The S Meter - Measuring Radio Signal Strength

They must be the most talked about topic on the air. True to say the figures over nine lend importance to a report and the graphic use of red in some designs make the listener feel something really special is going on, but they are for the most part only a guide.

Have fun with them by all means but please do not hold them up as gospel.

S meter scale

Only in a few classic designs has the law on which the S Meter scale is based been fully interpreted. Even then the antenna has to be an EXACT match to the input impedance of the set with no compromises.

Radio hams exchanging signal reports should realise that to be of any value, the two operating stations must be identical from the ground up. Quite literally, as soil conductivity around an antenna is a vital part of its characteristics. Having said all that, what kind of voltages exist at the antenna socket?

S Meter Specification

The International Amateur Radio Union (IARU) agreed on a technical recommendation for S Meter calibration for HF and VHF/UHF transceivers in 1981.

IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input power of -73 dBm. This is a level of 50 microvolts at the receiver's antenna input assuming the input impedance of the receiver is 50 ohms.

A weak signal with signal strength of S2 corresponds to received power of -115 dBm or 0.40 microvolts in 50 ohms on HF.

A strong signal with signal strength of S8 corresponds to received power of -79 dBm or 25 microvolts in 50 ohms on HF.

For VHF bands the recommendation defines S9 to be a receiver input power of -93 dBm. This is the equivalent of 5 microvolts in 50 ohms.

The recommendation defines a difference of one S-unit corresponds to a difference of 6 decibels (dB), equivalent to a voltage ratio of two, or power ratio of four.

Signals stronger than S9 are given with an additional dB rating, thus S9 + 20dB, or verbally 20 decibels over S9. Some signal signal generators are calibrated in dB above 1uV and have an ouput in EMF.

For example to set an HF receiver's S-reading to S9 set the signal generator output to 34 dB above 1uV

S-Points Below 30MHz

S-Points Below 30MHz
Signal strengthRelative intensityReceived VoltageReceived Power Across 50 Ohms

S-Points Above 30MHz

S-Points Above 30MHz
Signal strengthRelative intensityReceived VoltageReceived Power Across 50 Ohms

Where the signal is acutely strong, report the level but listen on the attenuator. With signals this strong, there is no need for the real performance of a communications receiver so avoid overload distortion with a prod at the ATTN button.

S Meter Readings - Myth And Legend

In an article written for AOR, there is one problem with having a calibrated S Meter.

If the meter in question is on an AR7030, then rest assured, it will be accurate as long as the antenna impedance has been matched to 50 ohms. But, once you know it's telling the truth, we all seem hell-bent on bending it around the end-stop.

Longer long-wires, better Beverages, radical rhombics, daring dipoles, quintessential quads, whimsical Windoms, gee-whiz G5RV's and indescribable impedance matching conspire to red-line the meter.

More is better, surely? Not necessarily. And don't call me Shirley.

Our ability to hear a signal is not just based on strength. It is the ratio of that strength to any noise degrading your enjoyment of it.

Therefore, go for the best signal-to-noise ratio. Users of loop and balanced aerials will have already noted that signal levels are generally down compared to a long-wire, but the noise levels are greatly reduced.

The signal-to-noise ratio has been improved not by increasing the signal level but by dropping the noise floor. OK, the S Meter is down but the signal is clearer. In real terms, audibility is up.

As long as the signal is strong enough to quiet the receiver - not really a problem with an AOR - then if it's there to be heard, you'll hear it. Having said all this, there is still something magical about going for out-and-out signal strength. And an AOR AR7030 can take it.