Wednesday, 29 January 2014

6m receiver sensitivity, or increasing noise levels

Are noise levels creeping up or are operators not taking notice of how deaf some commercial multiband rigs can be?
I operated for a while last night and heard dozens of UK stations calling on 6m. More than 25% of the stations I called with my 100W to 5 element yagi seemed not to hear me? Not even a QRZ!
I know that beaming north of west through north East from here on 6m the noise level rises by more than 10dB. I can see this quite clearly on my P3 display attached to my K3. This broadband noise is not so obvious just listening on the K3, but is easily seen on the display.
More worrying is that stations in a southerly direction from here, including Kent, were also having problems hearing me.
Now, I can see the signal levels on 6m on the display, I know the bandwidth so I know when a station similarly equipped to myself should hear me on a quiet band. Unless these 'deaf' stations are running considerably more than the UK legal limit, then they should be hearing me.
I should add that I have a low noise preamp on the K3 input ( not an ideal place for it, but it makes a noticeable difference).
So, is it that a lot of stations are actually 'deaf' on 6m or is the 6m noise level now getting so high that it is impacting our ability to work at typical terrestrial distances? SpE signal levels are generally so high that a 'deaf' receiver doesn't matter too much. Good performance on SpE is not really a good indicator of receiver performance.
In case you think this problem is limited to myself, I have discussed the observation with some other 6m operators and they seem to have the same complaint.
So, what is it ? Lack of attention to receiver sensitivity or increasing noise levels ( or both) ?

Sam




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4 comments:

  1. probably because they were calling CQ DX Sam, many stations on HF call CQ DX and ignore europe, sometimes to their own peril, as they get qrm'ed forever more. Its a growing disease on HF.

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  2. You forgot to factor in the extra 40dB to the "path loss" as you dont have a DX callsign?

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  3. You are not DX Sam and some people only care about that, it is surprising how many operators can pull 4U1ITU out of the noise once it appears on the cluster, but my perception is there are a lot of very deaf HF and VHF stations now operating. I put it down to noise levels. Here is a quiet rural area I see 10-20 dB noise lift on 2m beaming at a nearby house with LED lighting. It eliminates getting data from evening Funcube passes to the West.

    You are also not making it easy for yourself by using a K3. You have good filtering and low IMD noise, most don't. You are always going to hear better than someone without a K3.

    Mike G0MJW

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  4. Hi Sam

    I finally got a 4m antenna up and did some testing with the Nacton. The 4m noise from a load is 25dB below the noise floor from my Antenna beaming in its worst direction. With the antenna in its quietest direction to null the noise, the noise increases by about 5 dB vs the load. . I don't know what the sidelobe rejection of the antenna is but lets assume it is perfect. Further the noise figure of the Nacton is also unknown but maybe low enough to be neglected. A lift of 5 dB is maybe about what you would expect, but 25dB isn't. This is a point source and I think its coming from somewhere near the local power station. This means that the increased background noise is reducing my sensitivity by 20dB, so yes, if that is typical everyone else is probably now too deafened by noise to hear your signals and it really doesn't matter any more that commercial radios have poor noise figures.

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