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Everybody's S-Meter is Correct
Copyright 2018 by Don Keith
What is the first bit of information we typically glean or give when we begin an on-air conversation with another amateur radio station? The signal report, of course. Later, we talk about the weather, brag about our stations, decry the lousy lack of sunspots, but first, we want to know what kind of signal we are getting to the other guy's radio. And we want to make sure we give the other op an accurate idea of how well we are receiving his or her transmission as well.
It's a courtesy, of course, but it also helps us get an idea of how the band is behaving so we can gauge if we can continue the chat. It can certainly help each operator involved in knowing how well his station is performing. And in some cases, it gives us a chance to inform the other guy of a problem with audio, keying, or other aspects of the signal he is putting out there on the air.
Fortunately, there are a couple of standardized ways of letting a station know how well his signal is making the haul from his place to yours—the “RST” system and that dancing little back-lit indicator there on the front of your whizbang new radio…the S-meter.
“RST” originated way back in the early days of telegraphy as a way to quickly and concisely inform a station how well he was being heard. It is a 3-digit number with the numbers standing in order for “readability,” “signal strength,” and “tone.” Readability is a five-number scale while the other two parameters are nine digits. The scale, according to the American Radio Relay League, is:
Readability:
1 - Unreadable
2 - Barely readable, occasional words distinguishable
3 - Readable with considerable difficulty
4 - Readable with practically no difficulty
5 - Perfectly readable
Signal strength:
1 - Faint signals, barely perceptible
2 - Very weak signals
3 - Weak signals
4 - Fair signals
5 - Fairly good signals
6 - Good signals
7 - Moderately strong signals
8 - Strong signals
9 - Extremely strong signals
Tone (CW only):
1 -Sixty-cycle AC or less, very rough and broad
2 - Very rough AC, harsh and broad
3 - Rough AC tone, rectified but not filtered
4 - Rough note, some trace of filtering
5 - Filtered rectified AC but strongly ripple-modulated
6 - Filtered tone, definite trace of ripple modulation
7 - Near pure tone, trace of ripple modulation
8 - Near perfect tone, slight trace of modulation
9 - Perfect tone, no trace of ripple or modulation of any kind
Operators can also add a “C” if there is a chirp on the keying or a “K” to denote the presence of key clicks. There's also a quaint “X” indicator that can be included if the station's signal demonstrates the frequency stability of being “crystal-controlled.” And of course, the “Tone” number is not used for voice communications, though I think I could make a strong argument for making some kind of audio evaluation a third number for voice reports.
The obvious problem with this method—even a system that has stood the test of time for almost a hundred years—is that it is totally subjective. What exactly is the difference between a signal that is “barely readable” and one that is “readable with considerable difficulty?” And how do you bring yourself to give a guy a “4”—readable with practically no difficulty—when it's so close to being a “5”—perfectly readable? Oh, and how many of you could describe what a “rough AC tone, rectified but not filtered” actually sounds like?
One thing that prompted this article was hearing a station the other day give another ham a signal report of “Five by two!” I suppose it is possible to be perfectly readable with a very weak signal, given a quiet band, but hardly likely, and especially considering the conditions that particular day. I also have a QSL I received that has a CW signal report of “3-8-9.” Maybe it was my fist that made me difficult to read despite my strong signal with a perfect tone!
Of course, I am almost always “5-9-9” or “5-9” to DX stations in pileups, even if they have to ask me to repeat my call a dozen times or insist on calling me “K4NC,” so I'm obligated to give them the same report right back, right?
And I hesitate to even bring up those QSOs in which one station's audio is so distorted or hum-ridden that he can hardly be understood and the fellow on the other end of the circuit doesn't even mention it to him. I've even heard such accolades as, “Good audio!” when the offending station was using so much compression I could hear pots and pans rattling in the kitchen and he sounded as if he was talking through a pillow. He certainly should have been smothered by somebody! But is it his fault if nobody tells him he sounds like loud, distorted mush? If this overly diplomatic reporter told him how good he sounded?
So, thank goodness, we have another way to give honest signal reports, one that is totally accurate, unbiased, the same no matter what the make of the rig or the disposition of the operator. I speak of the S-meter. It is the ubiquitous little device available on most every commercially made piece of ham gear or SWL receiver, and since it is—by definition—a meter, it has to be correct. And since all the operator has to do is say where its needle points when giving a signal evaluation, it removes all prejudice from the resulting report.
Another thing that prompted this article was a discussion on the Yahoo Kenwood TS-2000 reflector—one that crops up on a semi-regular basis. Inevitably, someone will do an A/B check of different radios, using the same antenna, listening to the same station at about the same point in time. The result in the latest round of give-and-take was that the TS-2000's S-meter seemed to read considerably less than the other radio's. Was it possible that the 2K's receiver was that much worse? Maybe. But the station being sampled was just as readable on both radios. In some cases, the S-meter read nothing at all on the 2K and S-3 or S-4 on the other rig, yet again the station was equally readable on both.
Dang quirky radios! How is that possible?
Let's take a quick look at what an S-meter is. In actuality, most S-meters measure the voltage across the radio's automatic gain control (AGC) circuit, though that is hardly universal. Few are—dare I say it?—accurately calibrated. Likewise, few are linear across their entire scales either. They are actually designed to give the user a RELATIVE measure of signal strengths arriving at the receiver. More on the meaning of RELATIVE momentarily.
“S” stands for “signal strength.” Thank goodness! I'm still miffed that “E” means “voltage” and “I” is “current!” The markings on the scale of the meter—the “S” units—are based on our old friend, the “RST” system above. Things were pretty wild and wooly in the old days of the S-meter, but back in the early 1980s, the International Amateur Radio Union agreed to recommend that an S-9 reading on an S-meter, indicating “extremely strong signals,” would be defined as -73 dBm, or a level of 50 micro-volts at the antenna input to the receiver if the impedance presented there is 50 ohms. That's for HF frequencies. The recommended standard is different for VHF, but let's leave that alone for this article.
Furthermore, the IARU suggested that each S-unit gradient represented a difference of 6 dB. That is a power ratio of four.
So, since the IARU came up with a recommendation for a standard, the S-meter Police must guarantee that all S-meters everywhere work exactly the same way and are completely accurate devices. You know the answer to that! Without getting technical—and primarily because I don't understand it all either—suffice it to say that the manner in which most S-meters work makes them anything but accurate, rarely linear so that each unit on the meter equals a 6 dB difference, and almost completely inconsistent from one radio to another.
Also, the accuracy of a signal report, including an S-meter reading, is still dependent on the operator and his setup. Is my signal really 40 over S-9 when I hear that wonderful report, or does the guy just need a QSL from Alabama? Is that an average of the peaks or a momentary peak on that rapidly dancing little meter? If I'm not even moving the meter, could it be that the op over yonder runs with his pre-amp off, the way I usually do on 160 through 40, or does my signal really suck? And we haven't even gotten into other parameters that affect my signal strength at his place. Minor little things like antennas and propagation!
What the S-meter can do just fine is give a RELATIVE indication of strength between two or more incoming signals. “Don, you are peaking about S-9, but your buddy down the street…running the same power and about the same audio setup…is 20-over. You may want to do some antenna work!”
Your meter may be inaccurate and nowhere near the IARU's hopeful standard and dismally without linearity, but it is inaccurate and non-standard and non-linear for everybody. Just don't try to compare the S-meter reading on your Goshdarn 3000 Plus with your buddy's Heckfire Pro IV. They won't be the same…necessarily.
So here is the good news. Everybody's S-meter is completely accurate! Accurate as long as he is using it to give RELATIVE signal strength reports.
Permit me a brief illustrative fable. When I was a youngster, I was helping my dad (SK WA4AZJ) with a carpentry job. He asked me to cut him a 2X4. I asked him how long it needed to be. He answered, “About as long as a piece of rope.”I stood there perplexed, saw in hand, for a bit, trying to figure out where to start cutting, until he finally let me off the hook and gave me the exact measurement for the board he needed. Lots of things in life are relative.
Maybe I can apply that same bit of whimsy next time I begin a QSO.
“What is my signal there, old man?”
“Oh, you're peaking about two inches with peaks to the screw-head on the old S-meter. You even got up to the `n' in `Kenwood' a couple of times. Good signal! Now about that QSL I need…”
Don Keith N4KC has been a ham radio operator for more than fifty years. After a long career in broadcasting and advertising, he now writes full time and has published more than thirty books, fiction and non-fiction, on a wide range of subjects including amateur radio. See www.donkeith.com or www.n4kc.com for more info.