October 1976 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
It is always interesting to look back
many years ago and see what people thought were the big issues of the day.
According to this 1975 QST article, some amateur radio operators
thought the decades old S−scale system for determining signal strength using a
combination of objective and subjective measures was in need of modernization.
Nine separate levels of signal strength was, in this author's estimation, too
many to provide a meaningful report to the sender regarding how well his message
was being received. Five was plenty. He had a good point since, as pointed out,
can an operator really say whether a signal quality rated a score of S5 versus
S6? History has been the judge of the matter, and now 44 years later the
original nine levels has survived.
The S−meter and the S−scale - is it time for changing the rulebook?
By Jury A. Belevich*, UA1IG
A QSO without a signal report is a rare thing. And for good reason too - we all
like to know how our signal sounds at the other end. Years ago, hams were just as
concerned about their signals (if not more so), and a system, the RST (readability,
signal strength, tone) system, was conceived to standardize signal reporting.
1. Faint signals; barely perceptible.
2. Very weak signals.
3. Weak signals.
4. Fair signals.
5. Fairly good signals.
6. Good signals.
8. Strong signals.
9. Extremely strong signals.
Table 1 - Signal Strength
1. Very weak signal; impossible reception.
2. Weak signal; reception
with strained attention.
3. Satisfactory signal; reception without particular
4. Good signal; reception without strain.
5. Loud signal;
Table 2 - S−Scale
Table 3 - S Power Levels
Table 4 - S−Scale Corporations
Is the RST System Outdated?
But through the years, the S−scale of the RST system has shown its age. The scale
shown in Table 1, which spreads the estimation of signal strength over a very wide
scale, may have been convenient when radio reception had only natural (cosmic and
atmospheric) disturbances. Today, however, man-made and interstation (adjacent-channel)
interference prevails. The S−scale is almost inaccurate as a result of this interference,
because the difference between two adjacent reading points on the scale is so small.
What is the difference, for instance, between an S4 and an S5 signal?
When estimating signal strength based on the current S−scale, differentiation
between S2 (very weak) and S3 (weak) signals is difficult, if not impossible.
Better Signal Estimates with Modified S−System
We'll obtain more precision in signal estimation if we substitute instead a simplified
S−system and the following S−scale. Some intermediate S−meanings have been removed
from this scale so that it is simpler to estimate the signal strength more accurately.
It isn't long before most amateurs learn the great difference between estimating
signal strength by ear and doing so with the help of a receiver's S−meter. This
difference is caused by calibration variations and various S−meter amplifier tube
For example, the S−meters on two receivers which have different sensitivity levels
must be calibrated. It is recommended to estimate the S9 signal with an input signal
strength of 100 microvolts when the S−units are spaced at 6-dB intervals. The calibration
is made by the voltage applied to the input terminals of a receiver, as shown in
With this method of calibration, S−meter scale graduation marks on a receiver
with 1-microvolt sensitivity and a second receiver with 50 microvolts will proceed
from S3 to S8 (Fig. 1A and B), because these signal strengths correspond to sensitivity
of the receivers. With the same input signals (for instance, 100 microvolts) applied
to the receivers, the S−meters will indicate the same signal strength (S9) because
they are calibrated in the same way.
Fig. 1 - At A, S−meter scale for the first receiver. At B, S−meter
scale for the second receiver.
S−Meter Calibration Problems
There is a problem, though. When estimating signals by ear, an operator will
determine S6-S7 in the first receiver and not more than S1-S2 in the second. This
method of S−meter calibration allows for measurement of the signal level, but it
doesn't reflect real conditions of communication, which depend not only on signal
strength but also on the state of a receiver (and its quality too).
Often, a receiver has an S−meter amplifier tube with deteriorated characteristics.
This causes understated S−meter readings, such as "RS−51" or "RS−52." This is full
readability on a faint signal.
Another problem with measuring signal strength is that receiver manufacturers
use various methods of S−meter calibration. For example:
1) The National Radio Co., Hallicrafters. The use of 50-m V input signal to determine
the S9 point. Each S−unit equals 6 dB.
2) Swan Electronics Corp., Collins Radio Co. The standard for S−meters is 100
mV through a 6-dB pad for S9.
3) The R. L. Drake Co. The standard is 50 mV through a 50-dB pad for S9.
There is another method of S−meter calibration which this writer considers more
correct. First, the zero level of the input signal is determined, the level which
corresponds to the receiver noise level (it may also be called maximum or critical
sensitivity level). In acoustics, this level is considered the "audibility threshold
level." From acoustics, we know that normal speech exceeds the audibility threshold
by 60-70 dB. Thus, we adjust a receiver's S−scale so that the maximum signal strength
exceeds the receiver noise level by 60 dB. If we consider the maximum signal strength
to correspond to S5 (Table 2), each S−unit will be equal to 12 dB. As receiver sensitivity
contains the condition of necessary signal-over-noise excess to provide normal reception
(usually 10-20 dB), this value conforms well to the concept of receiver sensitivity.
With this method of S−meter calibration, an aural estimation of a signal is the
same as the S−meter reading. If the S−meter measures the value of the i-f voltage
at the output of the receiver, the S−meter readings will remain accurate despite
receiver deterioration (such as in the case of aging tubes). There will still be
some dependence of the S−meter sensitivity on its amplifier tube if the S−meter
measures the age voltage.
* a/box 20, Leningrad, M-244, 196244, USSR
Posted September 18, 2020