You Learn Something New Every Day: S-Meters

On the IC-746PRO mailing list, a fellow asked, “On the 2m band, why doesn’t the S-meter reading on my IC-746PRO match the S-meter reading on my 2m mobile radio?” My answer was that while there is a standard (it’s actually a recommendation), it’s rarely followed, so it’s not a surprise that the two readings don’t agree.

Mark, K5LXP, corrected me, noting that the recommendation specifies one value for an S9 reading below 144 MHz, and a different value above 144 MHz.  The recommendation reads:


  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.

This corresponds to a voltage of about 50 microvolts below 144 MHz and a voltage of 5 microvolts about 144 MHz.

It would be interesting to know how the software of the IC-746PRO handles this. I say software because the S-meter on the IC-746PRO is a digital meter whose readings are controlled by the rig’s microcontroller. Theoretically, the software could calculate the S-meter readings differently based on the band that the rig is set to, but I don’t know that the programmers would go so far as to do that.


  1. Dave New, N8SBE says:

    Elecraft goes to the trouble. It was discussed on their mailing list. Since the K3 is considered a state-of-the-art transceiver for use with transverters going up into the microwave region, the K3 can be calibrated for the power gain of the transverter, as well, so you can get a ‘true’ S-meter reading on all bands (including HF+6m), with all transverters.


    – Dave, N8SBE

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