Topband: Signal pickup mystery

[Pete Smith N4ZR]

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
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