The AR88 S-meter scale - UK Vintage Radio Repair and Restoration Discussion Forum

JanWra1 · 17th Mar 2018, 12:38 am

The S-meter on my AR88 had some difficulties to work properly but a change of R21 to a new one fixed that for abt £ 10:-, however I now have another question for you guys - the scale on the S-meter: As it goes from -6 to +100 (or is it 120?) which value corresponds to S9? Does anyone have a conversion scale?
E.g. S1 = ?, S2 = ? etc

Jan

G8HQP Dave · 17th Mar 2018, 11:21 am

There is not total agreement on the meaning of S9, and even less on the meaning of S8 etc. so a conversion chart from one ill-defined scale to another ill-defined scale might give the false appearance of precision where none is present.

Radio Wrangler · 17th Mar 2018, 12:07 pm

Don't worry about it.

as Dave says, the S1..S9 scale for signal reporting has no definition in terms of microvolts or dBm. There is a rough consensus that 1 S point difference should be 6dB, but this is rarely the case on the common transceivers with S scales on their meters.

Most of these meters are operated by the AGC voltage, and so their sensitivity and law are a function of the AGC characteristic of the controlled stages. This means that there is a large amount of variation from receiver to receiver even of the same model. On the AR88, you can't have the AGC on at the same time as the BFO because the BFO will create a lot of AGC voltage and greatly reduce the gain, so its S meter isn't useable when receiving SSB or CW.

Signal strength meters also vary a lot in sensitivity across bands. This is particularly true with sets like AR88s where coil sets are switched and the sensitivity varies further with the position of the tuning capacitor.

Also the value of a signal report is in the field strength received. The voltage at the antenna input of the receiver depends a lot on the antenna.

If I turn on the radio here on this bench I'm certain I'll hear plenty of QSOs and every report will be 5&9 or 599

David

Keith · 17th Mar 2018, 2:33 pm

It is my undertanding that the original AR88 S-meter is scaled in dB relative to 1uV emf. The general concensus amongst the vintage radio community is to set S9 at 50uV with 6db between the lower S units. This would correspond to approx +34dB on your meter. Even if you set this point correctly, however, moving either side of this, however, the scaling is very unlikely to be accurate.

JanWra1 · 17th Mar 2018, 3:49 pm

Thanks guys, basically it means that when the needle is vertical in AM mode the signal is abt S9 - that's good enough for me.

Jan

G8HQP Dave · 17th Mar 2018, 4:14 pm

Interesting that the Wikipedia article on S meters says that 6dB/division has been agreed, but has a picture of an S-meter which appears to be around 10dB/division (S7 to S9 looks about the same distance as S9 to S9+20).

dtvmcdonald · 17th Mar 2018, 8:21 pm

I measured the meter accuracy on all my boatanchors. This was done by feeding a known voltage into a 50 ohm step attenuator known to be accurate both in attenuation and impedance.

They all vary versus frequency quite a bit, like 10 dB even at best. The best are actually pretty accurate at S9. These are the R390A and the National NC-400. Next are the Hallicrafters SX-88 and the Hammarlund SP-600. The least accurate is the Racal RA-17C12 which is way off below S4 and above S9.

Except on the 7 MHz scale the R-390A is remarkably accurate. On that scale, which uses a crystal at the 3rd harmonic, it's worst off. Note that the AGC system on my R390A is highly modified, but the 3rd IF gain is adjusted so that the meter is correct and the audio output reduced.

17th Mar 2018, 12:38 am	#1
JanWra1 Tetrode Join Date: Jan 2018 Location: Stockholm, Sweden Posts: 78	The AR88 S-meter scale The S-meter on my AR88 had some difficulties to work properly but a change of R21 to a new one fixed that for abt £ 10:-, however I now have another question for you guys - the scale on the S-meter: As it goes from -6 to +100 (or is it 120?) which value corresponds to S9? Does anyone have a conversion scale? E.g. S1 = ?, S2 = ? etc Jan

17th Mar 2018, 11:21 am	#2
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: The AR88 S-meter scale There is not total agreement on the meaning of S9, and even less on the meaning of S8 etc. so a conversion chart from one ill-defined scale to another ill-defined scale might give the false appearance of precision where none is present.

17th Mar 2018, 12:07 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: The AR88 S-meter scale Don't worry about it. as Dave says, the S1..S9 scale for signal reporting has no definition in terms of microvolts or dBm. There is a rough consensus that 1 S point difference should be 6dB, but this is rarely the case on the common transceivers with S scales on their meters. Most of these meters are operated by the AGC voltage, and so their sensitivity and law are a function of the AGC characteristic of the controlled stages. This means that there is a large amount of variation from receiver to receiver even of the same model. On the AR88, you can't have the AGC on at the same time as the BFO because the BFO will create a lot of AGC voltage and greatly reduce the gain, so its S meter isn't useable when receiving SSB or CW. Signal strength meters also vary a lot in sensitivity across bands. This is particularly true with sets like AR88s where coil sets are switched and the sensitivity varies further with the position of the tuning capacitor. Also the value of a signal report is in the field strength received. The voltage at the antenna input of the receiver depends a lot on the antenna. If I turn on the radio here on this bench I'm certain I'll hear plenty of QSOs and every report will be 5&9 or 599 David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

17th Mar 2018, 2:33 pm	#4
Keith Heptode Join Date: Jan 2003 Location: Tewkesbury, Gloucestershire, UK. Posts: 693	Re: The AR88 S-meter scale It is my undertanding that the original AR88 S-meter is scaled in dB relative to 1uV emf. The general concensus amongst the vintage radio community is to set S9 at 50uV with 6db between the lower S units. This would correspond to approx +34dB on your meter. Even if you set this point correctly, however, moving either side of this, however, the scaling is very unlikely to be accurate. __________________ Keith Yates - G3XGW VMARS & BVWS member http://www.tibblestone.com/oldradios/Old_Radios.htm

17th Mar 2018, 3:49 pm	#5
JanWra1 Tetrode Join Date: Jan 2018 Location: Stockholm, Sweden Posts: 78	Re: The AR88 S-meter scale Thanks guys, basically it means that when the needle is vertical in AM mode the signal is abt S9 - that's good enough for me. Jan

17th Mar 2018, 4:14 pm	#6
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: The AR88 S-meter scale Interesting that the Wikipedia article on S meters says that 6dB/division has been agreed, but has a picture of an S-meter which appears to be around 10dB/division (S7 to S9 looks about the same distance as S9 to S9+20).

17th Mar 2018, 8:21 pm	#7
dtvmcdonald Pentode Join Date: Mar 2016 Location: Champaign, Illinois, USA. Posts: 227	Re: The AR88 S-meter scale I measured the meter accuracy on all my boatanchors. This was done by feeding a known voltage into a 50 ohm step attenuator known to be accurate both in attenuation and impedance. They all vary versus frequency quite a bit, like 10 dB even at best. The best are actually pretty accurate at S9. These are the R390A and the National NC-400. Next are the Hallicrafters SX-88 and the Hammarlund SP-600. The least accurate is the Racal RA-17C12 which is way off below S4 and above S9. Except on the 7 MHz scale the R-390A is remarkably accurate. On that scale, which uses a crystal at the 3rd harmonic, it's worst off. Note that the AGC system on my R390A is highly modified, but the 3rd IF gain is adjusted so that the meter is correct and the audio output reduced.