Re: LF: TX > RX isolation test

[N1BUG <paul@n1bug.com>]

Hi Stefan,

On 07/16/2018 08:00 AM, DK7FC wrote:
> What type of preamp are you using (schematic?). For a comparison with 
> and without antenna, you need to replace the antenna wire with a 
> capacitor of the same C (assume 5...6 pF/m antenna wire, i.e. use 56 pF 
> to replace a 10m vertical) connected to the preamp. If this shows as 
> e.g. -120 dBfs it should show -105 dBfs with the antennna connected.

I am not certain we are talking about the same antenna
configuration. This antenna is not an E probe, at least not in the
sense that I understand it. It's a 9m tall whip with 100:1 impedance
transformer at the base. The high Z primary winding connects to
antenna and ground. The low Z secondary goes to 60m of twisted pair
to the shack. At the shack there is a 2:1 transformer feeding a 50
ohm in/out preamp. This is the preamp I use:

http://w1vd.com/LFMFHFpreamplifier.pdf

I can not quite put into words what bothers me about my system but I
get a feeling something is wrong. I think I need to go back to the
beginning and try to understand noise floor / MDS at every stage of
the system... receiver without preamp, with preamp, etc. I am not
sure exactly how to proceed with this evaluation.

Paul N1BUG