6600M dBM vs S meter

fhmassey

I am trying to get myself straight on signal levels -dBm and S-units. In particular I'm concerned this evening about 80m.
I have set the rig to CW with all noise reductions off and a bandwidth of 500Hz.

With no antenna connected and 0dB preamp gain, I have:
-130dBm and S2 on the S meter. My understanding is that -130dbM should S0.

With the antenna connected and 0dB preamp gain, I have:
-108dBm and S6 on the S meter. My understanding is that -108dBm should be S3.

Any input is appreciated.
73,
Fred
AE4ED

Jeff_W4DD

Fred,

If you are reading the dBm off of the panadapter display, my understanding is that is the noise level in some "unknown" bandwidth based on the bin size of the horizontal axis of the panadpater display (i.e. the reading is really dBm/Hz but the Hz part is not shown). The S Meter is based on the bandwidth of the selected filter in whatever mode you are in. I've requested that the bin size of the panadapter be added to the panadapter screen somewhere, say in the bottom right of the window, but enhancements like that are not coming too quickly these days. It would add some meaning to what the panadpater shows (i.e. dBm in a given BW) and is likely known to the programmers just not currently displayed. Every time you adjust the Frequency spread of the panadapter, the dBm level changes because the bin size (BW) of the aggregated signal/noise changes.

Jeff, W4DD

fhmassey

Thank you Jeff.

KD0RC

I did a little experimentation and it looks to me like the dBm reading of the panadapter is taken at a 100 Hz bandwidth. I came to that conclusion by narrowing the bandwidth of the slice while watching the dBm meter on the slice (click the s reading so that it displays in dBm) until it matched the noise level being reported in the scale on the right of the panadapter. I found some nice noise peaks that were steady to look at higher levels, and it stayed true from -130 dBm (no antenna) to -90 dBm (horrible noise).

73,

Len, KD0RC

fhmassey

Thanks Len for taking the time to do the tests. My radio correlates pretty well.
73,
Fred, AE4ED

Stan VA7NF

Panadapter and slice readings are based on a different set of numbers.

Slice numbers should ALWAYS, for repeatability, be taken at 500Hz bandwidth - No exceptions

Panadapter/waterfall are based on a bandwidth of Panadapter width / 4096 bins. To make them the same, set your panadapter to 500Hz * 4096 bins. My calculator says this is 2Mhz.

This also means that if your panadapter is narrow (For example 5Khz around your receive frequency) then each bin, or the reading in dbm beside the panadapter, will be for a 1 Hertz bin, significantly lower than your CW bandwidth.

Jeff_W4DD

Based on Stan's useful comment on the total Bins (4096), I tabulated some Panadapter to Bin BW pairs along with the expected differential to the S Meter readings for CW (500Hz) and SSB (2.9KHz). A spot check seemed to confirm the values. Hopefully this table comes though intact.

Jeff_W4DD

To read the S Meter value in dBm, click on the S Meter value to the right of the bar graph and it will toggle between S units and dBm.

Mike-VA3MW

This might help a bit too.

https://youtu.be/GQdjoY91pyY

fhmassey

Hi all,

I listened to the video, and I think I understand now. The Panadapter is showing the power of each bin, and the S-meter is summing the number of bins contained within the bandwidth. The fact that I'm getting an s2 on 80m without an antenna connected is due to the presence of noise in the environment within my home.

Thanks for the responses,

Fred, AE4ED