Antennas for Receiving

Author: Edmund A. Laport

For receiving applications, certain points of design covered in the foregoing discussion will not apply. There will of course be no problems of potential gradients and high-voltage insulation. Conductor sizes and insulation will be determined almost solely by bandwidth and mechanical considerations.

In some locations precipitation static is serious at times. Sandstorms, dry snow, and wind-blown fog are well-known causes of precipitation static interference. A substantial reduction in such noise can be realized by embedding all metallic parts of the antenna and feeder system in a low-loss dielectric so that the charged particles hitting the antenna cannot discharge directly to the metal. The antenna and feeder wires can be rubber- or plastic-covered and the metal of insulators and fittings completely incased in paraffin, gutta-percha, polystyrene, or other conveniently cast material. With such precautions to prevent exposure of the metal of the antenna and feeder to the flying charged particles, several decibels reduction in noise level can be realized during conditions of precipitation static.

It must also be recalled that, in the receiving case, energy is flowing from the antenna to the receiver. This requires that the feeder be correctly terminated in an impedance match at the receiver end so that as much as possible of the received energy will be delivered to the receiver input.

Several receivers can be operated from a single antenna and feeder system if precautions are taken for proper impedance match of the feeder over the full range of frequencies to be received. Decoupling resistors can be used in the inputs to the several receivers provided that the power losses they introduce do not require extreme receiver gain and give rise to intolerable receiver-tube noise. In many regions of the world, atmospheric noise levels are at all times high enough so that one can never use all the intrinsic gain in a modern communication receiver. In such regions, decoupling resistors in the receiver inputs have no detrimental effects on the realizable signal-to-noise ratios. Figure 3.22 shows a method of using three receivers with balanced 200-ohm inputs on a 600-ohm feeder termination.

FIG. 3.22. Example of feeder impedance matching using three receivers.