By Barry Cleworth VK5BQ
There is little doubt that among the many modes of transmission available to radio amateurs, television has the potential to capture and maintain the interest of JOTA participants. This is particularly so when a full duplex system is employed, enabling guides and scouts at both ends of the link to see and hear each other simultaneously.
Following our previous years example, David VK5KK and I set up a wide band FM ATV link, using frequencies on the 1.2Ghz and 2.4Ghz UHF bands, between my QTH at Stansbury on Yorke Peninsula, and the SA VHF Group headquarters at the Elizabeth water tower. A back up link was also established at Davids QTH, where his superior elevation of 185 metres ASL provided continuously reliable results.
Although the path length to VK5KK exceeded 80 kilometres, P5 pictures were achieved for most of the time. The option to establish links between guide/scout halls was not considered, due mainly to anticipated propagation problems presented by their locations.
However a link between VK5BQ at Stansbury and Lee VK5YLE at Greenacres made possible a relay on 426.25Mhz to the OHalloran Hill AM ATV repeater. VK5RTV viewers across Adelaide were able to see some of the JOTA activity on the repeaters normal output frequency of 576Mhz (Channel 35).
This output frequency being on a public broadcast channel provided a window for the public to view JOTA activity. Reports of signal quality by some of the viewers were most favourable.
An engineering channel was also established on 70cm to provide liaison between the technical directors, Barry VK5BQ and David VK5KK.
The ATV Studio
Since the radio shack at VK5BQ is only a converted fourth bedroom in the house with quite restricted floor space, it was again decided to set up the lounge room as the JOTA studio, where the twenty young girl guides and their leaders could be more comfortably accommodated. However it became quickly apparent that the studio was just on the overload point, confirming the authors view that small JOTA groups are to be preferred both for ease of accommodation, and behavioral controls.
In regard to the last subject, I am aware that there are many amateurs who have withdrawn their assistance to JOTA through behavioral problems. However, I could not have been more satisfied with the behaviour of my group, which included girls from Yorketown, Minlaton and surrounding areas.
The girls watched their JOTA counterparts on a 63cm receive monitor with audio and video inputs from the 2.4Ghz receiver in the shack via shielded tie lines, under the floor. The 2.4Ghz transmissions from
the water tower emanated from a 25W transmitter and slot antenna. The backup signal from VK5KK radiated from a parabolic dish fed from a 5-watt transmitter.
In the studio at VK5BQ, two cameras were initially employed, utilising a video switcher routed to the shack (transmitter room). However, one camera was retired due to malfunction.
Audio via two microphones was fed through an audio mixer prior to being routed to the transmitter.
Two 34-cm monitors were deployed in the studio for monitoring camera output, and final program line output, prior to being applied to the transmitter.
The transmitter being fully home constructed from kits available in VK5, was set up for about 19 watts output, and fed via LD4-50 heliax cable to a fully home constructed 2.1 metre parabolic dish. The output frequency of 1250Mhz is an accepted simplex channel regularly used by up to about ten amateur stations in VK5.
Received signals, on 2.4Ghz, returned via a 1.2 metre dish also fully home-constructed. As most ATVers will be aware, activity on amateur television remains within the realm of the equipment home brewer. In fact, the only commercially manufactured equipment used for this event at VK5BQ were the camera and monitors, and of course the 70cm liaison transceiver.
Another small piece of equipment used was the vision distribution amplifier, (VDA) or video splitter if you like. These devices are used to obtain several video signals at 1V p-p, from the one source. In our case, it was required to simultaneously record both incoming and outgoing programs, necessitating multiple video and audio outputs. Two other monitors were also fed from one of these VDAs as it was required to monitor the off-air signal and the incoming received signals.
Lighting in the ATV studio is not something to be overlooked and after some experimentation, a big improvement over last years effort was realised. Fluorescent tubes were tried but abandoned in favour of incandescent lighting with daylight deliberately excluded, as a mixture of two lighting sources with differing colour temperatures is rarely satisfactory. The improved lighting, coupled with the excellent smooth camera work by my son Peter, drew several favourable comments from various viewers some of whom were not involved in JOTA.
Few technical problems became apparent, despite the very hot and windy weather on the Saturday and the resulting brief power outage, fortunately not requiring the generating set. Some SSB interference accessing the audio circuits at the water tower caused some concern, but was quickly dealt with by Dave VK5KK.
Interaction between transmitter and receiver was not a problem, no doubt due in part to the positioning of the parabolic dishes and, of course, their sharp radiation patterns.
Although propagation conditions were a bit variable on the hot and windy Saturday afternoon, a cool change going through the area brought improved signal stability on Saturday evening and Sunday morning.
At about 8PM on the Saturday evening the ATV gear was switched off to facilitate activity on the HF bands, but despite the 400 watts PEP on 20 metres fed to a 3 element triband beam, results were very disappointing. Quite a few contacts were made on 2 metre and 70 centimetre repeaters. Scouts and guides at the water tower however were involved with JOTI contacts and HF.
Publicity for JOTA
A surprise event emerged this year in the form of a visit to the water tower by a team from a commercial television company based in Sydney. Four staff members including cameraman, sound technician and interview officer was very pleased to record a segment for the Channel 9 weekly childrens program Squawk. They were quite impressed with our TV link for JOTA and perhaps it should be put on record here, that although the media and professional TV stations regularly involve themselves in two way (duplex) TV links, on a daily basis, it is probably quite rare among ATV operators, particularly over 80 kilometre paths.
In conclusion, I would like to thank my two assistants, Gordon Welsh VK5KGS, and my son Peter, for their excellent help. Hopefully next year more ATV stations with very small groups of guides or scouts may become involved, using some of our ATV frequency allocations on the UHF and SHF bands, perhaps with duplex cross-linking. It is certainly a challenge and some planning by David VK5KK and others may already be in the pipeline.