LENGTH OF AERIALS

Wanganui Chronicle, Volume 74, Issue 244, 15 October 1931, Page 10

LENGTH OF AERIALS

MERITS OF SMALL ANTENNAE. RELATIONSHIP WITH FREQUENCY. It is very commonly believed that a targe outdoor aerial is setter than a small one, and many people, particularly those comparatively new to radio, go to great trouble and much expense to set up a really large and elaborate one, states a writer in an overseas journal. It is obvious that in one sense a large aerial is bound to pick up more power from the ether than a small one, out what we have to consider is whether that power, when so picked up, can be properly applied in the desired way. The power comes in in the form of nigh-frequency waves, and therefore the relationsnip betwen the characteristics of the aerial and the frequency of the waves becomes extremely important, lu view of this relationship, it is quite possible, and, in fact, it often hajjpens, that a long aerial is actually less efficient than a short one for the particular wavelengtn which is being received. Apart from the question of sensitivity —which is not really so important nowadays, owing to the ease with which any lack of aerial sensitivity can be made up by means of high-frequency amplification—there is the much more important question of selectivity, and, in point of fact, when working on ordinary broadcast wavelengt, you will find that a standard 100 ft. aerial often requires drastic cutting down. Although you are so often advised to have a flat top or roof to the aerial, this flat top may do more harm than good. You are also commonly advised to have high aerials, and with this particular part of the popular advice I entirely agree—but not with that about having a flat top. This is because the signals which are received in tho top of the aerial may be out of phase with those on the vertical oi down portion of the aerial.

A good deal depends on tho length of the horizontal portion in relation to tho vertical portion, but it has been shown that, if the horizontal part is about two or three times the length of the vertical part, then it is rctually better to cut off the top part altogether, leaving the vertical part alone. In other words, the effect of the horizontal top is actually in certain circumstances neutralising the effect of the vertical part and you would be better off with the latter alone. Personally, I am always in favour of a vertical aerial strung as high as possil le without worrying about the horizontal part at