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:
STANDARDISATION OF S-METER READINGS
- One S-unit corresponds to a signal level difference of 6 dB.
- 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.
- On the bands above 144 MHz this available power shall be -93 dBm.
- 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.