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Re: LF: TX > RX isolation test

To: [email protected]
Subject: Re: LF: TX > RX isolation test
From: DK7FC <[email protected]>
Date: Tue, 17 Jul 2018 14:31:19 +0200
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Hello Paul,

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:

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

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