Twisted Pair Fed Antenna

Twisted Pair Fed Antenna 

A very effective one-band antenna for transmitting and receiving consists of a half wave flat top with a twisted pair feeder. The impedance of a twisted pair ranges from 80 to 175 ohms, depending upon the spacing between the conductors and the diameter of the wire. This impedance is low enough so that the feeders can be connected directly into the center of the antenna. In practice, the last few inches of the feeders are fanned-out into a small triangle, as shown in Fig.1

The feeder can be any length, and it can be carried around corners of buildings, through walls and along picture mouldings. Nearby objects have very little effect on the efficiency of the feeders because of their close spacing and the large number of transpositions or twists along the feed line. The losses in the feed line are exceptionally low, largely because the small spacing between the wires causes the line to have a very low characteristic impedance. 

This means that for a given amount of power the voltage between the two wires is very low, thus insulation and dielectric losses can be held to a minimum. Ordinary stranded electrical cord should be avoided because of high losses, but single conductor AWG 12 (2 mm)  to AWG 18 (1 mm) polyethylene twisted pair wire is satisfactory. 

Harmonic operation is not recommended because the line is no longer non-resonant for such operation, and standing waves will cause high RF voltage across some portions of the line. If much power input is used, the line insulation will break down and burn. Operation on the second harmonic is possible, but the efficiency of the line then drops approximately 50%. This antenna is excellent for receiving because of reduction in noise pickup. Two of these antennas placed at right angles to each other will provide transmission or reception in all directions.