Broadcast Antennas on Buildings

Author: Edmund A. Laport

When a broadcast antenna is to be placed on top of a building, as occasionally happens with low-power stations, there are special problems of feeding and grounding. The radiator is usually electrically short, that is, much less than one-quarter wavelength high. Its input impedance is therefore low in resistance and may have a substantial reactance, and tuning is accomplished with a series inductance. It is assumed that in any event a single insulated steel tower or mast would be used for the radiator.

FIG. 2.31. Combined networks and feeder circuits.

The ground terminal must be constructed on the roof. If the roof is of sufficient area, a symmetrical counterpoise of 20 or more radial wires (insulated from all supports with light-duty insulators) may be a satisfactory ground. Since these conditions are seldom present, the system shown in Fig. 2.32 may be used. This is an adaptation of the principle of the Brown very-high-frequency ground-screen antenna. From two to four horizontal radial wires are centered under the radiator, each having an electrical length of one-quarter wavelength when loaded with inductance as shown. Each wire, when tuned naturally or with inductance, brings a virtual zero-potential point at the center which is taken as the ground point for the system. The full wire length can sometimes be used by allowing the excess length beyond the roof limits to hang down along the side of the building, properly secured and insulated.

The ground radials can be tuned by connecting two opposite wires in series and making symmetrical tuning adjustments in each inductance. The wires are disconnected from the mid-point during the tuning. A power oscillator of the correct working frequency is then coupled inductively to the two wires. Tuning is done by maximum current in the two wires, using a center-connected ammeter or a wavemeter near one of the coils away from the oscillator to avoid direct pickup from the oscillator.

FIG. 2.32. Elementary grounding system for roof antenna.

This tunes two wires to resonance as a half-wave system. This is repeated for the other two wires. Then they are connected permanently to a common mid-point. No loading inductors are necessary in using full-length wires, in which case the wire length should be about 0.24 wavelength to the end insulator.

The antenna is then tuned, using an impedance bridge if one is available, by connecting the correct amount of tuning inductance between the base of the radiator and the ground point.

The best feeder to use structurally in such a situation is a flexible coaxial cable of 50 to 60 ohms characteristic impedance. The sheath of the cable is terminated by connecting it to the ground point. The inner conductor, extended beyond the sheath by a flexible extension and clip, is attached to some point on the antenna tuning inductance, where it will present the impedance needed for a satisfactory matching of impedance of the cable. For a perfect match, the feeder clip and the clip attached to the ground point both require adjustment, but the match will usually be very nearly correct if only the feeder clip is moved after the antenna is originally tuned to resonance. The feeder clip will be near the lower end of the antenna tuning coil and will have to be adjusted carefully by fractions of turns.

The antenna tuning inductor may be housed, or if properly designed for the purpose, it may be in the open.