What Does Your S-Meter Reading Mean ? (Two Receivers Compared)

A receiver's S-meter is valuable for comparing strengths of stations and for adjusting peaks or nulls produced by loops, remote terminations, and phasing units. It is also very useful in bench-tests of homebrew RF amplifier and filter modules as well as verifying the gain and directivity of various antennas being compared at the same receiving site (or for comparing properties of different sites with a given portable antenna such as an active whip).

If the receiver can be swept under computer control, the computer's logging of frequency versus S-meter readback can make the receiver usable as a "poor man's spectrum analyzer" for bandscanning or for making intermodulation distortion product measurements in an automatic test environment. Nick Hall-Patch has applied this methodology with a good deal of success in the scientific observation of Trans-Pacific DX fade-in and fade-out times.

The more sophisticated that one's uses for the S-meter get, the more important it is to know what the readings mean in terms of standard parameters such as power measured in dBm (decibels relative to one milliwatt in a 50 ohm system) or voltage (rms or peak-to-peak).

If you have access to precision equipment, an S-meter calibration table for a given receiver can be prepared. The now-calibrated receiver can be used as a "transfer standard" with measurements that have real meaning, rather than just being somewhat-blind relative indications.

Luckily I work in the field of automatic RF test equipment and have access to metrology-lab calibrated signal sources. I brought two of my receivers, a Drake R8A and a Palstar R30, to a test-bench at work. During lunch I tested these with a Rohde & Schwarz model SMT-06 signal generator.

Drake R8A readings were taken at 2.3 kHz bandwidth, AM mode, and internal preamp ON (except as indicated in "Note 1"). Palstar R30 readings were taken at the narrow Collins filter setting (2.5 kHz width), AM mode, and no internal attenuation. Tests were done at 1 MHz. Measurements at other frequencies were also taken: these showed little variation (~ 1 dB) in sensitivity up to 30 MHz on the Drake R8A. On the Palstar, response was flat to 10 MHz, about 1 dB down in the 15-22 MHz range, and about 2 dB less sensitive at 30 MHz.

Measurements in the table below indicate the input level in dBm required to produce the S-meter reading shown.

Note 1: Reading with Drake R8A preamp OFF; Palstar R30 attenuator ON. In both cases, sensitivity decreases by about 10 dB. If the R8A’s attenuator was enabled, its sensitivity would go down approximately 10 dB more.