Direction Finding with "Ultra Shorts"
February 1935 Short Wave Craft

February 1935 Short Wave Craft [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Short Wave Craft, published 1930 - 1936. All copyrights hereby acknowledged.

Anyone other than a complete novice (Ham pun intended) at radio and antenna work is familiar with the basics types of directional antennas presented here. However, the universe of people knowledgeable about such things was very small in 1935, when this article appeared in Short Wave Craft magazine. An effort to educate operators and designers was really just beginning. The ½-wave dipoles shown here for the transmitter are somewhat directional at 2 dBi, and full-wave square loop antennas for receiving have a gain of around 3 dBi. However, the most significant direction-finding feature of a loop antenna is the comparatively deep null in its radiation pattern in the plane of the wire. That explains why direction-finding equipment, such as those on aircraft, typically use a loop antenna. When maximum precision is needed, Yagi, log periodic, parabolic dish, or other high gain antennas will be used.

Direction Finding with "Ultra Shorts"

Recently in this page, we mentioned some experiments which appeared in Wireless World, regarding the reception of ultra-short waves with loop aerials. In this resume, we pointed out that much work remained to be done along the lines of development of aerials and radiating systems for ultra-short wave transmission and reception.

In a more recent issue of the above publication, some further details have been presented, including an explanation for the apparent discrepancy in the direction of transmission of the very high frequency waves.

It will be remembered that with a horizontally polarized transmitter, it was possible to receive signals with a vertically polarized loop, but with its plane at right-angles to the direction of the transmitter.

In the later article, it is shown how this is possible. If the loop is pointing toward the transmitter, as at A in the accompanying sketch, a signal will be picked up in the loop. Now, if the loop is maintained in the same position, but the transmitter polarized in a horizontal plane, the field is everywhere at right angles to the winding of the loop, and therefore produces no EMF in it; this position gives minimum signal strength. But if the loop is set as at C, the electrical field will be parallel to its top and bottom, and will induce in these two sides EMF's which are opposed to each other around the loop. Since all parts of the loop are now equidistant from the transmitter, the two EMF's should have the same magnitude and phase, so that the resultant EMF around the loop would be zero. But, if for any reason, the field is non-uniform in the vertical plane so that the EMF, say, in the top of the loop is greater than that in the bottom, there will be a resultant EMF around the circuit.

In practice, the presence of the receiver, just below the loop will probably cause sufficient distortion of the field, while there is also the effect of the earth to be considered, so that reception in this way becomes possible.

Posted August 4, 2017