This article first published in the CARC Newsletter of April 2000 remembers the pioneering work on LF done by Derek G3GRO and Lech G3KAU sadly both now SK.
Firstly, thanks to all those who participated and to all those sent in signal reports and messages of encouragement. The exercise was arranged at very short notice and word spead around the LF operators living within reasonable reach of the site to invite one or two groups of operators to take part.
Apart from the Decca mast and an RF thermo-couple ammeter , none of the original equipment such as loading coils etc was used. For 136Khz operation a relatively small variometer ( about 500uH max) and a tapped toroidal auto transformer were connected to ground from the copper pipe lead-in to the base of the mast which is supported on 4 massive ceramic insulators. The exploratory visit earlier in the week (11th April) allowed a purpose-built additional loading coil for 73Khz to be built by Lech G3KAU back at base in time for the main excercise at the weekend. Three transmitters were employed at various times, the G3GRO 300W much modifiedBKE linear amplifier used for the 1W ERP tests on 73/136Khz earlier in the week and on Friday and Saturday for running between 1 and 5W ERP plus the well tried G0MRF 500W set-up and the G3YXM 1Kw class-D rig used previously on many /P expeditions. During the overnight sessions on Friday in beacon mode the ERP was 50W and 100W ERP overnight Saturday. We had a few problems to start with in getting the variometer to handle the 500W RF and we had a few splatts and sparks resulting in VSWR trips until we realised that the capacitive voltage divider in the forward/reflected power meter inthe variometer was tracking over and by-passed it. Fortunately we had another SWR meter in line. We also found that an RF sampler unit brought along by Jim M0BMU very useful in setting up the matching and tuning in conjunction with an oscilloscope very useful. We realised on Saturday that the range of our RF ammeter was too small for the high power operation and rescued the original very large 30A RF ammeter from the pile of redundant scrap units outside in the rain and pressed it into service. During the beacon sessions that was reading 14A into the base of the mast which looks like 5ohms resistive in series with 3750pf ! That represents an RF power of 1Kw into the antenna or 100W ERP allowing for an overall antenna efficiency of 10% on 136Khz.
The receive system was a home brew up-converter with an input bandpass filter of about 3khz bandwidth on both 73 and 136Khz followed by a mini-circuits MAR6 preamp and MC1496 mixer IC to a 10Mhz IF feeding both aFT990 and IC756 transceiver for most of the time. The FT990 and converter stood up remarkably well to the very large antenna input with no sign of cross-mod. We had a switched attenuator at the input to the converter but for most of the time it was switched out. That attenuator was used laterfor some comparative tests with a small “back-garden “ type antenna nearby which hopefully will be reported on later. We had two operator positions side by side, one handling the 136 and 73 traffic and the second op also monitoring 136/73Khz in parallel but also handling the HF cross-band input mainly from 7.0Mhz.
Summary of Results : We had about 65 QSOs in total including those duringthe initial setting up period on Tuesday 11th of April already reported. Two-way contacts were made on both bands and cross-band contacts from 136Khz to 73Khz and to 7.0Mhz/ There have not been any reception reports from across the pond from VE or W which was always going to be a long shot, but the longest contact was cross-band to 7.0Mhz with Alex, UB5WF in Loc KN58JQ about 200Km north of Odessa on the Black Sea who gave us RST429. normal CW at a distance of2225Km ! This was over a daylight path at 12-32 UTC on Sunday 16th. It is not known yet what RX antenna Alex had for 136Khz but it is highly unlikely that it was a dedicated LF antenna since there is as yet no LF activity in Russia as far as is known. Other long distance QSOs were to Valerio, Ik5ZPV2-way on 136 who gave us RST589 IK7HSS cross band to 7.0Mhz, and NeilsOZ8NJ (2-way136) who QSPd to us that IK5ZPV was hearing us and would callus shortly. We also got a RST 599 on 136Khz from Christer SM6PXJ , OZ5Nand Toni HB2ASB. Two QSOs were also made on both 136 and 73Khz to Finbar EI0CF and Ray GI3PDN. Reports on 73Khz were about 2-S points down with Rayand Finbar by comparison with 136Khz. We also worked GJ4CBQ and GU3SQX cross band 136Khz/7.0 Mhz which was pleasing since due to Loran QRM from Lessay they normally have difficulty in hearing stations on 136Khz. Perhaps one of the most unusual QSOs was with Graham G3XTZ/M operating mobile on 136Khz CW whilst driving to the site to have a spell of operating! I will post a full list of the QSOs made in the near future when I have transposed the logs. We have also just had a report via E-mail from MarcF5MAF in Toulouse, JN03PP who was hearing us at 599+ on a 2 meter diam loop at a distance of 900Km and bemoaning the fact there is no LF activity inhis neck of the woods.
Thanks to all those who took part despite the very wet and freezing cold weather and also to those who took the trouble to give us reports whichhave yet to be analysed. At one point on Saturday as the shifts changed over there were 12 people in the ATU shack at the base of the mast which must represent a large slice of the active UK LF operators! They were :- G3KAU, G3XDV , M0BMU, G0MRF, G3YXM, G3XTZ, G3YSX,, G4GVC, Kate,G4JAI, XYLGVC, G4TSH, G3LHZ , G3GRO and not forgetting Peter G3LDO holding the fort back home and Walter G3JKV who’s efforts made it all possible.
73 de Derek Atter , G3GRO
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