In 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.
of Contents]These articles are scanned and OCRed from old editions of the
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QST articles I have already posted. As time permits, I will
be glad to scan articles for you. All copyrights (if any) are hereby acknowledged.
See all available vintage
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.
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).
R1, R2 -15 ohms.
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 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.
|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 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
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 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.
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
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.
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.
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 S12 simultaneously.
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.
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 immediate
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
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
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 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.