|To:||LineOne <[email protected]>|
|Subject:||Re: LF: TX > RX isolation test|
|From:||Andy Talbot <[email protected]>|
|Date:||Tue, 17 Jul 2018 13:21:21 +0100|
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|References:||<[email protected]> <[email protected]om> <[email protected]> <[email protected]> <[email protected]> <[email protected]> <[email protected]>|
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
On 17 July 2018 at 12:14, N1BUG <[email protected]> wrote:
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