Anomalous S meter reading on 6m band only

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vk6cpu
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Location: Perth, Western Australia

Anomalous S meter reading on 6m band only

Postby vk6cpu » Fri Mar 24, 2023 4:43 am

Firstly - this post has nothing to do with Ritch's meter enhancements. Ritchies improvement has only facilitated me gathering the data. The issue predates all this.

The S meter reading on 6m looks anomolous cf other bands for comparable uV/Dbm readings (see table).
Comparison of readings across all bands was done using same bandwidth/mode (3Khz/USB) and all antennas disconnected with exception of small dummy load on acitve antenna port.

Screenshot 2023-03-24 121156.png
Screenshot 2023-03-24 121156.png (11.15 KiB) Viewed 968 times


My expectation was that with no antenna/limited external noise the Smeter would be in the 0-1 range on all bands. Which it is, except on 6 meters. Switching the 6m LNA in/out made no significant difference.

I assumed the Smeter reading was calculated/interpolated from the actual voltage reading on the ADC - so that relationship shouldnt change with frequency/band.

Looking through the Thetis Manual and the 8000 manuals I couldnt find anything band specific on S meter calibration.
Note - The physical readings look fine (dbm/uV) Im not quibling about the physical calibration of the rig - just the S meter calcs on 6 mters.
I dont have a decent signal generator so the best I could do for a comparison across the bands was just active antenna socket terminated in a dummy load for equivalence.

Welcome any comments whether this is normal or why the relationship appears to breakdown on 6m. Is there anywhere in software to adjust this ?

I have had the Anan 8000 since 2019 used mostly on 6m and WARC bands (limited antennas living in an inner city suburb).
I upgraded from FT991 and initially noticed background noise level was >>7 S units where as FT991 was usually around 3-4.
Didnt pay much attention to it and just assumed for last 3 years I have a very noisey 6m environment.
On unplugging the antennas noted that S meter reading was still S4 on 6m.
Operate mostly digital modes and not really been interested in S meter readings. But with increased TEP activity on 6m , I have started using SSB again.
I noted that my signal reports were significantly and consitantly higher than others nearby (10Km) were giving (ie I was reporting S9 +20 and they S7) with better antenna systems and Flex 6000s.
Which made me question the calibration of the S meter on 6m. If its displaying high on no noise then I assume its reading high on moderate/strong signals too.
All the best
Nigel
VK6CPU

Anan 8000 DLE, Thetis v2.9.0.8, Protocol 2 V2 1.18
INTEL i9 12900 64.0 GB Ram
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DL2XY
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Re: Anomalous S meter reading on 6m band only

Postby DL2XY » Fri Mar 24, 2023 11:33 am

Your values reflect exactly the IARU recommandations for S-meters:

HF: S9 = 50µV = -73dBm
VHF: S9= 5µV = -93dBm

So everthing is perfectly fine.

Walter
DL2XY
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w-u-2-o
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Re: Anomalous S meter reading on 6m band only

Postby w-u-2-o » Fri Mar 24, 2023 1:57 pm

Walter is 100% correct.

There is an unfortunate issue in that that the IARU left the 6M band in limbo when they laid down the law on this. The original software developers during the pre-Apache days of the openHPSDR project chose to lump the 6M band in with the VHF standard. It has remained that way ever since.

In more detail...

S-meter standard reference levels:

The S-meter standard reference levels are defined by IARU Region 1 Technical Recommendation R.1, as ratified at both the Brighton 1981 and Torrelmolinos 1990 meetings. This standardization is documented as follows:

Code: Select all

STANDARDISATION OF S-METER READINGS
1. One S-unit corresponds to a signal level difference of 6 dB,
2. On the bands below 30 MHz a meter deviation of S-9 corresponds to an
available power of -73 dBm from a continuous wave signal generator
connected to the receiver input terminals,
3. On the bands above 144 MHz this available power shall be -93 dBm,
4. The metering system shall be based on quasi-peak detection with an
attack time of 10 msec ± 2 msec and a decay time constant of at least
500 msec.

Unfortunately, this leaves S-meter reference levels undefined for the 6M band! For 6M, Thetis currently uses the "VHF" reference level of S9 = -93dBm. Whether this is correct or not is hard to say since the IARU didn't define it. But that's the way it is and your hardware, firmware and software are all working as designed.
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vk6cpu
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Location: Perth, Western Australia

Re: Anomalous S meter reading on 6m band only

Postby vk6cpu » Sat Mar 25, 2023 12:13 am

Thanks guys for the quick response.

My limited googling/research had unearthed the HF S meter relationship but not the VHF relationship.
Further experimenting this morning on the Anan/Thetis this change over happens at 30 Mhz or slightly above.

I can sleep easier at night now - knowing my noise floor (in S units) isnt as bad as I thought it was and I am validly giving JA's 59+ reports on 6m SSB.

A couple of supplementary questions - if I may.
1) is the S meter relationship implemented in Software or Firmware?
2) From other Ops observations/knowledge have other manufacturers adopted the HF relationship for 6m ?
I'm surmizing Yaesu use the HF relationship.
All the best
Nigel
VK6CPU

Anan 8000 DLE, Thetis v2.9.0.8, Protocol 2 V2 1.18
INTEL i9 12900 64.0 GB Ram
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w-u-2-o
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Joined: Fri Mar 10, 2017 1:47 pm

Re: Anomalous S meter reading on 6m band only

Postby w-u-2-o » Sat Mar 25, 2023 12:36 am

It's defined in the software (Thetis).

Not sure about other radios.

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