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
April 1945 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
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 application.
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
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.
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
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 other antennas.
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.
Returning 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
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.
Connections 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
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
Referring 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.
To begin 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.
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 S12
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 row.
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 bottom.
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
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
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.
Since the 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.
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 frequency.
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.