Multiple Tuning

Author: Edmund A. Laport

The most extreme conditions of low radiation resistance and high reactance are encountered at the lowest frequencies, and some extreme measures are necessary to obtain acceptable radiation efficiencies. The antenna represents a very great investment in structures and rigging, usually many times the cost of the associated transmitting equipment.

The most successful method of improving the radiation efficiency is that of multiple tuning. The antenna consists of a large elevated capacitance area with two or more down leads that are tuned individually as indicated in Fig. 1.7. The total antenna current is thus divided equally among the down leads, each of which has its own ground system.

The down-lead currents are in phase, and because of their electrically small separation there is no observable effect on the radiation pattern, which is always nearly circular. Power is fed into the system through one of the down leads.

When arranged for multiple tuning, an antenna behaves as a number of smaller antennas in parallel, voltage being fed through the flat-top system. Thus, a system with triple tuning is essentially three antennas in parallel, one of which is fed directly by coupling to the transmitter and the other two at high potential (voltage feed) through their common flat-top.

FIG. 1.7. Principle of multiple tuning.

From a radiation standpoint, the same effect would be realized if the different portions of the antenna were not physically connected through their common flat-top but instead were separately fed from a common transmitter and feeder system in the manner of a directive antenna. Practically it is simpler to take advantage of the fact that almost all antennas for the lowest radio frequencies must of necessity employ flat-tops for capacitive loading and merely to add the extra down leads for multiple tuning. In this way, there is only one feed point, and the problems of power division, phasing, and impedance matching are automatically minimized.

A simplified explanation of the manner in which multiple tuning increases the radiation resistance as seen from the power feed point is the following: Assume a certain antenna system which, on the basis of ordinary single tuning, has a radiation resistance of 0.04 ohm but has a total resistance of 0.90 ohm as measured at the feed point. For a power input of 100 kilowatts, the antenna current with single tuning would therefore be 333 amperes. If now this same system is multiple-tuned, using three down leads with idential ground systems for each down lead, the current of 333 amperes is divided approximately equally among them, giving 111 amperes per down lead. At the one fed by the transmitter, 100 kilowatts input now produces a current in that down lead of 111 amperes. From this it is reckoned that the total antenna resistance at that point is about nine times its original value, or some 8.1 ohms. If the radiation efficiency were not changed, the radiation resistance at this point would be transformed to 0.36 ohm. However, the radiation efficiency is improved over that of single tuning by virtue of the decreased total ground resistance resulting from reductions in the density of ground currents of each collecting point. Thus the ratio of radiation resistance to total resistance is effectively increased by multiple tuning.

If N represents the number of multiple-tuning down leads carrying equal currents, the new radiation resistance R_ττ is related to that for single tuning R_τ by the equation

The total antenna resistance with multiple tuning R_ττ is, in general, less than R_tN², and from this R_ττ/Rtt > R_τ/R_t.

In addition to improving the radiation efficiency, multiple tuning also provides a more convenient input impedance at the feed point and increases bandwidth.

The full explanation of multiple tuning is much more complex than indicated here, where only the basic principle is explained. Some of the modifying factors are as follows: The effective capacitance of the flat-top is divided among the down leads so that each requires a larger tuning inductance than in the case of single tuning. The resistances of these tuning inductances are in series with the radiation resistance of each down lead, as are the conductor and insulation resistance components of loss, and these are also transferred to the feed point through the factor N₂. For the same coil Q, the larger inductance required for multiple tuning introduces a proportionately larger resistance per coil. However, the total loss in the inductances with multiple tuning is also less than in the case of single tuning, assuming equal Q's for all the inductances.

Multiple tuning is best adapted to operation on a single frequency. Where it is necessary to tune the antenna to several operating frequencies from time to time, single tuning is the most convenient.