424 York

Amateur Radio W3BAH

Jenkintown, Pa. 19046 • Phone 215-887-7350

Edgar Wagner G3BfD 5( Feroncroft Avenue London, 3, England

The Front-to-Back Ratio of an Automobile

Have you ever checked the antenna pattern of your mobile antenna?

We are, of course* accustomed to thinking in terms of the back-to-frout ratio of beams, but how about the directional properties of the automobile in mobile operation?

Some time ago I did some work on this subject willi a view to learning something about the effect of different positions for an antenna on a car.

My first experiments were carried out on a station wagon with a nearly flat metal



roof. 1 deliberately placed the antenna base on a bracket mounted at the left hand rear comer of the roof,

I soon founcl that the major lobe was in the direction diagonally across the roof of the car as shown in Fig. 1, Thus it appeared that tlie roof was acting as one part of a dipole (see Fig, 2) rather than as a ground plane.

On 10 metres this was very logical since the diagonal length of the roof of the car was roughly lA wave length on 10 metres.

On 15 and 20 metres a similar radiation pattern was found although, of course, I he roof of the car was too short for Y4 wave length on 20 metres. Probably the SWR was quite high but I never measured this, because owing to the very short feeder used in mobile installations I do not regard the SWR as very important. Provided the SWR is not so bad as to prevent the antenna loading, the actual SWR is felt to be of little importance. The losses on that length of feeder are small.

For some time 1 continued to use my mobile installation with the antenna placed unsymmetrically in this way and found a back to front ratio of about 3 dB on 10, 15 and 20 metres, This small beam effect was at first welcomed as it facilitated mak-

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