Problem

Dale Neace

Flex 1500 s-meter reading is 5 with no antenna connected I have had radio several years and the really bothers me as how can radio give me a signal reading with nothing to hear? I have never liked that and it makes no sense why that is,and please don't tell me about having a real s-meter and that is normal I don't think that u should have any signal with no antenna connected unless there is something wrong with the radio? For example I go to ten meters and I have 5 to 7 s unit reading with nothing connected to radio. I have looked at replies to this question and it seems no one is giving a strait answer and I have been licensed for over 30 yes so I'm not a beginner thanks for all replies (flame suit on)! lol 73 N4XLD

Bill -VA3WTB

It is very normal...
Tim response to this...

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!

KF4HR

Bill while I agree with what you're saying, I understand Dale's frustration.  Ham's typically expect to see a non-indicating S-meter when no antenna is attached and use that as a baseline for reporting signals or QRM.  Without an antenna my 6700 typically sits at S3, while my other transceivers indicate less than S-1. 

While Flex 6000 series radios may be accurately reporting the various types of atmospheric noise that does not help much when a ham is attempting to provide an accurate S-meter report to a weak station (especially if the meter sits at a minimum of S5).  Personally I'd like to see the Flex S-Meter mimic analog (ALC voltage based) S-meter readings. 

KY6LA_Howard

"ACCURATE" You must mean INACCURATE OR RELATAVISTIC to some arbitrary uncalibrated setting. Most S-Meters do not even come close to measuring 6dB per S-Unit which is supposed to be the standard. So any measure you give a weak station via most rice boxes is utterly meaningless as there is no absolute reference to measure against except that the neater is moving somewhat. Personally I would love to totally get rid of the analog S-Meter which I can't read because Redon black is invisible to me and replace it with a digital meter reading out digitally in TOTALLY MEANINGFUL dBm like we had in PSDR.