of the potentialities of optical hyperlinks. The hassle took almost 18 months..."

Mike's resultant article revealed in 1987 with revisions from 2005 could also be found at:

Essentially the most powerful beginner atmospheric optical communication exams in Australia round 1970 have been carried out by an Adelaide group and reported in an extraordinary Tasmanian journal. The Hobart-based mostly 'Electronics Exchange Bulletin' was published around this time by the Tasmanian workforce of Leo Gunther VK7RG and Rodney Reynolds VK7ZAR (now VK3AAR). Their extraordinary magazine inspired articles by native experimenters on each conceivable topic related to electronics and communications. Through the pages of 'EEB' between August 1968 and October 1972, two college college students, Kingsley Burlinson VK6ZEA and Robert Averay VK5ZGE described their experiments in modulating fluorescent and mercury vapour gasoline discharge lamps, attaining atmospheric ranges in excess of 3.5 miles (about 5 km), simply outside Adelaide in valleys shielded from the city lights.

Though their tools was massively bulky by fashionable standards, Burlinson and Averay pursued a novel line by driving their gasoline discharge lamps with audio-modulated 10 KHz pulse width modulation from 'class D' switching output transistors, thereby avoiding linearity and modulation effectivity issues. In this way, the efficiency was much like that of the celebrated (at the time) class-D British Sinclair 'X-20' transistorised audio amplifier of the mid-60s. I was not aware of the reviews of those tests until the start of 1976, in any other case I could have skipped the subsequent few steps in my own assessments...

OUR Switch TO Gas DISCHARGE LAMPS

A search for higher modulated gentle sources than incandescents drew my attention to neon lamps. Gas ionisation is a much quicker process than the incandescent heating of a filament. In those days, neons have been cheaply available in any measurement from a pea lamp to a full size "beehive" bulb from one marvellous source. Melbourne experimenters active in the 1960s will remember Waltham's Trading Company in Elizabeth Street, Melbourne. At the tip of a slim stairway leading down from the pavement was a somewhat grubby Aladdin's cave of tables laden with cable, war surplus junk (every battle except Vietnam), bins of valves and khaki-painted gadgets of indeterminate origin. Cash-strapped adolescents milled about with down-turned eyes, slowly sifting row after row of cartons and crates stuffed with technical solid-offs of each description. I acquired several bins of neons there, along with varied other fuel discharge units to strive. A few years later, I discovered that a few of these contained traces of radioactive material to help ionisation. Let the buyer beware!

I initially wired a neon lamp into the anode circuit of a Philips battery triode from the 1920s, type B406. The orangey-pink glow around the cathode on these neons is fairly intense, though it would not even approach the depth of fashionable LEDs or lasers. After organising the neon modulator I used to be rewarded with close to excellent audio from my OCP71 for the first time. Fortunately, a parental veto on constructing mains-fed power supplies ended on my fifteenth birthday (8 March 1969), so the required 300 volt rail was no downside.

SIDETRACK INTO 'MECHANICAL' Television - 1971

I used the neon modulator and OCP71 as the basis for a easy television system in 1970, by adding a few Nipkow scanning discs to the outfit. This diverted me from the modulated gentle communication checks for a few years as I delved into all the pre-conflict Baird television texts to bring the pictures to an appropriate commonplace. This brought me into collaboration with the late Dan Van Elkan (b.1952 - d.1986, name sign VK3UI) and Tony Sanderson (b.1945, VK3AML), now the moderator of the 'bluehaze' internet site. They have been the 'ringleaders' of a larrikin group of amateurs operating residence constructed AM transmitters on the 160 metre band, then occupying 1800 KHz to 1860 KHz. These guys' amateur radio interests were distinctive and individualistic. The majority of conservative hf (shortwave) operators had a narrow and obsessive emphasis on 'communication quality' modulation, often 300 Hz - 3 KHz, clipped, non-linear and cruddy. Dan and Tony were both hi-fi enthusiasts. Their transmitters have been relatively broadbanded and immeasurably low in distortion, exceeding the audio specifications of many broadcasters. AKG or Western Electric microphones and broadcast-high quality audio peak limiters with twin time constants were involved. The design and construction of their modulation transformers and amplitude modulators was nothing wanting an art. Their 160 metre receivers, additionally, employed biased, low distortion envelope detectors working by means of fastidiously designed audio amps into enormous speaker techniques with vented enclosures. The content and audio high quality of their in-depth conversations on communications know-how made listening to their transmissions an absolute pleasure. Even their standard of audio compression served to convey the listener into the acoustic atmosphere of their homes - an underestimated facet of making a sensible auditory illusion. Their activity ruffled fairly just a few novice operators' feathers on the time... which solely increased my youthful admiration for both of them.

I truly met Dan (3UI) - dare I admit it - on the air on thirty first December 1969, as the result of a brief dalliance I had with pirate radio transmissions on 1.Eight MHz. He lived in Hawthorn solely a mile from my home, near the corner of Glenferrie and Riversdale Roads. Naturally he was amongst the first to hear my feeble and unstable transmissions, and he encouraged me to experiment additional to achieve the data to get the amateur 'ticket'. We had been each given a 'stop it or else' ultimatum by an over-zealous radio inspector known to the locals as 'Uncle Ugh' (many will still know who I mean), however Dan and i turned great pals as a result. Dan was about two years older than I, and was highly influential on the quick future course of my life. Because the year 1970 progressed, I discovered myself spending more time with novice radio friends and fewer with Rowland and the old school mob. Rowland finally carved out a very profitable profession in the Victorian Police Force (ironical, isn't it?), and i imagine he nonetheless does, but his early curiosity in electronics declined...

In subsequent decades, the amateurs have legislated themselves into 'band plans' with 'accepted modes' and 'accepted bandwidths' for various frequency segments. As a result - and I'll specific an opinion right here - they have systematically eliminated the legal basis for the kind of justifiable experimentation that 3AML and 3UI used to undertake. Many 'hams' are now operators of business 'black field' transceivers which might elegantly present single channel phone high quality (or worse) on each out there band for $4000+, but which regularly can't be correctly tailored to another mode or form of experiment. From my perspective, the result is that amateur radio has had progressively much less attraction as a pastime, and I'm certain I'm not alone in expressing this opinion. For my cash, in the event you legislate against experiment you kill the one attraction that novice radio ever had. So long because the emissions do not unfold past the beginner band edges, what is the problem? Anyone who pushes the hoary previous argument that "bandspace is at a premium, so transmissions should be of the minimum possible bandwidth" must be deaf and blind to the steadily declining level of newbie band usage over the previous fifteen years. Who's to say what amateur radio ought to entail, so lengthy because it gives training and encourages experiment? Many individuals wonder why I've by no means bothered to pursue an newbie radio license. I hope that I've justified my position. Thank God for alternate options with more freedom of content and bandwidth, like mild beam communication!

Dan (3UI) and that i finally organized take a look at transmissions of slender band tv using mechanical disc scanners over his 160 metre transmitter early in 1972. The following couple of years gave all of us a good grounding within the principles of mild detection and modulation, video amplification and optics.

Eventually, with D B Pitt and others within the United Kingdom, we formed the Narrow Band Tv Association, nonetheless in energetic operation and now represented on an internet site:

http://www.nbtv.org

One notably type donation to this mechanical Tv scanner mission was offered by the late Kevin Duff, VK3CV (b.1927 - d.1996). Kev worked in telecine at Melbourne's government Tv station, ABV channel 2 in Elsternwick, operating an archaic monochrome Marconi 35mm film scanner. This was originally said to have been designed for the 405 line British service and used at BBC Tv's unique studio at the Alexandra Palace in London. By 1972 it was only being used for half-hour a day, at about 4:00 pm, to broadcast 35 mm film episodes of the kids's serial "The Cisco Kid" - the one common program material that they had which nonetheless used that gauge of movie. The EMI 6097 photomultiplers on this Marconi telecine were written off as quickly as they developed spots on their photocathodes - Kev called them 'dynode spots' - but they were nonetheless quite serviceable for gentle detection. In 1972 Kevin saved some of these from the dustbin for us.

Dan (3UI) and i tailored the EMI 6097 photomultiplier to be used in our experimental camera and for the modulated light receivers. Its sensitivity was such an unlimited quantum leap from the OCP71 that it opened a whole new world of technical prospects to us. Further photomultipliers had been acquired from the late A H 'Mac' McKibbin, VK3YEO, who used 931A's for sluggish scan television scanners in those days.

MOD Light ON 160 METRES - FIRST CROSSBAND Tests 1974

Throughout the Autumn of 1975, I built a modulated gentle communication link that was used briefly between two members of the 160 metre AM group, Paul Higgins (then VK3BEK, now VK3EN) and Dave Stewart (VK3ASE). Both had radio shacks at first ground degree, going through each other throughout suburban Glenhuntly with an uninterrupted line-of-sight, and separated by about 700 metres. The modulator from my Baird mechanical tv receiver by then used a 6L6 beam pentode in sequence with the neon, which was re-mounted at the focus of a rough 30 cm moulded glass parabolic reflector equipped by Tony (3AML). The reflector had initially been used in a traffic signal.

This optical transmitter was placed on Paul's balcony in Glenhuntly Road, on the nook of Clarke Avenue. The receiver was positioned in the attic window of Dave's QTH in Burrindi Road, Caulfield South. It used a 13 cm diameter magnifying lens focussing onto a 0.5mm focal plane aperture, with a 931A photomultiplier catching the transmitted light behind it.

Using Dave's 160 metre newbie transmitter because the return link - the final word in cut up frequency operation - Paul managed to hold on a crossband contact by the neon lamp. Sig/noise was poor owing to the very poor spectral match between the orange neon and the blue sensitive photomultiplier. The neon was pushed past its current scores, so that its bulb was rapidly blackened by cathode sputtering, however at least it could be absolutely modulated. I was later able to measure the bandwidth and was astounded to seek out that the neon may very well be modulated to round 500 KHz, which was a terrific enchancment on the torch bulbs. Distortion was extreme. We had been pushing the modulation quite laborious, owing to the poor sig/noise ratio. A pattern of the audio log of the contact is at the moment available on Dave's internet site. The results were sufficiently good for us to realise that we have been heading in the right direction for additional improvement. The contact was logged on tape and an extract could be heard on VK3ASE's net site:

[ Actually, Dave seems to have removed this one for now. (Tony, VK3AML) ]

The primary drawback to be overcome was the low characteristic intensity of the neon discharge. We also wanted a supply with vital blue output, to match the spectral response of the photomultipliers, which might only detect violet, blue or inexperienced gentle. Development was accelerated by the involvement of one other member of the 160 metre cross-band contact clique, John Eggington (then VK3ZGJ, now VK3EGG) whom I met at the top of 1975.

BREAKING THE ONE-MILE BARRIER - DEC. 1975.

At my dwelling, I was lucky in having an elevated place close to the top of a hill in East Hawthorn, South of Camberwell Junction and quite near the high copper dome of 'Our Lady Of Victories' catholic church. Standing on our roof, the view of downtown Melbourne to the West and of the suburbs around to the North was unobstructed, encompassing all of Hawthorn, Kew, Toorak, Kooyong in addition to parts of Malvern, Richmond, Abbotsford and Northcote in an unbroken 120 diploma arc. Just on my side of the Yarra, on the top of a rise in clear view, was VK3ZGJ, with his shack facing me at the third floor rear of an previous Victorian mansion at 29 Shakespeare Grove, West Hawthorn. The topography was best for optical communication exams, precisely two miles (about 3.5 km) on an East-West path.

In the early 1970s, before I met him, John (VK3ZGJ) constructed a collection modulator for fluorescent lamps, consisting of several 807 output valves in parallel, with the fluoro within the anode return. He additionally built a portable light dependent resistor (LDR) receiver with a FET preamplifier. The LDR operated with bias into a load of 10 megohms or extra, and with its low noise preamplifier it had significantly better sensitivity and spectral match to a fluorescent lamp's output than my old OCP71. You had to arrange for the picture of the fluoro to focus precisely onto the hole between the conductive combs on the LDR's sensitive floor. This concerned peering on the LDR by way of a 'spy-hole' in the optical mounting while deftly manipulating the receiver's alignment.

LDRs have a very slow response, rolling off at the least 6dB per octave above 50 Hz. Treble boost may only partly appropriate the issue, as John found. My photomultipliers provided the answer to John's receiver issues, simply as his fluoro transmitter modulator permitted advances over my feeble neon. We pooled resources over the next five months.

Late in December 1975, John and that i got the communication system working between our homes, utilizing a vertically mounted fluorescent lamp on the roof which "broadcast" gentle in all instructions. The modulator consisted of several (four, I believe) kind 807 beam pentodes feeding the 40 watt fluoro of their anode circuit. A rail voltage of about 600 volts DC was applied to the lamp. We did not use a lamp starter or a ballast choke to function our fluoros. Instead, we had a novel starting association consisting of a band of aluminium foil wrapped across the glass close to the cathode end of the tube, which was connected to the secondary of an automotive spark coil. To start out the discharge, you'd apply the 600 volt rail, then energise the ring around the cathode by applying a battery briefly to the spark coil major. The high-voltage spikes applied to the glass close to the cathode began a barely perceptible glow discharge inside that end of the tube, which might immediately unfold the whole length of the tube below the affect of the 600 volts DC rail. Standing current was diverse by altering the value of the cathode resistor on the sequence 807's. The filaments at either end of the tube have been never heated up with this association, which appeared to extend the working life of the tube.

Our first 40 watt 'fluoro' check transmission came simply after the Christmas of 1975. It was in one path only, with John transmitting and me receiving. 3ZGJ managed to arrange some quite elaborate music packages interrupted by announcements, one in all which I recorded on tape. Within the early 1970s a British group, calling themselves "Radio Love" (hey man, real groovy) had proposed a system of local gentle beam broadcasting. This appears to have been an try to bypass the heavy hand of British officialdom, which at the moment had forced several unbiased broadcasters (eg 'Radio Caroline') to transmit from ships anchored in International waters off the English coast. Nothing was heard from the group after the publication of its preliminary plans, including the diagram under which neatly summarised their concepts:

John 3ZGJ, in imitation of this 'Radio Love', jokingly introduced his programs as being transmitted from "Radio Hush". The name was a bit much less poofy, yet nonetheless retained the essential environment of the authorized fringe-dweller! Later we duplicated the system to provide full duplex communication (simultaneous transmit and obtain) in each instructions. A typical extract from these fluorescent mild transmissions within the last week of December 1975 might be heard on this tape, recorded from the photomultiplier output at my finish of the hyperlink, two miles from the transmitter:

RadioHushDec75.mp3

We used no reflector or collimator with these fluorescent lamps. The photomultiplier receivers only managed to realize 15 dB sig/noise over the 3.5 km range with this arrangement. Fluorescent lamps had greater than their fair share of problems as a modulated source. Their phosphor coating had time lag, the persistence limiting the upper modulated frequency to about 5 KHz. The time lag was not fixed with the wavelength of the emitted gentle. The crimson phosphor components had very lengthy persistence, while the blue phosphor was much faster. The system's higher audio frequency restrict due to this fact varied with the spectral response of the detector, however with a blue-sensitive photomultiplier it was greater than ample for audio.

The fluorescent lamp discharge would wander and 'snake' inside the tube, especially at switch-on, interfering with the modulation. For some purpose which we might by no means explain, the output at the cathode finish of the fluoro, and solely at the cathode end, was modulated by an erratic whine at about four hundred Hz, probably attributable to the discharge hopping about from one a part of the coiled cathode filament to a different. This always set a definite restrict to the sig/noise achievable, even where there was plenty of mild sign to demodulate.

The audio frequency response of the fluorescent tube was unexpectedly uneven, an effect undoubtedly brought on by acoustic resonances of the modulated mercury plasma column inside its tubular enclosure. One might certainly hear a faint acoustic radiation from the tube whereas it was in operation, as these acoustic waves truly penetrated the glass walls. Certain modulation frequencies, clearly related to 'organ pipe' resonances throughout the fluoro tube, would cause the mercury discharge to extinguish, or to break up right into a collection of spaced glow discharges along the tube. I later found that this downside had been noted by N C Beese, who wrote a chapter on "Light Sources for Optical Communication" in the e-book 'Infrared Physics' (Pergamon Press Ltd., London, 1961, Vol. 1, pps 5 - 16). To quote from Beese (pps thirteen - 14):

"Enclosed arc lamps operated on a.c. energy within the audio-frequency range, or on d.c. and modulated by a.c. currents could trigger sound vibrations to be produced inside the arc chamber. They're brought on by thermally induced variations in fuel pressure that consequence from changes in current density in the arc. At certain crucial frequencies, resonance of appreciable intensity is built up by reflection from the bulb walls. The size and form of the bulb, sort of gasoline or vapour filling, temperature and working conditions decide the frequency of the plasma oscillations which are much like standing sound waves in the discharge. Ordinarily this phenomenon is not noticed as a result of lamps are operated on d.c. or low frequency a.c. with ample ballast to make sure stable operation. In lengthy tubes the discharges assume a constricted, snakelike appearance at the important frequencies and are brought on by sound power reflected from the ends of the bulb. In a spherical bulb the sound waves spread to the bulb partitions and are then focussed back upon the arc to produce instability at the electrodes [...]"

Beese goes on to analyse a Xenon discharge lamp 1.5 cm in diameter and 13.Three cm long with three Amp d.c. current utilized with a 2 amp a.c. modulation utilized. He famous:

"[...] the arc showed violent distortions at 2250 Hz however was quiescent at 2000 Hz and 2500 Hz. With 5 A d.c. and 3 A a.c. modulation at 2500 Hz, the discharge again confirmed pronounced disturbances, however was stable at 2300 and 2700 Hz. The instability may begin at either electrode, whereupon the discharge constricts into a thin luminous ribbon with sinusoidal form, and the voltage will increase due to elevated arc size. [Instability at] harmonics of the basic frequency may even be noticed[...]

"[...] In a spherical bulb with electrodes at the centre [like a Xenon arc] acoustical resonance occurs when the bulb diameter is equal to one-half [of the acoustic modulation] wavelength [...] An arc centred in a spherical bulb will literally 'blow itself out' by its own sound waves if any of the strong resonance frequencies are utilized to the lamp for an appreciable time [...]

[...] In a low stress discharge lamp [eg fluorescent], assuming a median gasoline temperature of 250 degrees Centigrade, the velocity of sound in mercury vapour was 19,000 cm/sec, calculated by Laplace's method. At 600 Hz, the wavelength equals 31.7 cm [...] Maximum disturbance or turbulence on the electrodes happens at a half wavelength from the nodes which are on the ends and centre of the lamp."

Owing to the low intensity and prolonged supply space of the fluorescent lamp, the radiated flux could by no means be correctly collimated. These limitations, and the frequency response irregularities clearly indicated that our mild transmitter needed a change of strategy.

At this level, Rodney Reynolds VK3AAR drew our consideration to EEB's publication of the earlier work of Burlinson and Averay, whose modulator circuit and mercury arc supply was a completely novel strategy. Their optics, however, have been fairly crude, not nearly directive enough for our work in suburban Melbourne, where street lights and illuminated advertising indicators proliferate.

FROM FLUORESCENT LAMP TO MERCURY ARC

We would have liked the next depth source, no fluorescent coating, and an output wealthy in blue light to match the photomultipliers. From the electrical provider Arthur J Veall in Bridge Road, Richmond, John and i obtained some Philips excessive stress mercury arc lamps in January 1976, of the kind used for manufacturing unit lighting. Most of those had inconvenient fluorescent coatings, apart from the smaller lamps beneath a hundred watts output, which sadly had frosted glass envelopes. We purchased a few HP80 mercury lamps rated at eighty watts. Driving these was an actual downside. The fluorescent lamps that we beforehand used were low present, excessive voltage gadgets, modulated merely with normal output valves. By comparability, the high pressure mercury arcs ran at medium voltages (30 to a hundred volts) however with high current (0.5 to three amp standing current), they usually exhibited a severe unfavourable resistance characteristic - their voltage drop decreased sharply with growing present circulate.

Other problems arose owing to the instability of the mercury arc. Its putting voltage various broadly with ambient temperature, and its standing current diverse with time as heat brought about the mercury to vaporise and the arc strain elevated. At its peak, the quartz arc tube had to withstand inner pressures of around 30 atmospheres, so it was a system that you simply needed to treat with nice respect!

The slightest overmodulation peak would extinguish the discharge, and the entire device then needed to cool before it could possibly be re-struck at a moderately low voltage. The lamp polarity also had to be reversed at frequent intervals with a DPDT swap to keep away from premature cathode failure by ionic bombardment. The lamps were designed for AC operation, the place the results of cathode heating had been shared 50 instances per second by each electrodes. In spite of using DC bias on the tubes, for which they were not designed, we managed to get a couple of hundred hours out of them, which was Ok when the lamps only value $7 apiece. Today they value about ten instances that.

The modulator had to steer the current passing to the arc. Quiescent class-A amplifier conditions demanded that at least half of the supply rail must be dropped by the sequence modulator, the remainder being dropped by the mercury arc. The negative resistance of the arc was our stumbling block. As the current by means of the arc increased its voltage drop decreased. This dragged the voltage applied to the series management machine up towards the rail voltage as present reached a most. With 200 volts-on the rail and a peak current approaching 5 Amps, no transistor generally available in 1975 might handle the job. They could not take the excessive peak currents simultaneously with the excessive emitter-collector voltage that this load with its destructive resistance would current to the output transistor at school A. We had some costly pyrotechnic displays of 2N3055 and BUX80 transistor failure owing to this secondary breakdown point being exceeded. Like many foolhardly experimenters, we hardly ever used fast-blow fuses in our gear. Youthful arrogance, I suppose!

The elegant answer, prompt a few years later by Rod Reynolds (VK3AAR), was to run the control transistor in parallel with the arc, with a single sequence resistor as much as the provision rail from each. In that configuration, the transistor passed most voltage at minimal present, and vice versa, in order that a much smaller transistor may very well be used than within the collection modulator configuration, the tradeoff being only a decreased total energy efficiency. However, on the age of 21 in 1975 - and John was 20 - neither of us considered that...

Instead, we used a category-A vacuum tube amplifier of elephantine proportions in series with the arc. The ability provide was capable of 200 Volts DC at three Amps, and the reservoir capacitors totalled 1500 µF at a 350 Volt ranking! For the modulator, four hefty 6080 or 6AS7 high present, low gm triode regulators were positioned in parallel with low-worth balancing resistors in their cathode returns. The anodes had a nasty habit of glowing a dull crimson when the arc was initiated, because it took a few minutes for the mercury discharge to run as much as its typical 100 Volts drop. During that time, the most important part of the provision rail was applied to the valves, which were sent beyond their dissipation restrict. Under these situations, one might see alarming high resistance 'spots' scintillating with a brilliant and sparkling yellow mild on the massive oxidised cathode surfaces of the 6080s. I at all times anticipated a 'bang' however by some means I used to be at all times fortunate...

Each 6080 filament consumed 2.5 Amps at 6.Three Volts. The valve heaters alone consumed a complete of 63 watts! It was all brute pressure, ignorance, bulk energy and heat! Fan cooling was obligatory. We used a domestic fan of 1928 vintage with a bum sleeve bearing, which rattled continually throughout QSO's. Nevertheless the system labored remarkably nicely, and due to Melbourne's many disposals stores, it may very well be constructed for a number of dollars. Not so now! Electronic disposals stores of that kind are actually largely a factor of the past.

This mercury arc transmitter was quite unstable in operation, liable to permitting the arc to drift into a gradual thermal runaway. To right this tendency, one had to trace the current of the arc by manipulating the grid bias of the output tubes - therefore the rationale for the metering of arc current and voltage within the modulator circuit, above. The temperature of the quartz bulb was an important variable, as this and the arc's standing current were carefully associated. The bulb temperature controlled the mercury vapour pressure within the quartz phial, and therefore the electrical resistance of the arc. With insufficient common present circulate, the heat of the arc can be too low to maintain the mercury vapour pressure, causing the voltage across the arc to slowly fall, in turn leading to an additional fall of current. If the standing present was too excessive, the arc would change into so hot that it not only turned dangerous, but was impossible to modulate fully. One could not depart the system unattended for more than about 5 minutes. Arc current would fluctuate, particularly in the time just after 'switch on', because the log report under clearly indicates. Warm-up would take around 15 to half-hour before the system finally settled into static present circumstances, with the heat pumped into the arc electrically equalling the heat misplaced to the encircling surroundings. The average standing present varied with the audio program content and with the symmetry of the audio waveform applied.

A typical example is provided by the following log that I took of the arc transmitter's electrical standing, measured via the night of Saturday thirty first January 1976. Operation on that evening was finest described as 'intermittent'! Eventually, in response to 3ZGJ's insistence that I add a damaging peak clipper to my modulator, I fitted a diode to the grids of the output tubes to do the job!:

6:05 pm

eighty V @ 0.Forty two Amp, 33.6 Watts

REMARKS: Test TRANSMISSION Only earlier than nightfall. Arc has been on for 45 mins of heat-up, approx 60% peak modulation, with music from 78 rpm discs (avoiding copyright issues!). Ambient temp. 350 C. VK3ZGJ not listening yet. Power and heater transformers are each scorching to touch, but Ok.

PHOTOMULTIPLIER RECEIVERS - 1975/76

The receivers used on this system had been also designed round components obtainable from disposals sources. Photomultipliers are superbly sensitive, and never nearly so fragile or troublesome to set up as some so-referred to as specialists recommend. They require a clean a thousand volt supply at about 5 or 10 mA - simply organized with an old valve radio transformer working into a voltage doubler. The hand-held photomultiplier receiver was a reasonably hazardous device while you have been manipulating it at night from the top of an earthed steel ladder! Particularly so when the one thousand volt provide had 8 µF of oil-stuffed block reservoir capacitor behind it (ugh!) and was fed to the receiver by flat 240 volt twin lead flex. It turned even more dangerous when rain was falling! In my youth I had scant appreciation of my very own mortality. Few teenagers ever do!

The previous disposals photomultipliers typified by the 931A had just about no response to red mild, so they couldn't be used with LEDs or HeNe lasers. Newer photomultipliers with gallium arsenide photocathodes have good purple sensitivity, but at prices exceeding $one thousand they're out of my league.

Overall outcomes with this cumbersome hyperlink system have been remarkably good. Considering the potential theoretical problems of thermal lag and ionisation time, the mercury lamp's restrict of 50% modulation at 10 KHz was fairly usable, entirely acceptable for direct amplitude modulation at audio frequencies. Piles of signal were accessible. The output was terrific. At evening, with one's eyes dark-tailored, you needed to guard your eyes with darkish glasses or welding goggles. But information and narrow band video modulation revealed wild section shifts above 5KHz. I suspect that the mercury plasma, when thermally modulated at an audio rate, was trying to expand and shrink in opposition to its quartz tube housing with each modu1ation cycle, interacting with the arc cavity in a fancy collection of acoustic resonances. Indeed, one might actually hear the modulation coming from the arc, notably at excessive audio frequencies, by placing your ear down close to the quartz bulb. One undesirable side impact was that sure modulation frequencies near the arc cavity's acoustic resonance extinguished the discharge. Sibilant sounds have been especially dangerous. In reply to a query concerning the modulator I remember saying over the hyperlink "it is working completely!" At the opposite end, all John heard was "it's working perf - Click". The arc extinguished itself, right on cue!

Over the 4 months as much as April 1976 the system was in operation as much as six nights every week between VK3ZGJ and my QTH at Camberwell Junction. It was unaffected by all however the heaviest fogs and rain showers. On very hot nights, low frequency noise and quick flutter fade had been evident, however rarely obtrusive. The impact was as a result of hot air cells rising from the heat floor into the cooler evening sky. This brought about a heat shimmer, with a consequent scintillation of the acquired light. We considered the usage of FM subcarrier modulation to clip off the scintillation on the acquired light modulation, however the mercury arc lamp hadn't a sufficiently quick rise time to help this. Subsequently I found that the use of longer wavelengths - purple or infra-pink gentle - greatly scale back scintillation. FM technique could also be fascinating for links of over 5 miles range, but for shorter distances amplitude modulation's higher out there base bandwidth in all probability makes it the popular mode.

The old mercury arc/ photomultiplier system may have had considerable long range potential but we never had the opportunity to check it. The heavy energy calls for of the terminal tools demanded a mains supply at each ends of the link. We never successfully tried DX away from our respective properties and two miles (3.5 km) was the best distance examined. We may generally achieve 45 dB sig/noise over that distance.

Communication ceased during the day owing to the scattered blue light of the sky. The photomultipliers had been saturated by it. Interference filters may have been used to extract a dominant spectral emission line of the mercury discharge from the overall background radiation, but I used to be unable to acquire one till the 1980s. They're costly and suitable only for parallel rays of light. There are higher methods of reaching good sig/noise in the course of the day, utilizing a monochromatic source and a wavelength away from the peak output of the sun.

However, I would energy up the photomultiplier with a comparatively low voltage - about 500 volts - just before sunset, to catch John's first distinct phrases because the ambient light pale. On scorching evenings, the sign was all the time lower than anticipated. I couldn't account for this, until one evening I climbed the ladder to adjust the receiver and observed a wierd smell. The receiver's optical cavity was filled with smoke! I had forgotten that John was virtually directly West of me, and as the summer solstice handed, the position at which the solar set on the horizon was shifting North on successive evenings. That evening, it was setting directly behind John's home - so my thirteen cm receiver lens was focussing the solar's image on the again of the receiver housing, which was manufactured from black-painted particle board. Awk! We had been so lucky not to have set hearth to all the pieces - but I stored the incident a secret from my people. Things that my mother and father did not know could not hurt them! With some embarrassment, I fitted the optics with a removable lens cover.

One potential use of the mercury arc transmitter that LED's, Luxeons and most low energy lasers most likely could never rival lies in non-line-of-sight (NLOS) linking by reflection off clouds. Some years ago I acquired a pair of 1 metre diameter searchlight mirrors to try the thought. The mercury arc may be mounted in a single, photomultiplier in the opposite, each monitoring the same cloud. Street lights in the present day use the same type of mercury discharge as the modulated source, so that 100 Hz hum background plus harmonics would be unavoidable in an city setting. I've but to strive the idea, as direct linking is undoubtedly extra dependable and efficient, and has subsequently been my fundamental line of research.

OPTICAL COMMUNICATION'S 'Comic CUTS' - 1976

Almost as quickly as the optical hyperlink between John VK3ZGJ and myself was established, he relayed me to 144 MHz on the cross-band discussions radiated by VK3AML on 160 metres, several instances each week. For all practical purposes, I was on an intercom in 3ZGJ's shack, and there was no legislation in opposition to that. Legally and technically, I was a supervised (ie, pull the plug out if he misbehaves) customer who just occurred to be present near 3ZGJ's microphone!

Through the early a part of 1976, there have been many cross-band contacts on 160 metres with me "on lightbeam" which appeared so as to add technical novelty to the proceedings. The discussions had been completely open to any subject, provided that the due decorum of the medium was maintained. On some occasions, it wasn't! One evening we in some way drifted onto the subject of funerals, morticians, Egyptian mummification and the 'dear departed'. This contact, with many others from that 'mild beam hyperlink' interval, was logged on audio tape by Tony VK3AML and by other listeners, and extracts might be downloaded from Tony's 'bluehaze' internet site ("Multimedia Page 3") as "Death By Light Beam" (mp3).

Over time, the content material different from creditable technical depth to immature ratbag ramblings. I never stored observe of all the log tapes, and that i by no means know when bits of it's going to re-emerge on the weekly replays that have been maintained by VK3ASE - and others on Internet - haunting me ever since...

Most of our issues with the outdated fuel discharge lamp system have been more comical than theoretical, and most had been utterly unforseen. Moths, for instance, had been irresistibly interested in the extremely-violet output of the mercury arc. Bogong moths, huge Emperor Gum moths, Christmas beetles, flying ants - the total box and dice! They swarmed the transmitter arc on scorching nights, attenuating the beam flux and suiciding against the hot lamp. Every couple of hours I'd should scrape their smouldering bodies out of the lamp reflector. Their fluttering wings modulated the transmitted beam, and the light they mirrored formed a feedback path into the adjacent, constantly working, duplex optical receiver. If I had my transmitter microphone working near the monitor loudspeaker, the presence of an Emperor Gum moth (which, I should clarify for non-locals, are the dimensions of a small dinner plate) was signalled by ear-splitting feedback warbling at their wing-beat frequency of about 15 Hz. This was usually followed by our neighbours lobbing pebbles onto the tin roof of my radio shack to get me to 'turn my bloody noise down'! As we frequently operated between midnight and 3 a.m., I can hardly blame them. We referred to as the effect "moth-back" .

Another unexpected fault took ages to detect. After a seemingly harmless rainstorm, the mercury lamps would stop to function. The lamp itself seemed perfectly Ok, even on close examination. We finally found that water had drained down the bulb into to lamp's Edison screw socket. There, the DC bias arrange an electroplating motion, dissolving the wires connecting the bottom with the bulb. The issue was quite merely solved by mounting the lamps upside-down.

Our line-of-sight path, skimming the tops of suburban Hawthorn's buildings, backyard foliage and energy strains made continuous contact between the terminal stations the subject of persevering with concern - and a few unusual QRM. The scenario worsened in windy weather as there was a tall gum tree very shut the the optical path, in a backyard about a half mile distant from 3ZGJ. In a Northerly wind this massive nuisance had branches that would sway back and forth into the beam path, slicing words and phrases out of our conversations. Eventually the rattling thing turned such a drag that we made half-critical plans about sporting balaclavas on an illicit midnight raid on its owner's garden with ropes and a pruning saw! Another suggestion involved borrowing a surgical chopping laser from Melbourne University's medical school, and trimming the foliage from a distance - the gap of 3ZGJ's shack veranda, in reality. A couple of years afterward, I took a have a look at the same path with binoculars, and located that the nuisance tree, by then with several others, had grown to fully block the optical path. L.O.S. paths will be impermanent! The growing of tall trees in suburban streets and gardens is now way more fashionable than it was thirty years in the past. It could also be unfortunate for optical communication fanatics, but it surely gives an aesthetically pleasing manner of supporting h.f. wire antennas. (I'm humming a chorus of Monty Python's "Always look on the bright aspect of life" whereas I'm typing this)!

The colour of the mercury lamp additionally supplied problems. It regarded the identical as some other avenue mild in Melbourne. One night time, we drove up to at least one Tree Hill in the Dandenong Ranges outside Melbourne in an unroadworthy Austin with the portable LDR receiver, optimistically hoping to set a distance file. At the highest of the outdated lookout tower there (removed in 1981) we looked down on the lights of Melbourne, which have been as numerous as the grains of sand on a seashore. With John's optical receiver we searched the varied lights seen in the final path of Hawthorn for about half an hour, vainly making an attempt to listen to audio modulatio