This posts shows a measurement of ambient noise and comparison with the data given at Expected ambient noise and its more detailed references.
The test scenario is my 40m station, a G5RV inverted V dipole with tuned feeders, a balun and ATR-30 ATU. Antenna system losses are less than 1dB.
The chart above gives a range for expected ambient noise at 40m.
Above is a screen shot from a spectrum analyser measuring power in 1kHz bandwidth from 7.0 to 7.1MHz. The band is mostly unoccupied, and the mean noise power is about -99dBm, it would be 3dB higher in 2KHz bandwidth (ie -96dBm).
Above, the extrapolated -96dBm in 2kHz is plotted as a red dot on the chart, falling a little higher than the ITU’s Rural precinct. The antenna system isn’t perfectly efficient, but losses are low and the red dot could be up to 1dB higher for a lossless an antenna system.
In S unit terms, the extrapolated noise power in 2kHz is S5, the actual IC-7410 S meter reading was S3 but conventional S meters are notoriously unreliable at low readings.
Above is the relationship between S Meter indication (converted here to dBm on the basis of the IARU convention) and signal generator input for the IC-7410 used. S meter calibration of conventional ham communications receivers is rarely good and should not be relied upon unless verified by test.
Example 1: what if you see S9+10dB noise in 2kHz on 80m?
I mentioned that S meters are notoriosly unreliable at low readings, they are often better around S9, often reasonable from S7 to about S9+20dB. Be sure that the radio is properly configured for S meter calibration (that usually means preamps OFF).
So, if we take the S9+10dB figure as correct, from the chart above we would expect S6 to S8, so ambient noise seems to be perhaps 16-28dB dB higher than expected and begs the question whether some fairly local noise source is at play.
Example 2: my place is so quiet, noise is only S1 on 80m?
Again the accuracy of S meters comes into play, and it is likely that an S1 reading is low and that the actual value might well be in the range S4-S6.
Even so, S4-S6 is a lower than expected and begs the question of antenna system efficiency. Low antenna system efficiency will reduce the S reading of signals and noise (equally), and whilst that means you will still copy signals well, your transmitted signal will be degraded.
Conclusions
- The ITU predictions of ambient noise can provide a useful range of expected noise.
- S meters are not reliable for measuring received power in absolute terms, and will often read significantly low at the levels encountered in measuring ambient noise on the HF bands.
- Ambient noise that is too high or two low prompts investigation.