October 1932 Radio News
of Contents]These articles are scanned and OCRed from old editions of the Radio & Television News magazine.
Here is a list of the Radio & Television News articles
I have already posted. All copyrights (if any) are 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.
See all available vintage Radio News articles.
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 lead-ins.
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 generally
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 out.
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 better.
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 as possible.
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 water
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 trunk line.
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 topped apartment.
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 4b.
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