Monday, May 04, 2009

Ten metre madness

My intention to try PSK31 beaconing on 10m to detect sporadic-E propagation turned into something of a nightmare. What happened was this. I saw traces on the waterfall and decided to kill the beacon macro and make some contacts. I called Roberto, IV3KFV, and he came back with a report of 527 and said that my IMD was <-9db! I switched on the IMD Meter and sure enough, it was beeping away reporting IMD of around -15dB. Something was wrong and I would obviously have to investigate.

What then followed took up more than a day and had so many red herrings and false hopes it seemed as if someone "up there" was determined to drive me crazy. Many times it would seem as if I had found the cause of the problem, and then the next moment it was back.

Finally I got to the point where I had the K3 with just the power supply and the antenna connected to it. Nothing else. Using the internally generated PSK I was still getting poor IMD when transmitting into the antenna. I then discovered that putting clamp-on ferrite chokes on to the power supply cable affected the IMD. With 4 chokes on the cable the IMD was perfectly acceptable. I moved the K3 back into the operating position and ... bad IMD again!

At this point I was out of ideas. Changing the antenna or moving it further away from the transceiver was not an option. Due to back problems it's difficult for me even to work on antennas. Operating from the ground floor where a better RF ground would be possible was not an option either. With a house this small and antenna restrictions you don't get a lot of choices.

I decided that the K3 must just not like something about the situation here, so I thought I would try all my other radios to see if any of them was less fussy. I tried the the K2 (both with its power supply and running from its internal battery), the Juma TRX2 and the FT-817 (from power supply and battery.) All radios exhibited poor IMD when transmitting into the 10m antenna, compared to when a dummy load was used!

The obvious conclusion is that the IMD Meter is giving false readings. However, that doesn't explain the bad signal report I received from IV3KFV. Having a local who could come on and listen to my signal would be a big help at this point. Unfortunately there are no other active amateurs here in Cockermouth that I know of. The nearest amateur to me is a few miles away, on the other side of a hill, not a strong signal at the best of times, and I rarely hear him on the air.

So the problem remains unsolved and I am giving up. There is only so much time you can spend trying to fix something and getting nowhere. If I can't work on 10 metres then I guess I'll just have to live with it.

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