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MG MGB Technical - Period radio/aerial help please!

I have a period Motorola 1070X installed in my 67 mgb roadster. It is of the switchable polarity type. I have a two-part extendable steel aerial installed which does the trick when in a strong signal area such as London. However, in areas where the signal isn't extremely strong, you need to turn the radio volume almost to full before you can hear it. If however you go and touch one finger onto the aerial, the signal is boosted ten-fold. For information, if you then earth the clamp of the aerial to the body of the car, the signal dissapears completely! Can anybody suggest any solution that will help me get the best out of my radio? Is it a problem with the radio or the aerial? I have tried three different types of aerial and they all do the same.
Thanks in advance.
Martin Port

Martin,

I work in a large company with antenna engineers on staff, and I posed your question, since I had wondered about this in the past.

When you ground (earth) the antenna, the RF energy is shorted to ground. When you touch the antenna with your hand, you are adding a capacitance between the antenna and its ground. You can experiment with adding capacitances between the aerial center conductor(as near the antenna as possible), until you have achieved desired results. A tunable capacitor, such as an air capacitor, may provide the greatest convenince for determining the optimum capacitance. These capacitors have interleaved metal plates, one set of which is turned while the other set is stationary. These were most common in old radios, but are fairly hard to find, these days. But, you get the idea.
Fred Doyen

Many older car radios have trimmer capacitors accessible through holes in the back to tune the antenna to the radio and optimize reception. Don't know if your radio has one, but it doesn't hurt to look. You might also take it to an automobile radio specialist, and let him take a shot at it.
R. L Carleen

Martin,

Here's a link describing what R.L. suggested:

http://www.radiorepair.us/antennas.htm

However, you described your symptoms as gaining a 10-fold improvement in signal when touching your aerial, and a trimmer capacitor provides only limited improvement (it trims a capacitance already inherent in the system). Try the adjustment and see what happens, making sure you don't adjust it too far (note the caution in the link).
Fred Doyen

The warning is fine about not turning the screw anti clock but the effect of the trimmer is only apparent when the screw is turned fully clockwise. Tune the radio to a station at the lower end of the band and then move the trimmer screw clockwise until max volume is achieved. This will happen with the screw well tightened as the capacitiance will be greatly affected by the pressure between dielectric layers.
Iain MacKintosh

This thread was discussed between 20/06/2005 and 21/06/2005

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