this article the author describes the automatic antenna switching system
which was developed for controlling the forty-odd receiving antennas
at the FCC's Grand Island (Nebraska) Monitoring Station. With this system
it required only a matter of seconds for the operator to select any
desired antenna by simply pushing a couple of buttons on the control
panel. A similar system could easily be designed for a lesser number
of antennas either for a test range or an amateur radio operation.
April 1945 QST
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are hereby acknowledged.
See all available
vintage QST articles.
Automatic Antenna Switching
A Simplified System for Instantaneous Selection from a Number
BY Alfred K. Robinson EX-W7DX
The Field Division monitoring stations of the Federal Communications
Commission must be able to determine quickly and accurately the operating
frequencies of all classes of radio stations, including those of the
Army, Navy, and other government agencies. They must be able to make
bandwidth and modulation measurements and many other technical studies,
such as channel-occupancy surveys, wave-form analyses, keying checks,
emission tests, and a great number of other tasks of a highly specialized
nature. To accomplish these duties requires apparatus capable of the
greatest possible degree of accuracy. Since standard equipment is not
always available, the Commission's field inspectors often must design
and construct special apparatus which may be required for a particular
This has been true in the case of the antenna systems
required. The Commission's monitoring stations require a wide variety
of antennas. For instance, as many as seven medium and low-frequency
double-ended Beverages, nine double-ended high-frequency rhombics, four
multiple doublets, several simple doublets, beams, verticals, and long-wire
antennas are used at a single location.
The necessity for such extensive antenna systems is not
hard to understand when one considers the large number of stations on
the air with their multiple-frequency assignments and duplicated channels.
An example of how valuable these antennas become is apparent from the
fact that with a single Beverage, elimination of practically all except
those stations located on a line with the antenna is possible. Also
there is the probability of being able to select as desired either of
two stations on such a line if one is in one direction and the other
in the opposite direction, even though one of these stations is several
hundred miles farther away.
Fig. 1 - Antenna-feeder switching system.
Only one feeder wire is shown in each case; connections for the second
wire are duplications of those shown, and are made through a second
section of each of the three-gang switches. (Connections to the third
switch section are shown in Fig. 2.) R is a 200-ohm resistor.
Fig. 2 - D.C. circuit of the third switch
section. The negative terminal of the battery is grounded at the chassis.
C1, C2 - 0.5 μfd.
RFC - 80 turns No. 16 wire on
a 2.inch diameter form.
A, B and C are sections of S11,
while D and E are sections of S12 (see Fig. 1).
R2 -15 ohms.
Administration, which has made use
of such engineering, has eliminated the necessity for several times
as many monitoring stations as are now in use to monitor the bands properly.
Since measurements and observations vary over wide frequency
ranges and because antennas designed for low-frequency operation are
seldom suitable for high frequencies, there is a need for some type
of switching arrangement whereby any antenna may be made instantly available
if full use is to be made of the receiving facilities.
Such a switching
system, essentially automatic, is shown schematically in Figs. 1 and
2. These drawings show antenna-feeder and direct-current control circuits
respectively. Notably there are no "dead-end" feeder lines connected
to the receiver at any time. The system takes care of a total of forty-six
antennas with the possibility of adding two more immediately. By making
slight modifications the system could be extended to include eighteen
more if necessary.
As indicated in Fig. 1, the antennas are
classified and divided into eight groups with six of one class to each
group. This is done for several reasons: the inspector can more quickly
select the antenna group most likely to produce without having to set
up each separate antenna; switching is accomplished without "dead-end"
feeder lines; better isolation between each individual feeder line and
still greater isolation between groups of antennas is provided; the
use of comparatively small automatic switches becomes practical and
these can be mounted at the best possible point in relation to antenna
termination, with a corresponding decrease in antenna feeder-line interaction.
Another highly desirable reason for the grouping method of switching
is to permit the best antenna of any particular class to be compared
with the best antenna of other classes without switching through several
In order to keep the efficiency of the antennas
at a maximum, open-wire feeder lines are used throughout. A short distance
from where the transmission lines connect to the switches, the wire
spacing is reduced to 3/16 inch. Such a line is practical when using
dual-hole Lucite beads commercially manufactured for two-wire coaxial
cables, and these make a very neat installation. The impedance is maintained
approximately by the use of smaller-diameter wire. The advantage of
close spacing is obvious, since the possibility of interaction because
of the feeder-line connections at the switches is reduced to a minimum.
R.F. Circuit Details
|In this article the author describes the switching system
which has been developed for controlling the forty-odd receiving
antennas at the FCC's Grand Island (Neb.) Monitoring Station.
With this system it requires only a matter of seconds for the
operator to select any desired antenna by simply pushing a couple
of buttons on the control panel.
Each of the numbered
switches in Fig. 1 consists of three sections ganged together on the
same shaft. Each section has six positions and is operated by a 6- to
8-volt d.c. "stepping" solenoid which continues to rotate the switch
arm, contact by contact, so long as the solenoid circuit is held closed.
Fig. 1 shows only the first section of each switch. It is seen that
these switches control the selection of one of the two transmission-line
wires to the various antennas. The second switch sections (not shown)
control connections to the other transmission-line wire in exactly the
same manner. Connections to the third sections are shown in Fig. 2.
These third sections of each switch control connections to the vertical
and horizontal lamps on the control panel which indicate which antenna
of the forty-odd available is in use at any given time.
to Fig. 1, it will be seen that the various antennas are arranged in
groups. Anyone of six Beverage antennas may be selected by S1
and S2. Since a Beverage antenna is directional, its direction
depending upon to which end of the antenna the receiver is connected,
provision is made to switch the receiver to either end. S1
controls connections to the "forward" end or" each antenna, while S2
controls the connections to the "rear" ends. Connections to anyone of
four multiple doublets are made through S3 and blank contacts
are available for the addition of two more antennas to this group.
In the next group, controlled by S4, are four h.f.
beam antennas, a vertical antenna and a 250-foot general-purpose antenna.
Similarly, S5 controls a group of six half-wave doublets.
Connections to either end of anyone of nine rhombic antennas are controlled
by S7, S8, S9 and S10. The
switching of this group will be discussed later.
to any desired group of antennas are set up by the "group" switch, S6.
Thus, to connect the receiver to any desired antenna, the "group" switch
is first turned to the desired group and then the "antenna" switch for
that group is turned to the desired antenna.
Since more than
six connections are required for the rhombic antennas and since these
antennas are used to a considerable extent on high frequencies to determine
the "sense" of a signal (direction from which a signal is arriving at
a bi-directional antenna), this group is provided with a separate "group"
switch, S10, which connects to X on the first group switch,
S6 This permits around-the-compass rhombic directivity without
the necessity for going through other groups while doing so. S10
selects anyone of the three rhombic antenna switches, S7,
S8, S9, while the latter each provide for six
different connections to the rhombic antennas. Here also connections
may be made to either end of each antenna.
Transmission-line anchors, showing the
jack connectors used for "patching" circuits when required.
The control and indicator
panel is shown in Fig. 3. It is only 5 1/4 inches high and of standard
rack length (19 inches). Antenna designations are marked on the panel
in tabulated form with an indicator lamp opposite each row of "antenna"
designations and one above each column of "group" designations. This
system simplifies considerably the wiring, besides saving on those parts
which are so scarce and difficult to secure these days. The simultaneous
lighting of one lamp in the vertical row and one in the horizontal row
serves to indicate which antenna is in use. Thus, to select any particular
antenna, the "group" push-button switch, S13 (Fig. 2), is
held closed until the horizontal lamp lights indicating the proper group.
The "antenna" push-button switch, S14, is held closed until
the vertical lamp lights which indicates the desired antenna in that
group. Thus, when the fifth horizontal lamp from the left and the third
vertical lamp from the top are lighted, the 3.3-Mc. doublet is in use.
Fig. 3 - Arrangement of the control and
indicator panel used with the antenna-switching system.
When any of the rhombic antennas is to be used, the "group"
push-button switch, S13, is held closed until one of the
last three lamps in the horizontal row lights, indicating that the "rhombic
group" switch is connected in the circuit. Then the "rhombic" push-button
switch, S15, is held closed until the indicator lamp shows
the desired rhombic group, after which closing the "antenna" push-button
switch, S14, will select the desired antenna in that group.
Control and Indicator Circuits
to Fig. 2, two operating circuits are required, one for the solenoids
which operate the rotary switches and a second which furnishes voltage
to light the indicator lamps. The numbered switches in Fig. 2 are ganged
with the correspondingly numbered switches of Fig. 1.
with, it should be noted that when the "group" switch, S13,
is closed the solenoids of S6 and also of an additional three-gang
switch, S11 are energized, so that these two switches turn
in unison as though they were ganged on the same shaft. Lamp voltage
is fed to arms A and B of S11. Arm A, through its contacts,
delivers this voltage to the arms of the first five antenna-indicator
switches operated by S14 via S6, and thence through
the contacts of these switches to the vertical row of indicator lamps.
Arm B, on the other hand distributes voltage to the first five lamps
in the horizontal "group" row.
When section A of S11
is in the position marked X, it delivers lamp voltage to one arm of
an additional switch, S12, which operates in unison with
the "rhombic" switch, S10, and thence through the contacts
of S12 to the arms of the last three antenna-indicator switches,
S7, S8 and S9. The contacts of these
switches are connected to the vertical row of lamps as shown in Fig.
2. Voltage from X also is fed through the "rhombic" pushbutton switch,
S15, which energizes the solenoids of S10 and
When arm B of S11
is in the position marked X, voltage is fed to the arm of section E
of S12 and thence to the last three lamps in the horizontal
When the arm of section C of S11 is in the X
position, voltage is fed through the "antenna" push-button switch, S14,
to the arm of S10 and thence to the solenoids of S7,
S8 and S9.
One of the monitoring bays at the FCC
station at Grand Island, Nebraska. The antenna-switching panel is in
the right-central rack, just above the row of Key-type switches at the
Study of the diagrams will show
that very few parts are used, considering the job that is accomplished.
Twelve three-gang switches, three push-button switches, fourteen lamps
and sockets, a source of d.c. power, a few feet of close-spaced transmission
line and other wire are all that is necessary for a workable system.
The switches are of the three-section, six-position, low-loss wafer
type fitted with a "stepping" solenoid, as mentioned previously. They
are particularly well suited to this use, since two of the sections
to the rear of each unit are well removed from the solenoid winding
and have practically no other metal in the immediate vicinity.
A few refinements have been added, such as condensers C1
and C2 and the r.f. choke, RFC, to prevent any possibility
of clicks in the receivers from the "make" and "break" of the push-button
switches. Resistors, R1 and R2 in Fig. 2 are connected
in series with the lamps to prolong their life. The several units are
provided with octal plugs and sockets so that they may be removed readily
for service or rearrangement.
To expand further the usefulness
of the available antennas and to provide an auxiliary for the automatic
switching system, all antenna-feeder lines are brought in to General
Radio jacks, thus making it practical to "patch" to any antenna manually.
The transmission lines brought to these terminal jacks are well spaced
to avoid coupling. The jacks make excellent points to connect the close-spaced
lines from the switching relays.
The rhombic antennas
shown in the diagrams belong to the Radio Intelligence Division of the
FCC, which has been doing such excellent work in keeping radio channels
clear of unauthorized transmissions and locating sources of interference
to important wartime communication circuits.
rhombics are used by the RID independently of the Field Division of
the Commission, the connections to these antennas must be made in a
manner such as not to disturb in the slightest their use by this agency.
To accomplish this a 250-ohm carbon resistor is inserted in each leg
of the transmission line coming from the rhombic-group relay. Extensive
tests on several frequencies have shown no interference to the RID and
no perceptible loss of signal for monitoring purposes.
While the usefulness of an automatic switching system for rapidly selecting
the most desirable aerial is readily apparent, a few of the more unusual
results include those noted when propagation conditions are acting up.
At such times good reception of a wanted signal often is possible on
an antenna whose directivity and theoretical design are both unfavorable.
Normally such an antenna would never be selected by manual means, but
the automatic system is so rapid that an inspector having difficulty
with a signal will run through all antennas in a matter of seconds.
Quite frequently the switching system has been used to select one signal
free from interference from as many as five other signals on the same
Although many other similar instances could
be given, suffice it to say that the Field Division of FCC, through
its monitoring stations, has been able, by use of highly skilled inspectors
and such specialized apparatus, to reduce the number of spurious emissions,
off-frequency signals, and undesirable operating practices to a point
where the increase in efficiency of communication channels is a very
gratifying contribution to the war effort.