January 1960 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
At the end of the last century (the 20th), aside from the impending total collapse of the world's electrical infrastructure due to Y2K computer date issues, technovisionaries (a word I just made up) predicted the near-term demise of local over-the-air (OTA) broadcasting of both commercial radio and television. Cable and satellite was going to supplant it all. For a short while things seemed to be going that way, particularly as both forms of media (radio and TV) began being available via smartphones. The FCC (Federal Communications Commission) was so sure OTA television was dead that it wanted to reallocate unused spectrum (white space) for other uses. It also mandated a conversion of all TV broadcasting to be done in digital form. The plan forced either trashing of existing television sets and purchase of new models or the purchase of analog-to-digital conversion boxes. The compliant public folded like a cheap suit (the kind I own) and shelled out hard-earned (or welfare) money on new sets. To the surprise of those in-the-know, media consumers got tired of having to pay to watch basic TV and radio, especially for local interests, and began installing rooftop antennas again. There is now a bit of a renaissance occurring where in place of the former multi-element VHF/UHF antennas strapped to chimneys and mounted to roofs with or without separate antenna rotators we are seeing the compact antennas with built-in preamplifiers and rotators.
A New Antenna Rotor
Fig. 1 - Control unit (top) of the Tenn-A-Liner and motor housing (bottom).
By Harry Greenberg
Chief Electronics Engr., Channel Master Corp.
Better accuracy and reliability are objects of this automatic design approach.
For those many TV installations in which the rotator has become a necessary component, the most popular has been the automatic type since its introduction several years ago. The user simply sets the control dial to the best position for the particular channel he wishes to receive and then sits back as the rotator is moved to that position. However, dealers and consumers soon discovered that these automatic units had shortcomings.
Because they used solenoids, such rotators produced a noisy "clack-clack" during operation. They got out of step, requiring frequent service. They also produced electrical interference visible on the TV screen. Also, because they moved in step-by-step mechanical stops instead of continuous rotation, they could be set in position only in increments of, say, six or ten degrees. The latter limitation can be important in installations with reception problems using highly directional antennas to obtain maximum picture clarity, and more especially important with color TV. To meet the problems noted, Channel Master has introduced the Automatic Tenn-A-Liner (Fig. 1) , using a dual-synchronous motor design without solenoids. It works without producing noise or interference, relieves the service problem, and permits orientation of antennas within one degree of desired direction.
In conventional automatic rotator systems, operation of the roof-top antenna depends on whether two independently rotating discs in the control box are lined up ("off" position) or not lined up ("running" position). When the discs are lined up (Fig. 2A) a switch is held in an open position, preventing operation of the motors (M1 and M2).
When the user moves his control dial, one of two discs (the manual disc) moves with the dial. This disturbs the alignment between the manual disc and the rotator-controlled disc (Fig. 2B) and applies power through the switch so that motion of the rotator begins in the desired direction. Just as the manual disc moves with the control knob, the rotator-controlled disc must move in unison with the rotor, until they reach the position set by the viewer. When this position is reached (Fig. 2C), the two discs are once more in alignment and the switch again opens, stopping all motion. The antenna, fixed to the rotor, is now properly oriented.
Fig. 2. How an automatic rotator control moves an antenna into position.
Fig. 3. This synchronizing system permits continuous movement rather than the stepped motion provided by an escapement.
During this operation, the normally open switch in the rotator housing keeps closing momentarily, sending pulses of current to a solenoid in the control box. Through a mechanical linkage, the rotor-controlled disc slips through an escapement movement, like one used in a clock, permitting motion one step at a time. Thus each pulse permits motion of the rotor-controlled disc through a step of six or ten degrees at a time. Sometimes the linkage may jam, or a pulse may fail to move the disc, upsetting synchronism.
The system used for maintaining synchronization between the rotor and the rotor-controlled disc in the Tenn-A-Liner eliminates the need for the step-by-step intervals of movement, provided by the action of a solenoid and an escapement. Instead, the rotor-controlled disc is moved by the continuous action of a motor.
When the viewer sets his control dial, a pin (Fig. 3) is moved from its neutral position into a groove that rotates with the control dial. The movement of this pin closes a normally open circuit and simultaneously starts the rotation of two identical motors, one in the rotor and one in the control box. These two move the rotator and rotor-controlled disc in synchronism. The control-box motor moves the latter disc through a fine-meshed gear train. The pin moves in the groove of this disc until it can fall back into its neutral position, stopping the action. After the rotator and antenna have reached their pre-set position, more precise (one degree) adjustments may be made, if necessary.
Rotators are conventionally powered through a 29-volt step-down transformer rather than directly from the a.c. line. However, before this voltage reaches the motor, it must generally pass through a potentiometer in the indicating device, through which about 6 volts is lost. Since there is no potentiometer in the new system, the full 29 volts is used to produce higher torque in the motor.
Posted May 2, 2019