The Magic band
Achievements to date
125 DXCC countries worked (Several GM firsts) as of 17/04/05
Clean linear amp? Harmonic suppression
Dual band antenna? Diplexer 2m / 6m (updated 5th Feb 2005)
Need to combine several Power Amplifiers ? The Bagley Polygon
Real Audio Soundclip, GM4ISM - VK6KZ QSO on 8.2.92 …. 29Kb
Real Audio Soundclip, Pileup from the USA…. 48Kb
Potted history of GM4ISM's station
6 Meters is my 'primary' band. I first built a converter when the first UK 6m permits were issued, and was amazed at the dual personality of the band. On one side it behaves like the higher VHF bands, with similar propagation effects, on the other side, especially when Sporadic E is present, it behaves like an HF band. Later, with the peak of the sunspot cycle, I found this VHF band to be superlative. I like a band where conditions as a norm allow only limited range, but under those special conditions, allow the ultimate in DX. Success is a combination of luck, and operating skill, as some of the openings were so short, or the competition so fierce. It is possible to work thousands of Km on a very simple system. I used 20W RF to a whip (5/8 l on 2m, which happens to work well as a base loaded quarter wave on 6m).
I even built a diplexer to enable both bands to be used at once.
From the mobile I have worked most of Europe, a couple of African countries, South America, North America and Asia. I still need Australia from the mobile station HI.
From Home I used to run the same FT690 II (which long ago replaced the HB transverter that evolved from the original converter). Now I run an FT847, with the 4CX250B Pa if needed.
The antenna is a home brew 5 El Yagi to NBS, fed with a hairpin type matching system and phasing loop Balun (ARRL HB). It is over 10 years old and still works well, probably because the driven element and balun connections are designed to be open to the weather, and water cannot build up on the joints. Trying to seal a complex mechanical system often lead to trapped water and corrosion problems.
The other unique feature is the clamping method that attaches the boom to the mast.
An oversize clamp is used with a heavy rubber sleeve fitted round the boom. This allows the boom to move slightly and buffers the antenna and mast from gusts of wind.
Over the years I have had a number of Power amps. One of the early ones was a QQV0 6-40, which managed about 90W with a lot of HT and somewhat red anode colour.
I then converted a QY3-125 Band II PA, which was considerably happier running 125W out, and presently resides with GM4LBV. That amp has seen a lot of heavy service from here and from the Ariel Radio Group club station GS3RIJ.
During one of its rare failures, I was forced to hastily commission an old band I TV transmitter amplifier using a 4CX250B. Since the RF deck was already 'tuned' all I had to do was add bias & HT. A couple of hours work saw an amp capable of about 175W op.
Terrible efficiency.. A triple tuned anode cct designed for wideband TV signals was not particularly effective, but it worked, and because the sunspot cycle was in full swing, it took a couple of years for me to rebuild the tank cct.
I have refined the system a little, adding a 100W solid state reserve, which is always ready, even if the filament delay is not. The single tube runs about 300W under normal conditions. With 2700V on the anode I can get a little more HI, but the tube runs a bit hot!
I have recently tried PSK31, which requires quite a linear system and although the received signal degrades slightly above 400W op, the signal is perfectly decodable, suggesting moderately good IP performance. The only thing I found was that the 100% duty cycle on transmit causes unpleasant hot smells to emerge from the amplifier.
A more robust amplifier that will run full power at the antenna continuously is going to be required for this mode. WSJT modes which I have employed recently do not require high linearity (at all!) and the transmit duty cycle is OK. See my WSJT page for more detail of recent operation on this mode
I have started a dual band (2m / 6m) PA using a 4CX1500B. Maybe
I will get round to using the 3CX2500 eventually HI.
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