Ferrite rod antennas can be made extremely small, as this life-size photo clearly illustrates.
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
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
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