Re: HB9VQQ testing Kiwi SDR

Glenn Elmore

Using "Quiet receiving site", curve C and "galactic noise" curve D from ITU-R P.372.8 as a guideline, one can probably stand a system noise more than 20 dB above KTB before it impacts SNR  from a losslessly matched antenna. This presumes that not only receiver side noise and NF but also IMD, ADC non-linearity and common-mode and similar QRM ingress is no higher.  It also recognizes that ground losses and ground noise currents can be a significant part of the feedpoint impedance and noise floor even  up at 10 MHz and particularly for horizontally polarized antennas.

If the SNR present on the antenna being used is substantially due to far-field sources, propagated signal and propagated noise, then except for IMD and ADC issues the receiver used shouldn't impact things much. In real environments of course that's the rub.  Both near-field sources and strong signal overload from out-of-band signals can make this not the case.

It does say that for a well matched antenna (well matched to radiation resistance, not necessarily the presented feedpoint resistance),  NF of the preamp and rx probably don't matter much. When one starts using poorly matched antennas, particularly a problem for electrically small antennas, then the requirements on the rx system hardware and ingress mitigation get greater.

If you are seeing appreciable difference between receivers/detectors then I suspect you have a problem with ingress,ADC/IMD and that these need to be eliminated.

Just my opinion from HF active antenna, preamp and receiver investigations...

Glenn n6gn

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