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Author Topic: Tips for measuring local noise?  (Read 147 times)

AK4YA

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Tips for measuring local noise?
« on: November 07, 2022, 11:27:23 AM »

There is a chart on the wikipedia page for "S Meter"
https://en.wikipedia.org/wiki/S_meter

It seems to correlate levels in dbm to S readings.  Now I know S-meters get a bad rap based on vendors presentation of it, and that "technically" it represents current/voltage through an AGC @ 50ohms.

But this chart - does it also correlate to noise floor?  Like say I had an SDR hooked up to an antenna and its bandscope showed the noise floor at -109, would we then say "the noise floor is at S3"?  Or does no one think about things in those kinds of terms?

Im interested in measuring the SDRs noise with no antenna connected, with antenna connected, at different freqs, with different tuner settings, with different circuit breakers on & off at my house, different times of day, etc.  Mostly to see what all around my local environment in my house that I can improve.
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K1KP

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Re: Tips for measuring local noise?
« Reply #1 on: November 07, 2022, 12:23:40 PM »

If the SDR is calibrated to report the signal power in dBm, you can convert dBm to an S-value using the customary 6dB/S-unit and S9 = 50microvolts (-73dBm) conventions.

It is quite common for amateurs to comment on their local noise level in S-units.

You must note that the total noise power detected by an SDR (or other device such as a radio, spectrum analyzer, etc) will depend on the bandwidth. As you increase bandwidth, more noise energy is integrated by the receiver. The power will increase as the square root of the bandwidth, assuming uncorrelated noise. Therefore it is important to note the bandwidth of the receiver used anytime you record a noise power value for comparison.

For an excellent representation of typical ambient noise levels by frequency of various types of location (residential, rural, quiet rural), refer to a chart published in the ARRL Handbook, and included in a talk by Rob Sherwood, NC0B at:

http://www.sherweng.com/ctu2014/NC0B-CU-2014-8c.pdf

(see page 12). Note that this chart assumes a receiver bandwidth of 500Hz (typical CW case).

-Tony, K1KP
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WA3SKN

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Re: Tips for measuring local noise?
« Reply #2 on: November 07, 2022, 12:29:21 PM »

That article is not very good.
Do you have a calibrated frequency selective dBm meter or noise meter?  You will not get accurate readings with most of the amateur eqpt on the market.
Do you wish to measure HF noise or also VHF/UHF? Remember that bandwidth is also involved.

-Mike.
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