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Why does my S-Meter read S5 all the time?

Graham-KE9H
Graham-KE9H FlexRadio Employee ✭✭
edited December 2019 in SmartSDR for Windows
We get a lot of questions about why the S-Meter always reads some constant level, like S5 on a particular band, for the noise floor, without any signals present. 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
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Answers

  • K4EAR
    K4EAR Member
    edited April 2014
    A + S-unit shift from the 5000 with band noise. Mike, K4EAR
  • K4ELO
    K4ELO Member
    edited December 2019
    Thanks for that good info Graham. Fortunately I am Quiet Rural (I love it!) and the noise floor on 40M is usually about S2 unless there are thunderstorms.
  • K4EAR
    K4EAR Member
    edited April 2014
    I ran a few RX tests for $htes and giggles...20Meters...tightest X-Y axis' on the panadapter. Preamp "0" and display FPS @1 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. FWIW
  • George KF2T
    George KF2T Member ✭✭✭
    edited February 2019
    Would it make sense to allow S-meter "calibration" by creating a setup menu in SSDR where the operator could select one of those ITU curves? We'd still want the absolute values to read accurately, but a "calibrated" S-meter zeroing option could be nice.
  • Graham-KE9H
    Graham-KE9H FlexRadio Employee ✭✭
    edited March 2018
    Hi George: 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. --- Graham ==
  • George KF2T
    George KF2T Member ✭✭✭
    edited February 2019
    Thanks, Graham. You are, of course, quite right. My comment was more geared towards providing a "relative" measurement to compare one's environment on a daily basis to the norms. Funny, I had originally prefaced my comments with, "I'm not sure this is a good idea..." Thanks for ALL the hard work and attention to these details that makes the difference between a good product and an excellent one.
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited March 2018
    One thing to always keep in mind is the FLEX-6000 is a very accurate RF spectrum analyzer. This is a lot different from older radios that estimated the signal strength from a negative feedback loop (AGC). The result was changing the RF gain changed the S-meter reading. Personally I like having a calibrated S-meter that accurately tells me the integrated power within the RX pass band.
  • Wm McDermott
    Wm McDermott Member
    edited April 2017

    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


  • Graham-KE9H
    Graham-KE9H FlexRadio Employee ✭✭
    edited July 2014
    Wm:

    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.

    73's
    --- Graham / KE9H

    ==


  • AA0KM
    AA0KM Member ✭✭
    edited December 2016
    Flex-6500 antenna not connected.

    SSB 2.4 filter -10db pre-amp = s-4+

    CW 50hz filter -10db pre-amp =S-1+

    I tried  also using a dummy load and checked every band and they are all the same with or without dummy load.

    Also used different power supplies same results.

    For those reading this is (not a problem) just my recent observation.

    Interesting on how this all relates.
    imageimage
  • Walt - KZ1F
    Walt - KZ1F Member ✭✭
    edited November 2016
    Interesting! That procedure could be done programmatically, correct? I like that!
  • AA0KM
    AA0KM Member ✭✭
    edited December 2016
    Well, when your used to other radios that jump up the S-unit readings when you use pre-amp and this doesn't it fools you on how it all works..lol

    I have happend to have my antenns un hooked and saw the s-units thus the investigating of the why.

    A-true S-Unit reading.



  • Walt - KZ1F
    Walt - KZ1F Member ✭✭
    edited November 2016
    Frankly I think S-units is such a completely fictitious thing. There are so many variables that affect it, location, TOD, ok, rig, antenna, gray line. It's nice that S5 or 6 or 9 actually has some basis in fact but how many uVolts are at the antenna will be different in your QTH from the guy 2 doors down.
    Independent of 'faking' a signal report, which we all do, 5-9 good buddy, not to be out done by the last report, you give 5-9+10 and follow that with, could you give me your call again pls.

    Seriously, I do like what Gerald said, it quantifies the relationship that should exist between the effectiveness of your antenna system to your preamp level. I am going to add that to my code and see what happens. 
  • N3NER
    N3NER Member
    edited November 2019
    Not sure I understand Gerald but, maybe I'm missing something.  I checked my 6300 for the first time today on a dummy load and I always have a S-3 reading and that go up to a S-5 if I enable the RF gain.  
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
    Change the RX filter size.  The S meter is showing you the totla RF power inside of the filter.  The larger the filter, the more RF power, the higher the S-meter reading.

    If you are noise limited at the antenna, enabling the preamp will make the noise floor rise (a bad thing) and the s-meter will go up.  You want the noise floor to drop when you enable the preamp, which indicates you are not noise limited and RF pre-amplification is providing an increase in the signal to noise ratio.

  • EA4GLI
    EA4GLI Salvador Member ✭✭✭
    edited November 2016
    Hi Tim,
    Care to explain this a bit more: 
     "You want the noise floor to drop when you enable the preamp, which indicates you are not noise limited and RF pre-amplification is providing an increase in the signal to noise ratio."
  • EA4GLI
    EA4GLI Salvador Member ✭✭✭
    edited November 2016
    Thanks!
  • Bob W8RMV
    Bob W8RMV Member ✭✭
    edited March 2018
    When searching the prior posts for S meter operation, I came upon this.  My new 6400M reads ~5 S units even without the antenna connected.  I disconnected everything from the rig but the PS & I still see ~5S units at an RF gain setting of "0".  When I bump the gain up to +32 it reads 2S & at -8db gain it reads almost 6 S units.  Normal?  - Bob
  • Jay Nation
    Jay Nation Member ✭✭
    edited March 2018
    Bob
    Sounds a tad high, but might be completely normal. The S meter reading seen depends on, which band is selected, the preamp selection, and the filter selected,
    and also whatever local noise might be being picked up. Even with my antennas disconnected, I sometimes see some readings around an S3. Noise is always present. and your radio's good at hearing it. 
     
    #Flexradio

    73, Jay / NO5J

  • Bob W8RMV
    Bob W8RMV Member ✭✭
    edited March 2018
    Must be really sensitive.  Here it is with a shorted coax cable & still S5.  I own 2 old boat anchors & 3 contemporary rigs & none do that even with the preamp on.  I have
    have a relatively quiet QTH.  I talk to so many Hams with that have 2-5S units higher noise than I.  I know, not all S units are created equal.  But the proof is I hear folks they do not. - Bob



      image
  • Bill -VA3WTB
    Bill -VA3WTB Member ✭✭✭
    edited March 2018
    This is normal for an SDR radio, when no antenna is connected the receive bins are still being sampled. Steve from flex wrote a big article about this if I can find it.

    This is a comment from Tim He was answering someone who did not believe how the S-meter works in the flex.

    The S meter isn't broken so it doesn't need fixing.  The "fix" you are asking for would actually break the S meter by assigning an arbitrary signal strength value to the meter based on user perceived signal or comparison to an inaccurate standard (traditional radios).  Let me explain.

    First, there is always signal present.  Blackbody, atmospheric noise, cosmic noise and thermal noise are "signals" at RF energies and all contribute to the RF noise floor.  Even with a dummy load connected, there is still signal being digitized.  There is no such practical thing as "no signal" (unless the radio is in an RF isolation chamber at absolute zero).

    The signal strength meter design in the FLEX-6000 is based on how all lab grade digital spectrum analyzers operate; by accurately integrating the total RF power in the FFT bins within the receiver's passband.  This is a very important point.

    Where a traditional received estimates signal strength by calculating a value from the AGC voltage, the FLEX-6000 actually measures the signal power in each FFT bin. Since S units are a standardized unit of measure for RF power as defined by the IARU, we calibrate the S-meter based on a standard, making the FLEX-6000 very accurate in this regard.

    In the second paragraph, I emphasized an important point about measuring in the FFT bins, as opposed to estimating, RF signal within the receiver's passband.  This is very easy to demonstrate.  With your dummy load connected to the antenna port, change the slice to CW mode and change the filter to 50Hz.  At this RX filter passband size, there are only a few FFT bins measuring RF power and the S meter is barely moving.  If you put your cursor on the meter bar to display dBm, it may be as low as -134 dBm.  Now the IARU defines S0 as -127 dBm.  So the S meter reading in this case is actually less than S0!
  • Bob W8RMV
    Bob W8RMV Member ✭✭
    edited March 2018
    Thanx for that.  I look forward to seeing the article if you find it.  I'll do another search on it, too.  I saw this old post & thought I'd piggy back on it as it seemed we had similar observations.  I have two other SDR's and they & their s/w are different than what I am experiencing with Flex.  It has only been a ~week & hope to learn more in the coming weeks.   Thanx - Bob
  • Jay Nation
    Jay Nation Member ✭✭
    edited March 2018

    Bob

    Keep in mind your listening for signals, the AGC-T can be adjusted so that you actually hear no noise at all even when the noise floor appears to be S-anything.
    If it's noise only on that frequency, why worry, Actual signals, if they happen, will be S-more. and you can't actually work the noise anyway. The meter calibration is correct on a Flexradio, not so, on almost all other rigs. It does take some getting used to. The metering gets recalibrated every time the radio restarts.
     
    #Flexradio

    73, Jay / NO5J

  • KY6LA_Howard
    KY6LA_Howard La Jolla, CA. Paris and Sablet FranceMember ✭✭✭
    edited March 2018

    Welcome to the Modern World of dBm.  Your Flex is a laboratory grade spectrum analyzer which reads in dBm... for those who can't live in the 21st century, S0=-127dBM and S9=-73dBm...  Hopefully soon Flex will get rid of S meters and just give use much more useful dBm digital readouts.

    Most legacy radios are not laboratory grade instruments so their S-Meter reading as figments of the imagination of the circuit design.  Some read S0 with no antenna attached even though there is likely a signal present.  It's really hard to compare anything to a legacy S-Meter as they are virtually meaningless relative even to a radio sitting next to them.

    Looking at the screen shot above you seem to be reading about -103dBm which is in the range of the MINIMUM PRACTICAL DISCERNABLE SIGNAL for 80M.  Likely there is no point hearing below that point as the background noise (see graph at top) will cover the signals.

  • Bob W8RMV
    Bob W8RMV Member ✭✭
    edited March 2018
    Jay - As I said, I have other SDR's so I am familiar with AGCT.  In fact, if the IC-7300 had AGCT I'd probably still have it.  It had a horribly noisy receiver as the AGC was always pumped up it seemed.  I love AGCT and the Flex utilizes it very well!!  I love how quiet I can make it & still hear a signal.  But my question wasn't about AGCT. It was about S meter behavior, AGCT does not affect the S meter,  & I have not seen this S meter behavior on my superhet rigs other SDR's I have.  Since I do not have any Flex experience I am left to look for an explanation in the manual or search user sites like this.  I am not worried, I just want to understand & if it is normal.  We all use our radios in different ways & we have different expectations.  - Bob
  • Bob W8RMV
    Bob W8RMV Member ✭✭
    edited March 2018
    Howard - I was a little surprised when I saw the S meter on the Flex.  I was expecting a dBM display.  Maybe it is in the menu somewhere.  A friend & fellow Flexer used to talk to me about it.  I'll have to search more.  - Bob
  • Bill -VA3WTB
    Bill -VA3WTB Member ✭✭✭
    edited March 2018
    If you hover your mouse pointer over the graph on the flag, it reads dbm
  • Jay Nation
    Jay Nation Member ✭✭
    edited March 2018
    Bob
    My only point was noise is something best not listened to or even watched, it may just be only noise. To see the actual dBm level in real time try using Flexmeter, otherwise,
    you will have to hover over the indicator in SmartSDR.

    I'd also like a visible digital dBm readout to be added someday to both the Maestro and SmartSDR displays.

    With my radio, an S-0 reading would be an indication of a problem. The band noise never drops that low. So, I think your probably seeing normal behavior. 
     
    #Flexradio

    73, Jay / NO5J

This discussion has been closed.