I borrowed a graph from the Radio Amateur's Handbook, and marked it up with S-Units. It shows the relationship between atmospheric noise at the antenna versus frequency, for several environments.
Some things to note:
1.) The lower dotted line is equivalent to the quietest places on Earth. So if you have a reasonable antenna, you won't find anyplace that has less than S2 noise floor (in a CW bandwidth) on 40 Meters. Most suburbs will be more like S4 (or S5 in a SSB bandwidth.)
2.) Total noise is proportional to bandwidth. This chart is for 500 Hz receiver bandwidth. If you are using a SSB (3000 Hz bandwidth) filter the same noise floor will read/measure one and a third S-Units higher.
3.) In the US, where cities are not as tightly packed, I find the suburbs are closer to the rural line than the residential line in this chart.
--- Graham / KE9H
ANT1 terminated into a 50 ohm load, CW mode 100Hz filter. The panadapter grass is ~135dbm. The S-Meter reads +/- S-2.
In SSB mode 2.7kc BW, the noise floor is the same as with CW, and the S-Meter reads S-4.
The technical definition of S-units is directly tied to signal strength. S9 is 50 microVolts into 50 Ohms. S-units are 6 dB each, below that. So the chart was presented as a way of educating the users as to the nature of the atmospheric noise and the way it changes with frequency. If you wanted a 'sliding scale' then I suppose that is what the RST signal reporting provides. Relative 'readability' and 'signal strength' and 'tone' quality, irrespective of the number of dBm involved.
Thanks for ALL the hard work and attention to these details that makes the difference between a good product and an excellent one.
Personally I like having a calibrated S-meter that accurately tells me the integrated power within the RX pass band.
I'm just returning to amateur radio after decades.
My Flex 1500 shows S-2 with a dummy load connected and on some bands as high as S-4 to S-5 depending on bandwidth. Pre-amp is 0 dB. The power lead from a battery pack has 5 turns on a type 43 torroid and the USB cable has two extra ferrite beads and five turns on another type 43 torroid. I'm using the most recent software and firmware.
From the comments here, particularly those of K4EAR, these S meter readings are normal. Is that correct? I was thinking of returning it to the factory for a look see.
Your comments are welcome.
Wm McDermott AB9BE
If you see the noise floor rise when you connect a normal antenna to the radio, the radio is doing everything it needs to do, for you to hear everything you are going to be able to hear.
With a normal antenna, dipole, similar, or better, the FLEX-1500 should run 0 dB preamp from 160 meters to 20 Meters, then +10 dB up to 10 Meters and +20 on 6 Meters.
If you are in a real quiet location, then you might run +10 dB on 20 Meters. Judge by the action
of the noise floor when you connect a real antenna.
From your comments, I don't think there is anything wrong with your 1500. I suspect that you
just met your first honest S-Meter in your ham career.
--- Graham / KE9H
An easy way to check is to disconnect the antenna and read the dBm meter. Connect the antenna and read the meter again. If the noise goes up less than 8 dB, increase preamp gain. If it goes up much more than 10 dB, decrease preamp gain.
I’ve seen a number of different explanations in this thread of what the S-meter should be measuring, such as integrated (average?) power, and statements to the effect that if the RX bandwidth changes, the S-value should also change, even if there’s nothing but background noise.
I disagree. The S-meter should only report the peak signal strength measured within the audio passband. If the background noise is at an S-2 level then the S-meter should read “S2” regardless of how wide my receive bandwidth is. If the S-value is expected to vary as a function of receive bandwidth, then there would arguably come a point (stay with me here) where the RB is wide enough that the S-meter should be reading S9 +10, but the strongest signal in the RB might only be an S7! So how would anyone know the actual strength of any signal in such a scenario?
No, the S-meter should only be reporting the peak signal strength found anywhere in the audio passband. I don’t care how wide the passband is. It could be 500 Hz wide, or 2.7 kHz, or 250 kHz. If there’s nothing but noise in the passband, and the noise floor anywhere in the passband is at a steady -115 dBm, then the S-meter should never read above an S2 regardless of the width of the passband. The strongest measured signal strength (whether it be noise or a received signal) at any single spot in the passband is what the S-meter should be reporting. Nothing more, nothing less.
Perhaps there is some other information that some users would like to also see reported, but that’s not the S-meter’s job.
[edited to correct typos]
With our radios we measure the actual signal strength by integrating the digitized RF power contained within the FFT bins inside the RX filter. This is how a digital signal generator operates.
What we are saying is there is always RF power inside the filter and it is additive. The filter bandwidth is a dependency in the power measurement process. If you make the RX filter wider, the S-meter reading increases. If you make it smaller, it decreases. The standard is to measure integrated power in a 500 Hz filter. And it is frequency dependent too.
And you dBm to S unit numbers are a little off too for frequencies below 30 MHz
S1 = -121 dBm
S2 = -115 dBm
S3 = -109 dBm
The panadapter simply measures the signal in a given bandwidth and draws what it hears. If you look at any given pixel, it represents a certain amount of bandwidth. We call this the "bin size" of the FFT that is used to produce the display. If you cut the bin size into two pieces, the amount of noise in each piece goes down by half (3dB). In PowerSDR, the bin size is generally fixed for any given setup and does not change when you zoom. This is why the resolution gets worse as you zoom in on PowerSDR -- you begin to show one bin with multiple pixels. But for SmartSDR, we knew we wanted to have a larger range of zoom and this method was no longer acceptable. So we vary the bin size across a 1000:1 range. So the noise in each bin also varies. 1000:1 is a change of 30dB so from min zoom to max zoom, the noise in a bin will lower by 30dB and you see this change in the panadapter as you zoom in and out.
When people talk about noise floor in ham radio they are generally talking about the noise level with a 500Hz bandwidth. When the panadapter is zoomed in to the max level, the bin size today is about 5.8Hz. This is a 19dB difference in noise from where a ham would say the noise floor is to what you can see on the panadapter. This means that the panadapter can see 19dB below what most hams would call the noise floor. Your ear and brain are also able to hear below the noise floor in 500Hz because of how they work. But there are limits to how well you can hear. If you've ever worked JT65 or another long-term integrating mode, you have noticed that your computer can copy signals that you cannot hear.
So if you ask another ham "where is your noise floor on 80 meters" and he says "S5," what has he told you? Well with most hams, you don't know because you don't know the answer to these questions:
1. What bandwidth are you using to measure the signal?
2. Is your S-meter calibrated?
An S5 signal corresponds to -97dBm. And if he's getting this on sideband set to, say 2.8kHz bandwidth then the actual noise floor in 500Hz would be -97 - 10*log(2800/500) = -104dBm. There's nothing magical about 500Hz, it just happens to be the convention for measuring noise in the ham radio world. In SmartSDR, if you set the passband filter to 500Hz, the S-meter in the slice will show you the 500Hz noise floor.
If you start at maximum zoom and begin zooming out, you can see a point where the noise reading of the panadapter equals this number. What do you think this point is? ... if you've been following along, you will realize that this is the point where the FFT bin size is 500Hz. To get a rough idea if this is right, you could measure the width of your panadapter window and divide the amount of frequency displayed by this number. It should be in the 250-1000Hz range. The answer will not be exact because we do not continuously vary the FFT bin size -- we adjust it in steps and don't tell you where the steps are or what size they are. We do what's right for what you are viewing.
I know that was a long-winded answer, but I hope it provides some insight into noise and how it changes what you see and hear.
I've recorded two videos of me running a session of SDR Console V3 Beta 2, using an SDRPlay RSP1A with a 50 ohm dummy load connected to the antenna port. I varied the receive bandwidth everywhere between 10 Hz and 350 kHz, and the zoom between 200 Hertz and 1.4 MHz. The S-meter is reporting the maximum voltage measured anywhere in the vertical green receiver stripe in the display (the defined receive bandwidth).
Here are the videos (the S-meter is in the upper left corner of the band scope, and the Receive Bandwidth value is in small yellow font to the left of the S-meter):
Both videos show virtually no change in S-meter reading, regardless of any bandwidth or zoom changes I make. Which is exactly as it should be, because the noise level doesn't change despite any changes I make to the zoom or bandwidth.
Simon Brown, author of SDR Console, understands that concept well. His S-meter doesn't care what the bandwidth or zoom level are, it only cares what the maximum measured voltage is anywhere in the defined receive bandwidth, which is exactly what the IARU Region 1 Technical Recommendation R.1 says an S-meter should be measuring. The maximum voltage in the RB is, and should be, independent of any zoom level or bandwidth.
My apologies if I've been testing the patience of some readers, as that was never my objective. My only objective has been to clarify what an S-meter should (and should not) be reporting, per the IARU definition. For the S-meter to report anything else is to open up a whole new realm of latitude with long-accepted standards, with no limits of imagination to constrain it.
Disclosure: I don't own a Flex, but only because my good friend across town hasn't received his 6600M yet -- he's selling me his 6300 as soon as he gets notification that his 6600M is on the brown truck. :)
In SSDR as you zoom in and out, the bins change in size so you hear more noise as you zoom in.
Because SSDR can zoom in so tight they needed to make the bins change accordingly.
Rather than dealing with all this theoretical mumbo jumbo, I decided to do some experiments with a beam and a dummy load on 40M 7.074 to see exactly how the S-Meter actually worked in the real world
Obviously my dummy load (MFJ) is not a perfect Faraday Cage.
Here are the results
By VISUALLY LOOKING at the Panadapter screen I could definitely see signals peaking at the peak readings
So I guess I really do not understand what the controversy is?
Let's say )(it does) I have S5 on 40m SSb BW 2.7... IF there is a signal at S3 WILL I HEAR IT?
If not...FRS has a problem as many locations actually have a very low actual noise florr in there area at certain times of the year.
Will I copy the guy at S3 if all the lower the 6400 I have will go is S5???
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