Ham Radio Forum

Three phase motors are often used in high power applications. Is it possible to design three phase antennas? If so, what advantage would they have?

Many antenna designs employ multiphase radiating elements. Consider an ordinary three-element Yagi for example. In an ideal case the driven element is tuned to resonate at the transmitting frequency. The reflector is tuned to resonate below the transmitting frequency to make RF current changes in it lag in phase behind current changes in the driven element. The director is tuned to resonate above the transmitting frequency to make RF current changes in it lead current changes in the driven element. Consequently, RF currents in the three elements have different phases. Out-of-phase electromagnetic radiations from those three elements combine to give the antenna directivity and gain.

The distinguishing advantage of a Yagi, compared to many other multiphase radiator designs, is the use of radiating element detuning to phase the RF currents, so the antenna can be fed with a single feed-line. Other antennas that aren't based fundamentally on the Yagi radiating element phasing idea and that have multiphase radiators require multiphased feed-lines that are similar in principle to three-phase commercial power distribution lines (although current phasing between the lines usually is different). AM broadcast antenna arrays are good examples. In most AM broadcast antenna arrays each tower is fed by a separate feed-line and each feed-line is routed through an adjustable phase shifting network that is adjusted to obtain the desired antenna radiation pattern.

Eric

Late reply.

Although Erik805 discusses phased currents or voltages in antenna operation it is quite different from 3 phase power applications.

The purpose of 3 phase 50/60 Hz power systems is to economize on the hardware needed to transfer a given power level over large distances.

Three phase power is utilized in the power supplies of high power commercial broadcast transmitters for the same reasons (and to ease the DC filtering problem) but not in the RF sections to my knowledge.

In RF transmitter applications the RF power source is usually rather close to the antenna system and single phase RF power delivery (via coax or open line) is apparently quite adequate economically. However in multi-tower broadcast antennas, the various towers are fed by single phase RF currents having various phase angles at the various towers to provide a directional radiation pattern. This is not a multi-phase power delivery method in the sense of "three phase power".

Jess, w4pqk

This is very interesting to me. I think there should be more experimenting with multi-phase ham antennas to learn the real possibilities.