Shunt-fed Radiators

Author: Edmund A. Laport

A grounded vertical radiator may be shunt-fed as shown in Fig. 2.8. With shunt feed () the radiator is grounded at its base, and the system is excited at the point where the shunt-feed wire is connected. By using a sloping wire, as shown in B of Fig. 2.8, a number of wire lengths and tapping points are available. The feed wire acts as a transformer which is adjustable over a certain range. In typical use, the feed wire is adjusted to bring a predetermined value of resistance at the input, which may be the value necessary to terminate a given transmission-line characteristic impedance. There is always an inductively reactive component of impedance present also, which is neutralized by a series capacitance. Any radiator can be shunt-fed in this way, provided that an adjustment can be found that will give a desired input impedance, or more usually it is necessary only to provide a given resistance component.

A shunt-fed vertical radiator does not use base insulators and therefore does not require any isolation circuits for tower-lighting circuits or for any top-mounted very-high-frequency antennas that may be present. By virtue of its direct grounding, it is somewhat less vulnerable to lightning damage than a series-fed radiator. However, with a direct lightning hit on the tower, destructive potentials are sometimes transferred to the input to the feed wire, so that safety from lightning damage is not as complete as might be supposed.

A given vertical radiator, arranged for series feed, will have a series impedance at its base, which we shall designate as Z_ab at a specified frequency. When this same radiator is grounded and fed with a shunt-feed connection, it is adjusted to give an input impedance Z_as. The shunt feeder thus acts as a transformer which converts Z_ab to Z_as. An equivalent circuit of such a transformer can be derived as a network, in the form of a T or L.

FIG. 2.8. Shunt-fed vertical radiators.

While the transformation ratio is known, the phase shift between the current at the feed point and the current in the radiator is indeterminate.

The optimum application for shunt feed is with a vertical quarter-wave radiator working into a low-impedance feeder. When the radiators are considerably more or less than 90 degrees high, the feed-wire adjustment requires that the loop formed between the feed wire, the radiator, and ground become relatively large, and this loop becomes a considerable radiator itself, modifying the intrinsic radiation properties and the current distribution of the radiator below the tapping point.

The quarter-wave shunt-fed radiator is the unbalanced analogy to the delta feed so commonly used for horizontal dipoles at high frequencies. In the latter system, the shunt feed is balanced. In both cases, the transforming action of the shunt feed is relatively small, and the reactive component introduced by the feed wire is not excessive, giving input impedances of relatively high power factor. Furthermore, the feed loop is not sufficiently large to cause excessive radiation, though there is some.

The amount of parasitic radiation that can be tolerated from the feed loop is something the designer must decide.

In broadcast applications, radiation from the feed loop causes the vertical radiation pattern for a single radiator to be distorted and eliminates the cone of silence directly above the radiator. High-angle radiation is therefore increased in all vertical planes, especially at the very high angles where, with series feed, the field strength would be zero.

FIG. 2.9. Comparison of measured vertical patterns from series-fed and sloping-wire shunt-fed radiator of same height.

Figure 2.9 shows the measured vertical-plane patterns for a shunt-fed vertical half-wave radiator in the plane of the feed wire and in the plane normal to the feed wire. For comparison, the normal series-feed pattern is also shown by the dotted line. Figure 2.10 shows the measured current distribution along the vertical radiator which gave these radiation patterns. The distortion of the current distribution is rather extreme owing in part to the shunt feed and in part to the structural taper for this particular tower, which was self-supporting and tapered from 40 feet per side at the base to 1.5 feet per side at the top, 800 feet above ground. These data were obtained by model measurements by Brown and Epstein of the RCA Laboratories (not published), simulating an actual tower under study. They found that the area of the feed loop for this antenna could be greatly reduced by running the feed wire up the center of the tower, as shown in Fig. 2.8A, instead of using the usual external sloping wire. This made a considerable improvement in the measured vertical radiation patterns, proving that radiation from the sloping feed-wire loop was an important modifying factor in the entire radiating system. The current distribution for these two types is illustrated in Figs. 2.11 and 2.12. Running the feed wire up the center of the tower seems very desirable when shunt-fed radiators are used in a directive array requiring moderate or high degrees of radiation suppression at some angles.

Shunt-fed antifading antennas introduce three factors that require special attention in design. One is the modification of the current distribution in the radiator below the feed point, which causes the current at the base of the radiator proper to be many times that present with series feed.

FIG. 2.10. Measured current distribution on vertical radiator shunt-fed and produmug the patterns shown in Fig. 2.9 (curves 2 and 3).

This requires attention to reducing ground-system resistance as much as possible to maintain high radiation efficiency. This can be done by using a larger number of longer ground wires. The other point is the appearance of relatively high potentials at the feed point due to its high reactance when adjusted for the usual 50- to 70-ohm resistance to terminate a coaxial feeder. The high potential encountered at the feed point is the consequence of the feed current flowing into the high reactance of the input impedance. Precautions must be included to accommodate this condition, which in itself does not present a very difficult problem. The same factors that give a high input reactance will also contribute to selectivity. Attention must therefore be directed to this aspect of the application whenever bandwidth has to be considered. The shunt-fed radiator is a system which appears to be more simple than it actually is.

Fig. 2.11: unbeschriftet

FIG. 2.12. Measured radiator current distribution for system producing the pattern of Fig. 2.11.

It must therefore be applied with caution in exacting circumstances. For instance, the use of the sloping-wire feed on an anti-fading radiator nullifies some of the important properties for which such a radiator is used. As an element in a directive array, it is cumbersome to design because all available reference data on which the performance of a directive array is predicted are for series feed. Shunt feed introduces the impedance-transformer action, which is difficult to predict, especially when mutual impedances are taken into account. The sloping-wire feed, in addition to modifying the vertical pattern of each radiator, will introduce mutual impedances between feed wires which will, in general, be indeterminate during the design stage of the work. The coupling circuits must therefore be designed after measurements have been made on the final radiator system. There is also the complication that the phase and amplitude relations of the currents at the feed points will not be those prevailing in the radiators, and it is necessary to monitor radiator currents on the radiators above the tapping point. The design of a directive array of shunt-fed elements will usually require an enormous expenditure of engineering effort that may offset any structural economy. However, these remarks should not discourage the application of the shunt-fed radiator in cases where its simplicity and economic advantages can be realized and where the detrimental factors discussed are of minor importance.