| Tech Talk From the Breakfast Table |
October, 2006 |
Defining Signal Strength (S-units)
One thing that every modern receiver has is some sort of meter on the front panel. Some of the newer LCD display rigs such as the Kenwood TS-2000 have a "virtual" meter that can display several different measurements at once. By virtual, I mean that it isn't a traditional meter with moving parts but rather a display that simulates the traditional meter. Some even have a "needle" although most either use a series of vertical bars or a semi-circular sweep display.
The instrument we'll focus on for this article is the S-meter. Over the years, the S-unit which is a standard of measurement has sort of drifted into a hazy reference point that doesn't mean what it used to. As an illustration, listen to a DX pileup sometime; everybody is S-9 even though the other station has to repeat his call 7 or 8 times!
In the early '80s the International Amateur Radio Union (IARU) adopted a set of standards which described the S-Unit and the method of calibration. One of the earliest receivers using these standards was the Drake R4 which used a high quality analog meter that was very accurate.
The IARU defines S9 to be the value displayed when the antenna input level is 50 microvolts (µV) for HF bands and 5 µV for VHF and UHF receivers. This standard is further assumed to be for antenna inputs terminated at 50 Ohms.
So what is an S-unit? By definition an S-unit is a difference of 6 dB from the reference point. If the reference point is S9, or 50 µV, then S8 would be 6dB less or 25 µV. Come again? Remember the chart from last month's newsletter? Here's the line for -6 dB:
| dB | P2/P1 | V2/V1 or I2/I1 |
| -6 | 0.2512 | 0.5012 |
In the last article we were discussing power loss so we used the P2/P1 column. But this time we're talking about voltage so we'll use the V2/V1 column. Multiply 50 µV by 0.5012 and you get 25.06 or 25 rounded. Ok, what would S7 be then? Multiply it again (or cut it in half). We now have 12.56 µV. And so it goes until you get to S-0 which is no signal, or very little.
So now, let's go the other way and talk about really really strong signals. If S-9 is 50 microvolts, what would we measure on the antenna jack for a 10 dB over S-9 signal?
| dB | P2/P1 | V2/V1 or I2/I1 |
| 10 | 10.000 | 3.162 |
| 10 Over | 10 dB over S-9 (50 x 3.162) | 158.1 microvolts (µV) |
| 20 Over | 10 dB over 158.1 (158.1 x 3.162) | 499.9 round up to 500 µV |
| 30 Over | 10 dB over 499.9 (499.9 x 3.162) | 1580.72 µV (yikes now we're in millivolts) 1.59 mV |
| 40 Over | 10 dB over 1.58972 mV | 4.998 mV round up to 5 mV |
| 50 Over | 10 dB over 4.998 mV | 15.804 mV |
| 60 Over | 10 dB over 15.804 mV | 49.97 mV round up to 50 mV |
Do you believe it? Can there actually be that much voltage on your antenna? Sure, and more. If your receiver is properly designed it will handle it although there's no telling what that guy's signal is going to sound like. Usually a 60 over signal only means a couple of things:
Your meter is whack-o, a distinct possibility or you could have your RF gain control backed off.
The guy's in a mobile parked nearby. Is that him behind you?
The guy's across town and running a Texas Kilowatt. (they used to call that a "California Kilowatt".)
In reality, does all this really matter? So much of the information gleaned from your meter is subjective. For one thing, if you are in a normal RF environment you will be operating with your AGC turned on. The AGC often masks the true S-meter reading even assuming that it is properly calibrated. In the end, it's up to the operator to evaluate the incoming signal. It is in our best interest to give the other guy the best and most accurate signal report we can. If his signal is truly "loud and clear", say so. If it isn't, or if it sounds like he's got something out of whack (technical term) tell him.
Another important thing to remember is the importance of good operating practice. If someone tells you that your signal is "20 over 9", ask him what his noise level is. If he has a typical noise level of say S-5 then you may be running way more power than you need to. Unless fading is especially bad you could probably throttle back.
Recap
Things to remember:
S-9 equals 50 microvolts at the 50 Ohm antenna jack.
1 dB: The smallest change detectable by the human ear.
6 dB equals one S-unit.
6 dB: Twice the Voltage, or if negative, half the Voltage.
10 dB: 3.162 times the Voltage, or if negative, Voltage divided by 3.162.
numbers after the decimal point don't mean much; round off.
Next Month: Antenna gain; fact or fiction.