Re: LF: TX > RX isolation test

[Andy Talbot <andy.g4jnt@gmail.com>]

What's wrong is the mismatch between antenna and feed.  Assuming 50R receiver load, your combination of transformers is presenting the antenna with a 20k load.

Assuming 8pF per metre your antenna has 72pF, or a reactance of  16k at 137kHz which is commensurate with the load, so about 3dB loss there.

The radiation resistance is something like 0.004 ohms in series with that 16k reactive. So all you are seeing is the voltage corresponding to 9m up ( 9 *  E  in volts per metre)    applied across the 20k Rload then, reduced by 3dB due to the reactance.

So a lot of loss before your preamp.   All active E-probes give an input resistance of at least 1M, often 10M, sometimes even more and usually dominated by input C, of a pF or 2 perhaps.  As the probes are shorter than your 9m, they have less series C so the amplifier input R needs to be correspondingly smaller to allow this to be insignificant.  

It's a different take on an LF receiving antenna, but whatever happens you need a low noise preamp.    Conventional E-probes do it in a J-FET, you in a matched amplifier that doesn't look particulary low noise

Andy

www.g4jnt.com

On 17 July 2018 at 12:14, N1BUG <paul@n1bug.com> wrote:

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