Thoughts regarding the S-Meter

Sometimes I was thinking about signal reports I got during various QSOs (ssb). I would like to dig into this topic since I now have an RF  signal generator and I am very into measurement and lab experiments on antennas and rigs (I do not have any perfection claim, but it’s just to confirm my builds with numbers)

So, I got my Yaesu FT-991 and its service manual (“technical addendum” as per Yaesu naming convention) and I started to verify if actual alignment values were measurable with various tools I had in my lab. I started to check oscillator frequency and then the S-Meter. Here what the manual says about it:

IARU region 1 definition (coinsidered as de facto standard) regarding this scale of values (1981) tells that for S9, in HF bands, is -73dBm to the antenna feedpoint having the source a 50 ohm impedance. Each “S” point variation corresponds to 6dB (so, from S1 to S2 there are 6dB more, the same from S2 to S3, and so on…) Above 30Mhz frequencies, IARU standard for region 1 fixes S9 to -93dBm. The 20dB difference between HF and VHF is because of the different noise temperature that decreases passing from the first to the second frequency range. Also was observed that using a transverter with an RTX calibrated for S9=-73dBm showed usually a 20dB gain.

Coming to the unit of measurements, when using dBu(V) and uV should be specified if they refers to open (EMF) or closed (PD) circuit. Otherwise the measurement is wrong. On the opposite, dBm(W) (refers to power, not voltage) is always related to closed 50ohm system (PD), so we do not have any misunderstanding. Furthermore this is the preferred way to go!

Decent RF signal generators, give the choice to select between the two EMF or PD scales

A side note: this means that if during a QSO (without having any sort of condition change) I receive a 59 report using 100W in antenna, reducing my power to 25W the new report should be 58. (remember that 3dB is the half in power scales; and it would be 6dB in Voltage scale, that is not our case here).

So 6dB means halving two times the initial 100W… did you ever realized this ? Can you imagine how many times, explaining this, the typical OM looked me with a suspicious gaze ? Yes, many times…

Coming back to our service manual, there is a note saying that it considers 0dBu=0,5uV closed circuit (50 ohm) (1), that correspond to 1uV EMF (open circuit) or -113 dBm. Here my doubt was: ” ok then , 0.5uV are “PD”, but the mentionend “0dBu” are EMF or PD too ?”

Not having the immediate answer written in the manual, I performed an “ad absurdum” thinking:

Let’s assume that 0.5uV are “EMF”, so calculating “dBu” it would be -6.03dBu EMF or -12dBu PD. This is against the manual assumption (1) stated above and then wrong. This lead to deduce that 0dBu are EMF.

…..therefore all “dBu” values found in Yaesu table are “EMF” (…they love to complicate things….)

(for math part see for example this site: https://www.eeweb.com/tools/rf-unit-converter)

Summarizing, here the differences between IARU and Yaesu normalized in dBm, so absolute values:

S-Meter  IARU dBm Yaesu dBm value type

difference dB Yaesu (respect previous S value)

1 -121 -99 explicit -1
2 -115 -98 hypothesis -3
3 -109 -95 hypothesis -3
4 -103 -92 hypothesis -3
5 -97 -89 explicit -3
6 -91 -86 hypothesis -3
7 -85 -83 explicit -3
8 -79 -80 hypothesis -3
9 -73 -77 explicit 0

Therefore if I take S9 values, +36dBuV (EMF), this corresponds to -77dBm

Considering S7, +30dBuV (EMF) is -83dBm.

Wait a minute, -77dBm ?? this is very different from IARU recommendation.

Ok, nobody is following the common criteria…

Concluding: do you want to calibrate your S-Meter ?

Then use IARU values !

 

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