Please pardon my repeating myself, but this thing has really got me
buffaloed, and I've found that this is the place where the most
knowledgeable people about this sort of thing hang out
I am feeding DC down my feedline to the ARR preamp in the RX antenna
hub, and also to the relays and the logic inside it (my cheapo Chinese
relay board). I now have the hub sitting out there with no antenna
connected, so it is effectively just the preamp and the relays on the
end of the coax, plus common mode pickup on the coax.
On 1550 KHz (my local 70-over-9 broadcast station), this combination is
> 70 dB down as compared to my 160M shunt fed tower. However, if I go
up to 20 meters and find a strong station, then the feedline-cum-hub
combination receives about as well as the tower, and is only ~20 dB down
from a single small tribander.
Now here's the mysterious part. If I remove the DC power from the
preamp, the 20-meter signals drop from S9 to barely audible. This is
also noticeable, but just barely, on the 1550 KHz signal. Is it possible
that the preamp, which is between the feedline and the primary of the
binocular matching transformer, is somehow amplifying the common mode
signals? The shield of the coax connects to the shell of the preamp, and
from there to the secondary of the matching transformer (the 75-ohm
side). Is it possible that common mode signals are getting back into the
preamp input through the matching transformer primary? If so, any ideas
on how to clean it up? Or should I just get rid of the preamp out there
and do my amplifying in the shack?
--
73, Pete N4ZR
The World Contest Station Database, updated daily at www.conteststations.com
The Reverse Beacon Network at http://reversebeacon.net, blog at
reversebeacon.blogspot.com,
spots at telnet.reversebeacon.net, port 7000 and
arcluster.reversebeacon.net, port 7000
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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