With the local temperature finally under 95F, it's time to work on low band
During the middle of last season, I noticed the "Rotatable Flag" RX antenna
performance dropping off. Towards the end of the season, it suddenly didn't
work on 160 at all. A little troubleshooting showed that the transformer
went through a melt down. I still don't know what happened, maybe lightning
or 40 meter RF, but the small gauge windings were burned through! Easy
enough to wind a new transformer, but that didn't solve the slow
degradation that was still happening.
To make a long story short, today I found that the termination resistor had
changed value. It was made from three parallel 1 watt carbon comp resistors
to give about 920 ohms originally. When I measured it today it was 1550.
F/B with this termination was, at best, about 13 dB. After replacing the
resistor with a single 820 ohm, 1 watt, carbon comp, the F/B measured
between 34 and 23 dB, depending on frequency and sky/ground wave. The rear
null is very sharp again. As soon as I can, I will replace that carbon with
a good metal film job.
Now I could tell that I had other problems! The coax needed some attention
as well as the connectors. Although not on the flag, one piece of RX
antenna feed line had taken on water and the shield deteriorated to the
point where it acted like an antenna. BC birdies and other trash were very
bad until that coax was replaced.
It was found that an additional 10 dB of common-mode rejection was obtained
in the flag feed line when a second isolation transformer was inserted at
the receiver end of the line. This coax is in good condition and run
through plastic conduit underground for 200 feet. It still picked up a lot
of crap even when terminated at the far end with a dummy resistor.
It would seem like a good idea to check for some of the above problems
before blaming a RX antenna for poor performance. Too bad I didn't do it a
couple of weeks ago, I might have actually heard the ZD9!
Larry - W7IUV
arry - W7IUV
FAQ on WWW: http://www.contesting.com/FAQ/topband
Administrative requests: topband-REQUEST@contesting.com