Topband: Where to place a preamp? Switching Beverages?

[Greg - ZL3IX]

Hi Pete,

The common mode issue doesn't really affect your decision.  You will 
need to ensure that common mode pickup on the coax doesn't get into the 
antenna, which ever position you choose for your preamp.  This is really 
important for low output antennas like the Pennant

The decision on whether to place it at the Rx and or the antenna end, 
depends on the expected signal level at the antenna, the loss in the 
coax, and the sensitivity of your receiver.  These are exactly the same 
parameters as you would use to make the decision at UHF and above.  Here 
is an example.

Typical band noise output from a small Rx antenna in a quiet location on 
160m might be -127 dBm in a 500 Hz BW.  A typical noise figure for an HF 
preamp might be 6 dB, putting the noise floor at -168 dBm/Hz, or -141 
dBm in 500 Hz.  With no additional coax loss, your Rx will degrade the 
available S/N by 0.1 dB.

Now with 350 ft of good quality RG6, you can expect around 2 dB loss, 
dropping your antenna noise down to -129 dBm, still 12 dB above the Rx 
noise floor.  The Rx now degrades the available S/N by 0.3 dB.

Provided your coax loss is not much worse than this, I wouldn't worry 
about it and put the preamp where it's convenient for you, but I advise 
you to do the maths and insert the actual figures for your own situation.

73, Greg, ZL3IX

"My choice is whether to put it at the antenna end, incurring the added 
complexity of sending 12V DC to it via the coax, or to put it in the 
shack. "