28MHz background noise levels

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28MHz background noise levels

Although propagation at 28MHz is sporadic at the moment I've now used WSPR enough to see that most stations spot me with higher SNR than I spot them, even after allowing for differences in transmitter power. So I wondered if I had an unusually high background noise level. First check with a coaxial switch in the feed showed that the noise level I get with my antenna connected was typically 6-8dB higher than the level that I get using a 50ohm load. I thought that was reasonable but since discovered that the noise floor of my FT-897D was dominated by its built in power supply. Running the radio from a battery the antenna/load noise ratio is 22-24dB.

The antenna noise shows no patterning on a waterfall display and no particular peaks when tuning across the band. Although I live in a populated area it does not seem to depend on beam heading. I have some good choke baluns optimised for 28MHz and the antenna return loss is 20dB. So it seems unlikely that the noise is common mode stuff going up the feeder, or being picked up by it. All evidence so far is that this is natural white noise picked up by the antenna.

Does anyone know if this kind of level is normal for 28MHz? The Noise Figure of my FT-897D seems to be about 9dB so this result means I'm getting over 30dB of external noise from the antenna. It is a 2-element Moxon about 8 metres above ground. Has anyone else done measurements on natural background noise levels? I'd certainly appreciate numbers from anyone who has. I am getting some nice SNR spots and it would be nice to give some good ones in return.

- Nick.