April 1939 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.
Antenna radiation (beam) patterns published by
manufacturers are obtained under ideal - or close to ideal - conditions with a carefully prepared
and calibrated open air test site (OATS) or an enclosed anechoic chamber. Multipath, imperfect earth
ground, obstacles both manmade and natural, misshapen elements, poor VSWR, antenna orientation (in
both azimuth and elevation) are among the many factors which produce real-world operational results
that do not jive with a manufacturer's datasheet. Without employing some far field 3-dimensional
field strength scheme (see
Drone-Based Field Measurement System™), there is no way to obtain a complete picture of
how your antenna performs in all directions. This article presents a practical procedure for making
measurements that will at least collect useful data for specific orientations.
Checking Beam Antennas with the S-Meter
Converting Meter Beadings to Decibels for Signal-Strength Comparisons
By S. Gordon Taylor,* W2JCR
The "S" meters now so commonly employed in communications-type receivers are valuable primarily as an
aid in giving uniform signal strength reports, and in the constant check on the operating condition
of the receiver. But with the growing use of rotary beams for transmitting and receiving such meter
equipment is finding a new and important application in checking the radiation characteristics, and
even in plotting radiation patterns.
Fig. 2 - Voltage, power and db scales for quick conversion. The S scale of the receiver
in use may be plotted as shown in the right-hand column so that reports can be given readily in terms
of db gain or loss.
Fig. 1 - S-meter calibrations of various receivers in terms of decibels. The S scales
are arbitrarily chosen by the manufacturers, hence the wide variation in "db per S point." All values
are relative to the lowest S-scale reading on the receiver, and are independent of the receiver sensitivity.
That is, the fact that, in this chart, one receiver may show a reading of S7 while another shows S4
for the same db value above does not mean that the signal input would be the same in both cases.
Many hams recognize the possibilities in this latter application but fail to take advantage of it
because they (1) are unfamiliar with the procedure, (2) are reluctant to undertake the calculations
ordinarily involved, or (3) lack data from receiver manufacturers which permits variations in S readings
to be interpreted in terms of decibels, power ratios or voltage ratios.
It is the purpose of this article to provide data on a number of standard receivers, describe simple
methods of checking characteristics and plotting patterns of rotary beams, and provide a chart which
reduces calculations to a matter of simple arithmetic - with not an awful lot of that.
If you have a rotary beam and ask different stations for checks, the results will normally be of
only the most general value because for the most part these reports will be given in terms of the number
of S points difference between your minimum and maximum signal. A three-S difference on one standard
receiver may, in terms of power ratios, equal a difference of five S's on another receiver, four S's
on a third, and so on. For this reason the S reports from different receivers are not directly comparable,
and it is impossible to arrive at any sort of average suitable for use in plotting antenna characteristics.
Nor can this problem be solved by arbitrarily assuming some standard db value per S point, a practice
which is quite common. Measurements show that the "db per S" may average anywhere from 3 to 6 on different
standard receivers. This is another way of saying that with one receiver model S9 may be around 50 db
"up" from S1 while in another model it may be only 25 db up. What is more, the db difference between
S3 and S4, for instance, may be quite different from that between S8 and S9 on the same receiver and
What is needed is knowledge of the actual db calibration of the S meters for the different receiver
models, and this data will be found in Fig. 1 for nine standard receivers. The data on which this chart
is based was obtained from the individual manufacturers, for this purpose (or from their literature
in some cases), and most of it appears in print here for the first time.
The utility of this chart is obvious. It serves to provide a useful calibration for owners of any
of these receivers. Of equal importance, it enables the ham to interpret, in terms of db, the comparative
reports received from others who are using any of these receivers.
With rotary beams sprouting like mushrooms, the receiver S-meter takes on a new importance in furnishing
a means for giving information on radiation patterns - provided its readings can be reduced to some
standard. The important thing, of course, is the relative signal strength, easily expressible in terms
of decibels. Since no two receiver S-point calibrations are alike, the information in this article is
particularly timely and useful, and gives the beam owner a means of correlating signal reports.
Perhaps your receiver has a meter which you installed yourself and on which you scaled off your version
of the S scale. In that case you can obtain the decibel calibration by comparing your readings on given
signals with those of a friend who owns one of the receivers of Fig. 1. This comparison must of course
be made with the two receivers in the same location and switching the same antenna. It will be most
simple if you calibrate your meter to correspond with the S scale of the other; then its decibel calibration
as given in Fig. 1 will apply to yours also.
It is perhaps well to point out that the varying heights of the columns in Fig. 1 have nothing to
do with the relative sensitivity of the different receivers. In each case the column height simply represents
the relative values which each receiver manufacturer chooses to employ in designing and calibrating
his S meter circuit, and it is obvious that the different scales are not in agreement. The fact that
some calibrations start at S° and others at S1 is of no importance; this again simply represents
the manufacturer's choice of zero db level but does not in any way alter the utility of the db data
of this chart.
When the owner of a rotary beam receives reports on his front-to-back ratio in terms of decibels
these reports from different stations can be directly compared with a far greater degree of accuracy
than would be the case were the reports simply given in S points. For example, suppose two receiving
stations gave him reports. One, equipped with a "Sky Challenger" receiver reports readings of S9 for
the front and S3 for the back. The other, using an "NC-100" reports S9 for the front and S6 for the
back. On the basis of straight reports (without the data of Fig. 1) it would appear that the signal
variation as observed at the first station is twice as great as at the second. Convert both to decibels,
however, and the reports are nearly the same - 19 1/2 db and 18 db respectively.
A Translation Scale
Table I - Typical Beam-Pattern Work Sheet
Fig. 2 provides another tool of great value in checking beams and plotting radiation patterns. Here
the decibel scale again appears, and related directly to it are power- and voltage-ratio scales. In
the last column is the S scale of the Skyrider SX-17, which happens to be the receiver used at W2JCR.
Those having other receivers should substitute the appropriate S scale.
To illustrate the use of the chart, assume that a rotary beam is being checked. "On the nose" a reading
of S8 is obtained and off the back end the reading is S5. The db equivalents taken from column 2 are
36 and 21 and the difference is 15 db. Now referring to column 1 it is seen that 15 db represents power
ratio of slightly over 30, and this is the front-to-back ratio of this particular beam. Thus a specific
and decidedly useful report can be given to the station being checked.
The power ratio could be arrived at immediately without the intermediate conversion of the S readings
to db, but if this is done we get a ratio of something like 3600 to 120, which is rather unwieldy. To
reduce it to the simpler terms involves some mental gymnastics which are much more arduous than the
S-db-power conversion scheme. Although beams are evaluated by hams almost entirely in terms of either
db or power ratios, there are occasions when the voltage ratios are desired and for that reason they
are included in the chart.
Connecting his rotary beam to his receiver, the owner can check its characteristics by tuning in
some other station and noting the meter readings as the beam is rotated. The conversion to terms of
power ratio is then made as described above.
Fig. 3 - A sample rotary-beam directional characteristic determined by the method
described in the text. The curve could be smoothed out if desired.
Obviously the only additional work necessary for plotting the radiation pattern of a beam is to take
a number of readings as the antenna is rotated, instead of just the two needed to check front-to-back
ratio. There are several precautions to be taken in such checks, however, and the procedure followed
at W2JCR may prove helpful.
The first thing is to determine the antenna position which puts the strongest signal into the receiver.
If this is above S9 the antenna coupling to the receiver should be reduced so that the meter reads S9
or slightly lower, thus keeping within the range of the db calibration. Having found this position for
the beam, and with the receiver antenna coupling suitably adjusted, the exact course of procedure is
then agreed on between the two stations: The beam to be rotated in steps of not more than 22 1/2 degrees
and to be stopped in each of these positions for at least 30 seconds; at each stop the position of the
beam to be announced, then the carrier left unmodulated for the balance of the period.
At the receiver end the selectivity should be high to reduce the possibility of QRM, because an interfering
signal will make the check valueless. Leaving the antenna coupling at the adjustment mentioned above,
at each position of the beam make a note of the announced direction, then during the unmodulated period
make sure the signal is exactly in tune and note the resulting reading. This retuning is important because
changes in signal strength may otherwise tend to throw the receiver oscillator slightly out of tune,
making readings unreliable. If modulation were present, that too might confuse the readings by "wobbulating"
the meter.1 The meter readings are noted in fractional S points; usually it is possible to
estimate to one-tenth of a point.
Table I is the work-sheet of an actual check made by W2JCR and will be briefly described to illustrate
the orderly method of recording the measurements and compiling the desired data.
The directions as announced were entered in column 1 and the S readings in column 2. The transmitting
station was then asked to stand by for a few minutes while the desired information was worked out in
columns 3, 4 and 5.
In column 3 the db values for the various readings are noted. These are taken from the second column
of Fig. 2. We note that the lowest value here is 15 db, and inasmuch as we are interested only in the
relative values shown by this particular beam we adopt this as a new zero level and deduct 15 from each
of the values in column 3, entering the result in column 4. From these figures we see that the maximum
signal is 30 db above the minimum; that the "head-on" signal is 13 db above the back (southwest) signal,
etc. Incidentally, we note that this beam is apparently not functioning "according to Hoyle" because
it should have minimum radiation off the ends, which would be the SE and NW positions.
In this particular report the power ratios are entered in column 5. Had the owner of the beam been
interested in voltage ratios these would have been shown instead. In either case the data are obtained
directly from Fig. 2. If a power unit value of 1 is given to the minimum signal position, then the maximum
signal, which is 30 db up, would represent 1000 times this power, etc.
With these figures completed a detailed report was given to the owner of the beam by reading off
to him the figures of columns 4 and 5. Later, as a matter of interest, the pattern of his beam was drawn
up as shown in Fig. 3, using polar graph paper.2 Lacking this it could have been drawn on
plain paper on which a dot is first placed to represent the transmitter and lines drawn radiating from
this at angles of 22 1/2 degrees - like the cuts of a pie that has been divided into 16 pieces.
The scale selected for use depends on the ratios involved. In this case the maximum is 1000 to 1,
so the scale used was 200 per inch, making the front lobe 5 inches long. The other values were laid
out on each of the lines corresponding to the beam positions and the resulting points were joined together
with a line and this made up the pattern of the beam.
Had more readings been taken, perhaps at every 10 degrees, this pattern would have been less angular
in appearance and more truly representative, because obviously the actual radiation pattern is not likely
to have sharp points such as those shown at the front and back. Even in drawing the present pattern
it would have been entirely legitimate to round off the corners; in fact this is usual practice although
it is easier to draw as shown in Fig. 3.
Attention has been centered, in this discussion, on the use of the S meter and the data of Figs.
1 and 2 as aids in checking beams. But they have other applications as well. Power gain or loss for
different degrees of antenna coupling at the transmitter, and different adjustments of the transmitter
or transmitting antenna can be checked in the same way providing the" before and after" readings are
made under similar conditions of receiver adjustment, line voltage, etc. The effectiveness of any two
fixed transmitting antennas can likewise be determined, especially if they are so arranged that the
transmitter can be switched from one to the other quickly. Such comparative checks obtained from several
receivers located in different directions from the transmitter will provide helpful information, but
the radiation pattern of a fixed directional beam can be determined in this manner only if the antenna
used for comparison is non-directional or of known directional characteristics.
In conclusion, it is well to emphasize the fact that measurements such as those described in this
article are not perfect. The human element plays an important part, as in accurately reading the meter,
determining the exact angle between different positions of a beam, etc. A bad tube in the receiver may
alter the meter calibration, but fortunately many of the ailments to which receivers are at times heir,
while they may change the reading of the meter for a given signal voltage input, do not materially alter
the relationship within the scale itself and therefore do not change the db calibrations given in Fig.
1. In any event, the system outlined represents the most accurate, generally available method of checking
and is far superior to the old system of "three S's down," "two S's up" now in common use.
In plotting radiation patterns it should be borne in mind that two patterns plotted on the same beam
but by different receivers in different directions will not necessarily be the same. There are numerous
factors, such as reflection, absorption, and refraction, which may differ in different directions and
different locations. But by obtaining checks from several different stations it is usually possible
to strike some sort of average which will not only indicate the characteristics of the beam but may
indicate specific faults such as a nearby structure which is affecting its operation.
* The NC-100 utilizes an electric eye in conjunction with the r.f. gain control, which has a 0-10
scale. With a signal tuned in the gain control is backed off until the signal barely registers on the
eye, then the S reading is taken from a curve plotted in terms of gain-control settings. The combination
is therefore the equivalent of a meter.
** The Super-Pro meter is not calibrated in S-points but instead has a 0-5-milliampere scale. With
the antenna disconnected and no signal tuned in this will read somewhere around 4. As signals are tuned
in the reading drops and it is the amount of this drop from the normal no-signal level that appears
in the figure. Thus if the normal level is 4.1 and a signal drops it to 3.6, then that signal is 15
db above the "zero" level, etc.
1 Obviously, when making such a check with a distant station, it is also necessary to take the effects
of fading into account and, if necessary t make the measurements over a sufficiently long period so
that fading averages out. The measurements in such a case should be attempted only when conditions are
relatively stable. - Editor.
2 Polar Coordinate paper No. 358-31. Keuffel and Esser, 127 Fulton St., N. Y. C.
Posted March 31, 2016