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Topband: Flag Notes

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Subject: Topband: Flag Notes
From: (Larry Molitor)
Date: Sun, 24 Sep 2000 00:50:45 +0000
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

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