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Vintage Radio

Ever wondered about those mysterious antenna coils used in vintage radio receivers, or about those old-style loop antennas? This article unravels some of the mysteries.

By Rodney Champness

Click for larger image
Ferrite rod antennas can be made extremely small, as this life-size photo clearly illustrates.
Click for larger image
This small AM/FM receiver uses the PC board shown above and is intended for use in strong signal areas only. It's miniature ferrite rod antenna means that its AM performance is pathetic compared to larger sets.

Broadcast-band radio signals are radiated from tall masts that are fed from nearby transmitters. One way to receive these radio signals is to put up an external wire antenna that’s as high and as long as possible. One end of this antenna wire is brought down to the receiver and attached to the aerial terminal, while an earth wire is connected to the earth terminal (valve radio chassis were often not earthed back through the mains).

This type of "long-wire" antenna system largely responds to the electric component of the radio waves.

Another way to intercept these signals is to use a loop antenna. They vary widely in size, ranging from antennas consisting of several turns of wire which form a coil about one metre in diameter to very small, ferrite-cored loopstick antennas. Loop antennas couple to the magnetic component of the radio waves.

Both loop and long-wire antennas have been used since radio began.

Long wire antennas

Many vintage radio restorers have probably been puzzled as to why some receivers need only a small antenna to perform well, while others need a large antenna to give the same result.

The simple answer is that some sets require large antennas because they are either low-performance types or because they have faults which seriously degrade their performance. However, if we assume that a set is well-designed and that its sensitivity from the input of the converter onwards is good, then the only component that should further influence performance is the antenna coil.

By necessity, antennas are something of a compromise between size and performance. The best antenna for a broadcast radio is theoretically a quarter-wave unit fed against earth. However, this is hardly practical as at 531kHz, a quarter-wave antenna would be 141 metres long.

In fact, our so-called "long wire" antennas are still short when compared with a quarter-wave antenna at 1602kHz, as the latter is 47 metres long. This means that various techniques must be used to increase the effectiveness of wire antennas that are much shorter than the optimum length.

One simple method (as used in my crystal set in the April 2007 issue) is to employ an adjustable coil in series with the antenna. This adjustable coil resonates the antenna to the frequency being received and is commonly called a "base-loaded antenna" system. It worked well in my crystal set which was able to receive stations up to 300km away at comfortable headphone volume.

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