How was the special event station GB50 designed?
07 November 2002
At Greenwich, during the Millennium event, Cray Valley Radio Society had the luxury of two 70ft guyed crank up towers to support the HF antennas. These were placed at opposite ends of the building about, 150ft apart. At Windsor, tall antenna supports were deemed to be inappropriate. However the North Terrace location benefited from being high above the town, with a clear takeoff from west through north to east. Fortunately these were the favoured operating directions for GB50!Three HF stations operating in close proximity required careful design to minimise mutual interference. To this end each station had a set of Dunestar bandpass filters that automatically switched when a band was selected. Only HF1 had an amplifier running 400W, the other stations using transceivers at 100W and this further reduced the chance of inter-station interference.
All transceivers at GB50 were brand-new Icom rigs loaned by Icom (UK) Ltd. The station was designed to ensure that antennas and filters were switched automatically which removed the need for plugging and unplugging of antenna leads, Cray Valley designed the three HF stations with fixed configurations and limited them to certain bands, as listed below.
HF1 was the main demonstration station for the general public. Located on a large, curved table, the transceiver was the impressive, top-of-the-line Icom rig, IC-756PROII. The IC-756PROII uses DSP for improved receiver performance and clear transmitted speech. The Icom IC-PW1 amplifier was used on this station and was automatically switched to the correct band by a connection to the transceiver.
Only the three main HF bands 10, 15, 20m were used on HF1. On the tower close to the marquee entrance was a Force 12 C-3SS tri-bander supplied by Vine Antenna Products. This antenna uses full size elements on 10 and 15, two elements per band, and linear loaded elements on 20m (an efficient method of shortening antenna elements.) The C-3SS gave monobander performance on all three bands. The rotator for this antenna was a Kenpro KR-800 giving 450 degrees of rotation. The tower, belonging to a member of the team, also supported one VHF beam, dipoles for 40 and 80m used on HF3, and a small 2m beam for APRS.
HF2 was dedicated to the 12, 17 and 30m bands. This station also used an IC-756PROII, and a set of individual Dunestar bandpass filters switched automatically.
Antennas for this station were on the tower at the corner of the terrace. This tower had been used by the Granta Contest Group and was loaned by Plextek Ltd. The H.F. beam was a Force 12 WARC 2/2 loaned by Vine Antenna Products, it has full-sized elements on 12m and 17m. The 30m dipole was also supported on this tower, together with a 15m loop used by HF3, the 2m beam and a 70cm yagi for access to the packet cluster node GB7DXH at Harrow.
HF3 used a different transceiver, the IC-7400, known as the IC-746PRO in some countries. This rig is a development of the IC-746 but like its bigger brother uses DSP for selectivity. Whilst it does not have dual watch, it does cover 6m and 2m as well as HF.
HF3 was used mostly on 40m, this band gives excellent coverage of Great Britain and Western Europe throughout the day. There was also the option of operating on 80m and 15m. Space limitations led to the decision early on in the project plan to not operate on top band. The addition of 15m to HF3 gave the team extra flexibility and meant, for example, that if HF conditions were good GB50 would be able to ‘run’ contacts to the United States on all three bands 20, 17, 15m simultaneously. It also gave rise to a potential problem: if HF1 was already on 15 and the operator on HF3 selected the same band, damage could occur. Therefore an interlock was used between the two stations to prevent this happening. This arrangement also worked effectively where both 10 and 15m were shared.
6 metres The IC-7400 was used at 100W. The antenna for this band was mounted on the front of the tower near the marquee entrance. This was a 5 element Yagi on loan from G4EGU, the rotator was borrowed from Gravesend Radio Society. There were no band-switching or filtering issues with the VHF stations as there was a dedicated transceiver for each band. There was a limit to what could set up at Windsor and the decision was taken early on not to try to cover 4m or 70cm.
2 metres The IC-7400 could have been used but it was decided instead to use the Icom IC-910H. This is a dedicated VHF/UHF All Mode transceiver, capable of running 100w on 2m. FM was used from time to time but operation was mainly on SSB, with a little CW as well. The antenna was an 11-element Yagi mounted on the WARC tower to maximise separation from the APRS antenna. The G-450 rotator was supplied by Nevada Ltd.
APRS – the APRS demonstration used an Icom IC-2800 transceiver, with a TNC provided by M0BGR. The UI-view software was kindly supplied by G4IDE. A small HB9CV 2m beam was all that was needed to give excellent access to the nearest node.
Call logging was achieved using the DXpedition version of Turbolog written and supported by John Linford G3WGV. This is the same version that was used on the D68C operation from Comoros. Each of the five stations had a PC running Turbolog, with an interface to its transceiver. Turbolog generated voltages to drive antenna and filter switches via a small interface circuit. Each logging PC was itself connected to a central server via Ethernet, in this way the server managed to keep an overall ‘master’ log in addition to the log kept locally at each station. This allowed the team to very quickly gain GB50 operating statistics from the server. Some of the details such as QSO totals, countries worked were made available and displayed on the ‘DX Monitor’ screen in the Meet & Greet area.
Operation ceased at 10pm each day, a file containing all that day’s contacts was extracted from the server and sent to the web site www.gb50.com so that those who had worked GB50 could check that they were in the log.
Another feature of Turbolog is that it contains an integrated CW keyer, so operators did not need to bring their own keyer or use the rig’s internal keyer. The keyer unit also had an input for a straight key. Turbolog can also be set up to send standard messages such as CQ calls.
The Greenwich station, M2000A, was limited to SSB, CW and a little FM on VHF. GB50 however had the added capability of RTTY and PSK31. Priority was given to phone and CW but datamodes are often a very useful way of demonstrating different aspects of the hobby, and these modes gave extra opportunities to work towards the GB50 Points Award.
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