The Crawley Amateur Radio Club hosted another very successful and enjoyable Microwave Table on September 12th, when yet again we were favoured with good weather and a good attendance which encouraged participants as in previous years to spill out during the informal sessions into the open air on to the forest area immediately surrounding the Clubhouse for ‘eyeball’ chats or outside demonstrations etc.This event is one of several organised each year at different venues in conjunction with the UK uWave Group and we again had visitors from a wide area of the South and South-east, Oxfordshire and East Anglia with a total attendance of around 45 including CARC members which meant that we had pretty well a full house for the formal technical presentations in the afternoon session.
The general format was similar to that of previous years with an informal session in the morning with tables laid out for the ‘bring and buy’ sale in parallel with an exhibition of home constructed equipment being entered into the competition for the annual G3VVB trophy.
For those not familiar with the history of the trophy, it is awarded annually in memory of Cyril, G3VVB by the judges who in their opinion, entered the best made piece of home construction in the contest. The actual trophy is a beautifully engineered microwave power amplifier cavity in manufactured in highly polished brass intended to accept a ceramic 2C39 valve representative of the many that Cyril used to make each year for the amateur fraternity which enabled many would-be “microwavers” to get on air on 23cm or in the case of your scribe, on 13cm with the moderately high power level for those days of 35W SSB before the days of an affordable solid state PA! 
After the lunch break, the results of the constructional contest are usually announced followed by the presentation of the trophy to the winner and then the main technical session with several technical talks until the close at around 16-30pm.This year the doors opened at 10-00am as usual and by the nominal start time of 10-30am, the tables were almost full of a wide variety of items being offered for sale and the Coffee bar was in full swing with cups tea and bacon butties etc. disappearing at a rate of knots to satisfy the pangs of hunger of the early starters. As usual our two stalwarts Ted Macdonald G4TTY and Dick Lupton M0RXZ manning the coffee were kept pretty busy
Meantime, the informal chat session was well under way with old friends meeting up again and bargains on offer being picked over, - this being a good place to pick up microwave bits not readily available through main dealers.
Around a dozen exhibits were entered into the constructional display with again some excellent examples of home brew in the microwave field on display for the Judges including the trophy itself. Covering a wide range of complexity, the items being judged ranged from a nicely made Mini VNA network analyser through to a magnificent 500W 23cm MOSFET linear amplifier for masthead mounting with a remote a control unit and calibrated power meter.
During the lunch break Ted and Dick were again kept busy in serving their famous burgers and bacon rolls etc. after which the result of the G3VVB constructional contest was announced by the chairman of the judges Allan Wyatt, G8LSD and the trophy presented to the overall winner of the competition, John G3XDY for his 23cm amplifier.
There were three technical papers presented in the afternoon session. The first of these was entitled “What To Do With a Big Dish” by Mike Willis who works at the Rutherford-Appleton Research Laboratory and who described how he made the most of an unusual opportunity recently in having access to one of RAL’s large steerable microwave dishes to use on the amateur microwave bands for a weekend and some the results of the troposcatter and EME signal paths he explored .
The second paper was given by Sam Jewell, G4DDK describing the latest modifications to fine tune the design of his ultra-low noise 23cm preamplifier design which uses a MGF4919G HEMT device in the input stage to achieve a remarkably low repeatable noise figure of around 0.25 dB with an associated overall gain figure around 36dB.
The mounting of the input HEMT device is unusual in that rather than the source leads being soldered directly to the PCB earth plane in order to obtain minimum inductance, the leads are raised up above the PCB with small inductances connected to ground to provide some series negative feedback which optimise the noise figure. There are also small inductances in the gate input circuit to improve the input match and in the drain lead to optimise the output match. The profile of the HEMT looks rather like a small praying mantis insect sitting on the PCB!
Sam reported some impressive repeatability statistics for noise figure varying from a minimum of 0.2dB to a maximum of 0.3dB over a sample size of 25 units. Sample measurements have been verified with NF measurements taken at various EME conferences.
More information is available on Sam’s website at www.g4ddk.com
The final paper was delivered by Brian Colemore, G3NNS who described some of the more recent weak signal modes such as WSJT and JT65 etc. now available for use via a PC and sound card for the enhancement of weak signals. These modes employ DSP techniques and waterfall displays for Troposcatter or EME or JT6M for Meteor Scatter which will reliably decode data signals many decibels below the noise floor and use multi-tone encoding with FEC or forward error correction. These methods allow decoding even when the signal is so weak that it cannot be distinguished by ear alone.
Earlier weak signal modes developed for VLF using very slow morse (QRSS) do not cope with the slowly changing propagation effects experienced such as with EME or with burst-mode propagation as with meteor scatter.
The Round Table formally closed at around 17-00 and judging by the closing comments made by the departing participants, was felt to be an enjoyable and successful event.
One piece of post-meeting feedback was that unfortunately there had been major accident eastbound on the M23/M25 causing long tailbacks and several hours delay for our guests starting from almost immediately after their exit from the Club.
Fortunately for your scribe, realising that the A23 was already blocked northbound from Cheals roundabout, my driver managed to turn immediately right at the Football Stadium roundabout and went up through the town thus avoiding all the local blockage!
Note: the date for the 2011 Roundtable has already been provisionally set for Sunday 11th Sept 2011- Put it in your Diary.
73, de G3GRO
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