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Date: Tue, 17 Jul 2018 14:31:19 +0200
From: DK7FC <selberdenken@posteo.de>
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Subject: Re: LF: TX > RX isolation test
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Hello Paul,

Interesting.
I think that anyway the antenna will have a certain feed point impedance 
to the preamp input. It can be replaced by a certain R and C, discrete 
elements. C may be 56 pF and R may be 200 Ohm for example. But the 
discrete elements will not generate a RX signal voltage. For the preamp 
it should be equivalent. So you will see the system noise floor 
(preamp+soundcard/RX etc).
It is a bit like a dummy load for the RX. :-)

Is your 9m vertical standing on a free field or on the top of a house or 
close to trees and other greenery? That will make a difference!

I did not yet try a 100:1 transformer and i would expect that the 
impedance of that wire at LF is much to high (abt 20k Ohm) to be 
converted with a normal transformer. Maybe the core material is not well 
choosen or the internal capacity of the primary winding  is to high, so 
you're bypassing a lot of the signal? What happens when you connect the 
wire directly to the preamp?

73, Stefan


Am 17.07.2018 13:14, schrieb N1BUG:
> 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
>
>