Confused About S-Units?
April 1966 Popular Electronics

April 1966 Popular Electronics Table of Contents Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, published October 1954 - April 1985. All copyrights are hereby acknowledged.

S-units are probably not familiar at all to non-Hams since they refer to receiver signal levels (the "S" stands for "signal"). It is a relative unit of measure rather than absolute. Technically, the dBm unit of power is also a relative unit, but it is referenced to a fixed power level of 1 milliwatt - traceable back to primary standards at NIST or any other country's standards keeper. By contrast, the S-unit - at least originally - is relative to the strongest useable signal level at a particular receiver's input. An indication of S9 meant a maximum signal level was present at the input based in part on the receiver's dynamic range at a certain frequency. Each S-level step indicated a signal decrease of 5 to 6 dB below the previous S-level, again depending on the manufacturer's preference. As you might expect, confusion ensued. Nowadays there is an unofficial but generally adopted definition of S9 through S1, beginning at -73 dBm for S9, and then a 6-dB step size down to S1 level of (8 x -6 dB = -48 dB) = -121 dBm. It applies across the specified receive band(s), independent of frequency. Author Marshall Lincoln reviews S-units and S-meter circuits.

Some answers to why an S9 signal is sometimes not even S7 much less 10 dB over

By Marshall Lincoln

It might happen like this. Joe Ham, W0XYZ, is chewing the rag with his buddies on the air, and they get to comparing signal reports. Bob, who lives about a mile away, says Joe's signal is 10 dB over S9. Sam, who lives clear across town, says Joe is pounding in at 30 dB over S9, and Ed, just down the block, says Joe's signal is just S9. Which one of these guys is wrong, or has a bum receiver?

Maybe none of them. Or, maybe all of them!

The reason is that even if these guys lived side by side and were using the same model of receiver and the same type of antenna, they still might not get the same readings! Using different receivers and different antennas, they definitely will get different readings.

How come? It's just that an S-meter (signal strength meter on the receiver) doesn't really measure actual signal strength. But, it does indicate the relative strength of signals getting into the receiver. This means that a signal is shown to be stronger, weaker, or the same strength as another signal, without a definitive measurement of the actual strength of either signal.

AA signal which deflects your S-meter to S7 is two S-units weaker at the input to your receiver than one which deflects the meter to S9. Since most manufacturers calibrate S-meters so that one S-unit represents either 5 or 6 dB change in signal strength, the S9 signal is 10 to 12 dB stronger than the S7 signal at your receiver.

Calibration Variations. An important variable which causes S-meter reports to be inconsistent is the fact that receiver manufacturers do not have a common calibration standard. Also, each manufacturer has his own calibration method. Plus, variations in components may cause S-meters in two receivers of the same make and model to perform differently.

The folks at the National Radio Co., for example, calibrate S-meters so that S9 indicates approximately 50 microvolts input signal to the receiver. This company told the author that, "strictly speaking, the decibel readings over S9 should be in a nonlinear scale, but ours are put into a linear scale for simplified reading and for averaging purposes."

Swan Electronics Corporation says that its meters are calibrated to read S9 with an input signal strength of 100 microvolts at 50 ohms at 14 mc. The S-units are spaced at 6-decibel intervals. This company cautions that "production variations, particularly in tube characteristics, will cause considerable change in these figures ..."

The R. L. Drake Co. uses a 50-microvolt input signal at the antenna terminals to determine the S9 point. Calibration is at 50 ohms impedance, but each S-unit equals 5 dB!

Hallicrafters also uses 50 microvolts at the antenna terminals of the receiver to set S9, and makes each S-unit equal approximately 6 dB. Measurements are made at 50 ohms impedance at 5 mc. The company adds: "This approximation will have a variation of plus or minus 5 dB on a new receiver, and, as the tubes age, the variations may be still greater."

The standard for S-meters at Collins Radio Co. is approximately 100 microvolts at 50 ohms through a 6 dB pad for S9. This may vary, says the firm, from about 90 to about 115 microvolts, depending upon adjustments in the i.f. section of the receiver.

More Woes. As if there aren't enough variables in the S-meter situation already, let's see what else can affect S-meter readings. How about the S-meter adjustment pot on your receiver - how long has it been since you checked to see if it was set properly? This potentiometer is adjusted to produce a zero reading on the S-meter scale under certain conditions - generally with the antenna terminal shorted to ground, or with the r.f. gain control set at minimum, or both. Check the manual for your receiver for the exact procedure. You may find that your S-meter has been off by several S-units, because of tube and component aging.

While you're looking in the manual, check to see what the manufacturer has to say about the setting of the .f. gain control during receiver operation - this can have an effect on S-meter readings. Generally, the r.f. gain control must be in the maximum gain position for the S-meter to read according to the manufacturer's specs. With the r.f. gain reduced, a stronger input signal will be required to produce a given S-meter reading. However, even with the r.f. gain reduced, you still can use the S-meter for relative indications just as you would with full r.f. gain, as long as all readings are made with the same setting of the r.f. gain control. Change this setting and you change the meter indication.

How about the condition of the S-meter amplifier tube? When it gets weak, the meter indications are affected, even though overall receiver performance doesn't change. So don't overlook this tube whenever you check your receiver tubes.

How S-Meter Circuit Works. Generally, the S-meter is connected, through an amplifier tube, into the a.v.c. line. When a.v.c. voltage increases, as it will when a stronger signal comes into the receiver, the meter reads upscale.

Two typical S-meter circuits used in communications receivers are shown on p. 55. Essentially they are the same, except that one circuit has the S-meter, usually a 0-1 milliammeter, in the cathode circuit, while the other has the meter in the plate circuit.

The amount of current flowing through the S-meter amplifier tube, and hence through the meter itself, is determined by the a.v.c. voltage, which is applied to the tube's grid as well as to the r.f. and i.f. stages controlled by a.v.c. action. A strong signal produces a larger negative voltage on the a.v.c. line than a weak signal, and reduces the current flow through the S-meter amplifier tube. In this case the S-meter will read backwards - strong signals will be indicated on the left end of the scale and weak signals on the right end of the scale.

One way to make the meter indicate increasing signal strength as the meter needle deflects from left to right is to use a special meter built backwards from the conventional meters - one in which the needle rests normally on the right end of the scale, and moves to the left as current through the meter increases. Another way to achieve the same result is to use a conventional meter. but mount it on the receiver panel upside down, while inverting the meter scale so it is read right side up. This explains why some receiver S-meters are pivoted at the top of the meter face, and why others, although pivoted at the bottom, deflect to the right when you turn off the receiver.

A more elaborate, but forward-reading S-meter circuit is also shown to the bottom right. This is essentially a bridge circuit (see familiar version below) where R4 and R5 are equal in value. The other legs in the bridge are R2 and the equivalent of tube resistance plus the zero-adjust potentiometer R1. The pot is adjusted for zero reading on the meter (balancing the bridge) with no a.v.c. voltage present. Application of a.v.c. reduces the amount of current flowing through the tube, thus increasing the tube resistance and unbalancing the bridge, causing the meter needle to deflect upward.

Regardless of the kind of circuit your S-meter has, always remember to use it as an indicator of relative signal strength in the receiver, and nothing more, except for tuning and alignment of the receiver and transmitter. When you tell a guy that his signal is 30 dB over S9, you are actually indicating how well his signal is getting through your rig.

Posted March 6, 2018