October 1932 Radio News
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
QRM and QRN (manmade and natural interference,
respectively) has been a problem to be dealt with since the
beginning of radio communications. Amplitude modulation
(AM) was and is still the most vulnerable
because there are so many sources of electrical and electromagnetism
generation - both intentional and unintentional. Filters can take
care of out-of-band noise, but inband noise needs to be dealt with
differently. Some inband interference can be reduced in effectiveness
with circuits using specific time constants that address specific
noise types. One of the most successful methods for mitigating generic
noise is to limit the opportunity for noise signals to enter the
system by employing directional antennas. Focusing
(literally) reception in the direction
of the preferred signal can cause other sources to be rejected.
Reflecting surfaces like parabolic dishes and phased element antennas
are the two basic choices. In some cases, as is the subject of this
article, noise is picked up on the ground and lead-in wires of the
antenna system. Constructing a balanced system will cause such interference
to cancel before it ever reaches the receiver input.
Using a Balanced Aerial System to Eliminate Interference
By Thomas C. McClary
Figure 2 - shows the balanced aerial system installed
from a diagrammatic viewpoint.
There is no doubt that even with the increased power of broadcasting
stations today, radio interference still spoils reception in many
localities. Part of this interference is due to natural causes such
as static, but part of it is also due to inductive interference
sometimes called "man-made" static, and still in part some is caused
by trouble developed in sets, loose connections, worn out batteries
and tubes, etc. A further source is that set up by poorly constructed
antennas running in the wrong direction or with improperly erected
However, even with the most carefully constructed antenna, with
thoroughly protected lead-ins, properly checked tubes and latest
hook-ups, noises often make for poor reception in some instances.
Particularly is this true in metropolitan centers where man-made
static has come to have a special meaning of its own to listeners-in.
These noises in radio reception are due to the filtering into the
receiver circuits and inductive impulses coming from elevators,
household electrical machinery and sometimes commercial manufacturing
plants near by.
Up until recently, engineers have shrugged their shoulders at
thoughts of further eliminating these man-made static noises. "The
noises are there. If the set, the antenna and the tubes are in proper
condition, if the proper filters are installed, what more can be
done?" This has been their attitude. Some laymen answered by shutting
off their sets entirely. But the technically minded research men
have gotten busy with new circuits and antenna systems to try to
eliminate trouble. Engineers of the General Motors Radio Corp. have
been at work on this problem and have set about to study the causes
first and to try to develop means for overcoming interference. At
the present time, with many thousands of the installations described
in this article in service, they feel they can pronounce their solution
Their analyses show that between 60% and 90% of man-made static
interference in radio reception is picked up by the lead-in and
ground connections and that only a small percentage is picked up
on the flat-top antenna proper. Interference noises from refrigerators,
door bells, elevators, telephones, etc., are constantly struggling
to get in, therefore, on the lead-in wires.
The new system is a simple affair involving nothing more complicated
than a double antenna installation, with a twisted-pair lead-in
feeder connected to suitable balancing transformers at the set.
In other words, by balancing out all of the noises picked up on
the lead-in and ground system equally, the interference is practically
eliminated; the balanced transmission line offering a guarded path
through the noise field for the radio signals that are picked up,
high in the air above local interference. Local "strays" and inductive
noises from the general run of household machinery are thus immediately
ejected from the feeder circuit and passed to ground, allowing only
the radio signals to operate the sensitive receiver amplifiers.
The question, "How can the radio signal, itself, get through
this transmission line," may present itself. The answer lies in
the difference in potential between the top antenna wire and the
low counterpoise wire on the roof, as seen in Figure 1. This potential
difference results in an unbalance which permits the fields surrounding
the antenna wires to produce an electric potential which will pass
the system to the receiver. Fields, however, that are immediately
adjacent and surrounding the lead-in wires will cancel each other
Figure 4A (above) shows the transformer connected to
receiver while Figure 4B (below) shows the transformer
to be connected on the aerial mast.
When the engineers started their investigation the most pressing
relief was needed by apartment houses and business buildings where
the length of the lead-in could not be governed. The difficulty
was overcome by using inductive coupling so that the length of the
lead-in with this type of aerial did not matter.
The author investigated one of the apartment building systems
in operation in Dayton. Ohio. High over the building, above the
noise field, is the antenna on tubular masts. Two wires are used,
one directly above the other, about 10 feet apart and 50 feet in
length. as shown in Figure 1. The engineer explained that the distance
separating these wires must be at least 10 feet, and that the lower
wire must be at least 3 feet above the roof - 5 feet is preferable.
The higher the set of wires are, above any surrounding wires, the
The antenna wires should be stranded, enameled copper of at least
seven strands of number twenty-two for the best results. They should
be at least thirty-five feet in length, longer if possible. One
hundred foot is the best length, if space is available. Here the
important part of the system begins. The wires must be of exactly
the same length, otherwise the counter-balance of outside noises
is not accomplished. If guy wires are used for the support of the
masts, glass insulators should be used at the ends nearest the mast
to break any collected energy from being passed on to the antenna.
The antenna should, of course, be placed as far from the noise field
A twisted-pair lead-in of number nineteen "outside" weather-proofed
wire is used, connected to the antenna wires at the ends farthest
from the noise field. Mechanically twisted wire is essential as
hand-twisted wire is not sufficiently accurate and a loss of sensitivity
and noise elimination results. The lead-in wires are anchored to
the mast at a point midway between the two antenna wires so that
the length of each lead-in, from anchor to antenna, is exactly even.
A porcelain cleat attached to a piece of wood serves as an anchor.
Data For The System
However, as two lightning arresters will be used. the better
plan is to mount them on a solid block of wood anchored to the mast,
and use the binding posts of the arresters for the lead-in, being
careful to see that the length of the lead-in from its anchor is
exactly even. One post of each arrester is used as a terminal for
the lead-in wires, while the ground terminals of the arresters are
joined together by a jumper wire and grounded preferably on a cold
The twisted-pair lead-in is carried on to the receiver and connected
to an antenna coupler which should be mounted as near the receiver
chassis binding-posts as possible. The lead-in wires are attached
to the coupler, which in turn is joined to the receiver's "ground"
and "antenna" binding posts by means of twisted-pair wires.
This same system may be used for any number of receivers up to
twenty-five, the only difference being that for a multiple installation
a master-coupler should be used between the antennas and the leads-in.
In a multi-installation job, the antenna wires should be as long
as possible, never less than fifty feet.
They should be spaced not less than fifteen feet apart and the
lower wire should be not less than from six to ten foot above the
roof instead of three to five as is all that is necessary for a
single installation. Where a master-coupler is used it is installed
on the mast, midway between the two antenna wires - see Figure 2.
The lead-in wires are taken from the lightning arrester into the
coupler. The two black leads of the master-coupler are then spliced
to the main twisted-pair, lead-in wires which connect with the master
Installation on a Private Dwelling
Figure 3. This drawing shows the main details for constructing
the new double antenna system and placing it on a home.
Double Antenna On An Apartment House
Figure 1. The essential details for setting up the new
interference eliminating balanced antenna system on a flat
To establish the proper polarity, make tests with the first receiver
attached to the trunk line of the system. Reverse the green and
black leads from the individual coupler to the chassis binding-posts.
If there is no difference in volume, the polarity of the master-coupler
is reversed where it connects with the two antenna wires. If the
polarity of the master-coupler is correct, there should be a decided
difference in the volume when the leads from the individual coupler
to the receiver are reversed. To change the polarity of a master-coupler
it is only necessary to reverse the connections to the antenna wire.
For houses with a sloping roof, or any other location where the
tall mast type of aerial is impractical, the system described is
merely laid on its side, see Figure 3. Two wires are used as in
the other system, the upper antenna being ten feet or more higher
above the ground than the lower one. This is particularly important
to remember when installing the antenna on a broad roof with only
a slight slope. The lower wire is laid parallel to the upper along
the lower edge of the roof.
Single System For Homes
Four masts should be used, set so as to keep both antenna wires
at least three feet above the roof at all times. The greater the
height of both of the antenna wires and the greater the vertical
distance between them, the better the results. The twisted-pair
lead-in should be anchored midway between the antenna wires so that
both wires are the same length from the point of anchorage to the
far insulators. Both antenna wires must be exactly the same length,
exactly parallel, and connected with the lead-in wires at the point
farthest from the noise field. No master-coupler is needed for this
single installation; only the type 1050 coupler at the set.
Once the antenna is completed, continuity tests of the antenna
and all lead-in wires should be made. It is important that there
be no grounds on either of the two antenna or lead-in wires or between
the wires of the twisted-pair lead-in wires, and that the lightning
arresters are not short-circuited or grounded. It is also advisable
to run a common ground wire to the frames of all metal signs, cornices,
etc., on the roof of the building and that conduit or "BX" cables
in the building are grounded properly.
Both types of couplers are illustrated in Figure 4a and in Figure
It is important to remember that such a system as this is not
necessarily a cure-all for all radio interference. Its purpose is
to eliminate man-made static or interference originating in the
house or in the immediate vicinity where it would be picked up by
an ordinary lead-in wire. Also - and what is particularly important
- it avoids he transfer, to the antenna system, of interference
brought into the house over the light lines. Such interference usually
is not picked up by the antenna proper but is picked up readily
by the ordinary lead-in wire. By eliminating the possibility of
pick-up by the lead-in, noises of these types are effectively eliminated.
Posted August 21, 2014