April 1968 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
The purpose of
directional antennas is not just to increase gain along a particular radial in
order to enhance weak signals, but also to reduce gain in all other directions
so as to minimize interference. Interference can be both signals from unrelated
emitters and from multipath signals that originated from the same emitter.
Hence, a directional antenna can be advantageous in an environment where a
relatively strong signal from the intended emitter is surrounded by other strong
signals. Even if the interfering signals are not on the same frequency, the
effect of raising the overall noise floor and/or generating intermodulation
products can degrade the intended signal significantly enough to bugger audio
and/or video. The solution is to mount the receiving antenna on a rotator so
that it can be pointed in a direction which results in the best signal. Here is
Alliance U-100 Tenna−Rotor write−up with photos. RCA is one of the few
companies that still manufactures an antenna
rotator of the type primarily intended for TV antennas. Many companies make
models for Ham radio use.
Fig. 1 - A thrust bracket is used to take the antenna load off
By Ron Roberts
As antennas grow in size and increase in gain, the need for rotators also grows.
Gain is inversely proportional to the width of the antenna's forward lobe. In other
words, the greater the gain of a given antenna, the more likely the need for a rotator
to pick up channels transmitted from different directions.
The evolution of FM antennas is an excellent case in point. Before FM stereo
became a fact, most people used nondirectional "flying S" or "turnstile" antennas.
While gain was relatively low, these antennas picked up FM broadcasts from all directions,
without the need of a rotator. FM stereo has changed all that. Not only are FM stereo
signals weaker than monophonic, they are more susceptible to multi-path distortion.
Thus, you need a very directional antenna with high gain for good FM stereo reception.
But, if a number of stations are coming at you from different directions, you'll
also need a rotator to pinpoint the incoming signals.
Color TV is analogous to FM stereo. Ghosting - the TV equivalent of multipath
distortion - is far more objectionable in color than in black-and-white. Color also
requires stronger signals. In fact, if you mount a good directional antenna on a
mast with a rotator, you'll find that color can be received only over a very narrow
arc. You can swing the antenna many degrees to either side of the good color arc
and still get excellent monochrome pictures. That is why you need a rotator to receive
good color pictures.
Antenna rotators can be used anytime you want to receive signals from a number
of directions. Rotators, however, are not an unmixed blessing. For one thing, they
complicate the antenna installation. For another, rotators present a problem in
multi set installations. If Dad wants to watch the baseball game coming from one
direction, and the kids want to watch cartoons transmitted from a station in the
opposite direction, it's obvious that the rotator can't point the antenna in both
One solution is to use two or more separate antennas, mounted on the same mast
and combined by a hybrid splitter or coupler. Unfortunately, this can get to be
even more expensive, complex and unwieldy than a rotator installation, in many areas.
Alliance Tenna-Rotor can be operated with any of three control
units, T-45, top left, is a manual control, V-100, top right, is automatic, and
the C-225 control unit, is transistorized for automatic, step less synchronous action.
Cornell-Dubilier's AR-10B (right and above) is an automatic rotator.
Antenna turns until its position matches that indicated on compass rose on control
box. Cornell-Dubilier HAM-M. Heavy-duty motor will handle up to 1000 lbs. The calibrated
meter control unit indicates antenna position in degrees of rotation.
A rotator adds much weight to an antenna installation. Therefore the mast should
be braced securely.
For example, installers usually try to keep the mast short to make the installation
as solid as possible. In a non-rotator installation, you can often use a 5-foot
mast with nothing more than a chimney mount. However, the rotator adds both height
and weight. Therefore, guy wires are recommended even for the simplest rotator installations.
Use three or even four chimney straps rather than the usual two, and make sure the
straps are rustproof stainless steel.
Another good practice is to use a thrust bracket (see Fig. 1) in every rotator
installation. The thrust bracket takes all the weight of the antenna, prolonging
the life of the rotator.
For safety and convenience, do as much as possible of your work on the ground.
Chances are you can wire the rotator and mount it, along with the thrust bracket,
before you even climb the ladder. You may also be able to attach the antenna to
the rotator mast and the lead-in wire to the antenna, all on the ground.
Once you do get up on the roof, be careful not to lose your balance. A big antenna
on a high mast with a rotator is very heavy and cumbersome. It's a good idea to
have a helper, especially on windy days. Try to keep the mast balanced and under
control, with your feet planted firmly on the roof at all times.
Inexperienced installers often have trouble with the rotator wires. It is easier
to twist the leads and tin them before attaching them to the rotator terminals.
This reduces the likelihood of stray wires shorting between terminals or touching
Also, if you use twin-lead, keep it away from the rotator wire. Some standoffs
accommodate both rotator wire and twin-lead, but it is definitely bad practice to
run these two cables closely in parallel. To avoid interference, tape the rotator
wire directly to the mast. Use weatherproof vinyl tape, long standoffs for the twin-lead,
and twist the twin-lead. Of course, if you use coaxial cable, interference pickup
is no problem and the dual cable stand-offs will be convenient.
The most common consists of a top-of-the-set control unit and a mast-mounted
motor, connected by four- or five-conductor wires. One major difference between
various rotators is the control unit. With manual control units, the user pushes
a button or a bar and waits till the picture looks sharp. On automatic types, a
knob can be set to a desired direction and the antenna will automatically aim in
the direction indicated.
Posted June 6, 2023