Vintage Alliance Model U-100 Tenna-Rotor Installation Kirt's Cogitations™ #301
If you grew up in the era of rooftop television
antennas, then there is a good chance you are familiar with the electromechanical antenna
pointing systems that were often installed as well. Alliance, Channel Master, Cornell
Dubilier, Radio Shack, RCA, Winegard, and others made low cost, light-duty rotators for
television antennas. Ham radio antenna rotators were/are more robust in order to handle
higher weight and wind loads. Many television antennas also cover the FM radio band (88-108 MHz),
allowing them to do double duty.
There are a few companies (Audiovox,
RCA) who still sell antenna rotators that are modern transistorized
units with digital readouts. Some allow you to store channel number / antenna direction
combinations rather than having to remember the antenna direction for a particular channel.
Customer reviews of the new units reveal dissatisfaction with the weakness of the rotator
unit that mounts on the antenna mast, and the inability of it to prevent the antenna
from being rotated in a strong wind (no brake or poor brake).
Being an unapologetic technology renaissance
man, I recently purchased (on eBay) a vintage Alliance Model U−100 Tenna−Rotor that was
unused in the original box. The seller found it in his father's attic. It was advertised
as NIB (New-In-Box), which typically means it includes all the original parts, but it
arrived missing a couple U−bolts (no big deal) and the cover for the control cable junction
box (a bigger deal). I waited for many months for such an example to appear at a decent
price because of wanting to document the entire system as originally supplied, so it
was disappointing to have the cover missing. I plan to make the best of the situation
and exploit my local library's MakerSpace facility's 3D printer to make a replacement. A photo of
the cover will be added here when it is available.
Upon receiving the U−100 Tenna−Rotor, the first
order of business was to open the enclosures and do a visual inspection. Everything looked
good except the grease in the rotator was hard caked and needed to be cleaned off and
replaced, which I did. As you can see in the photos, the cast aluminum parts are very
robust. The "brake" built into the Tenna−Rotor is accomplished via the worm drive. A
worm drive is like a mechanical diode in that only a rotation of the worm input
motor drive shaft will cause the output shaft to rotate. Applying a torque to the output
shaft, which is connected to the antenna, cannot cause the worm gear to rotate. It is
possible for the wind or some other force to prevent the antenna shaft from turning as
the worm gear tries to turn, which would strain or stall the motor, but it cannot be
forced forward or backward. Load bearing surfaces are simple bushings rather than ball
bearings, but the shaft will turn so little in a typical lifetime of use that it shouldn't
Mechanical drawings were located on the
Norm's Rotor Service
Bougetoline has schematics, so I thank them for that. Lower quality
copies are posted here in case they ever disappear from Norm's Rotor Service website.
Unlike the newer systems that use a 3-wire control interface, the Alliance U−100
Tenna−Rotor uses a 4-wire cable. Control and feedback is about as simple as it gets with
a motor turning the antenna mast and a solenoid in the control box that advances the
dial as the antenna rotates. Cams on both ends open and close contacts as required. The
rotator box motor rotation direction is determined by which set of field coils is energized
by the control box; i.e., no fancy reversal gears to fail.
Do you remember the signature ker-chunk, ker-chunk sound of that solenoid
turning the controller dial? That was one of the reasons I wanted this old system, just
to hear that sound again.
When I first began planning an antenna installation,
I was going to put it on the roof like in the old days, but I decided I would try it
in the garage attic first to see how well it worked there. Doing so wouldn't get me the
classic antenna-on-the-roof look I wanted, but it would keep the antenna and rotator
unit safe from the weather and it would probably last the rest of my lifetime. The antenna
is a top-of-the-line
Channel Master CM−5020 model that covers the VHF and UHF television
bands, and also the FM radio band. My primary interest is being able to pick up radio
stations since I rarely watch TV. A half-wave folded dipole has been used for many years,
but because of where my house is located reception quality varies throughout the year
and according to the weather. Often time I would need to re-orient one or both sides
of the dipole to get good pickup. Sometime I couldn't get even relatively nearby stations
no matter the orientation. Now with the mighty antenna and Tenna−Rotor I can dial in
any station I want and get super strong, static-free reception.