17 Antennas (AM/FM DXing)
Description
This article is from the Shortwave radio FAQ, by Ralph Brandi rbrandi@lucent.com with numerous contributions by others.
17 Antennas (AM/FM DXing)
For mediumwave reception, most receivers have a short internal ferrite
rod. This will provide acceptable signals for high-powered distant
stations. Ferrite rods are quite directional, and the radio can thus be
turned to null out strong interfering signals, or to improve reception
of the desired signal.
For more advanced DXing, external antennas offer certain advantages. The
most common external antenna is a simple random wire, 15m (50 feet) or
more run out the window and then as high as possible (up in a tree, for
example). The wire can be connected to the external antenna terminal.
If none exists, you can open up the radio and wrap the wire a few turns
around the ferrite rod inside. It is also possible, although less
desirable, to simply wrap the wire around the entire radio. If the
radio has a terminal marked "ground" or "GND," another wire can be run
from this terminal to a copper rod driven a meter/a few feet into the
earth.
One problem with a random wire antenna for mediumwave work is it's
inability to reject strong local signals. Most receivers today lack the
dynamic range to effectively deal with the extremely strong signals from
a local mediumwave broadcaster as picked up by a random wire antenna.
Thus, some sort of tuned antenna is best for all but the most isolated,
rural locations.
The most popular antenna for mediumwave DX today is called a `loop'
antenna, and can be either of two types: ferrite rod or air-core wound
wire loop. These antennas are small, 25-100 cm (1-3 feet) in diameter,
and sit on the DXers desk or shack table where they can be easily turned
by hand for optimum peak or null of a signal. Each design works with a
tuned circuit before feeding the signal into your receiver, and usually
this circuit includes a small powered amplifier. Generally speaking, the
longer the ferrite rod, or the larger the diameter of the aircore loop,
(to a point), the sharper the null of the antenna. 45 to 55 cm (18 to
22 inches) would be optimum for a ferrite rod antenna.
Air-core loops need to be made by hand, as there are none on the market.
Ferrite loops, however, are available commercially from at least two
manufacturers; Palomar Engineers and Radio West. Unfortunately, these
two antennas do not have very long ferrite elements; and reviews of
their performance in the mediumwave press tends to be mixed. Ideally,
one would build their own antenna, or try to find either a used, older
Radio West loop or Space Magnet antenna, both pre-1980. Plans for
building all sorts of mediumwave loop antennas are available through
National Radio Club publications. The address is found later in this
FAQ.
A more advanced antenna is the "beverage" antenna. This is a length of
wire 300 m (1000 feet) or more, with extremely high gain and narrow-beam
directional characteristics. It is usually, but not always, terminated
at the far end with a 450 ohm resistor connected to a metal stake driven
into the ground. It should be pointed in the direction of the desired
station. The beverage antenna can, under good conditions, be used for
transatlantic and transpacific DX.
For VHF-FM, the important factor is height. The higher one can place an
antenna, the better reception will be. A multielement Yagi antenna,
which can be found in Radio Shack or similar stores, will often produce
excellent reception. Since a yagi is quite directional, the use of a
rotor is essential for reception of stations in different directions.
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