PART B: PROPERTIES
Have you ever rotated a portable radio to find the best position to hear the station? If so, you
are an experienced radio direction finder. The equipment was not very complex, but it met
your requirements to get the results you wanted.
If you had turned the radio on full circle you would have heard two maximum and two minimum
volumes. The reason for this is quite simple. The antenna in the portable radio is usually a
bar- type antenna which is highly directional.
The pencil can be used to demonstrate the directional properties of the bar antenna. Hold a
pencil in front of yourself with the pencil parallel to the ground. (See Figure 1-11).
Figure 1-11. Pencil parallel to ground.
Keeping the pencil parallel to the ground, rotate it one full circle. Note that you see each end
once and each side once, or put another way, the pencil is sideways to your twice and end on
twice. In a bar antenna the sides give maximum reception (signal is loudest) and the ends
give minimum reception. The minimum reception points are called NULL points and are
important to direction finders because they are easier to hear and locate accurately than are
the maximum signals.
The response pattern (a diagram showing the maximum and minimum areas of reception) for
a bar antenna is a figure eight. The diagram in figure 1-12 indicates that the end points of the
bar receive almost no signal, while the sides receive maximum signal. Since we are using our
ears as aural indicators for signal reception, and since a minimum signal or NULL is easier to
locate accurately than a maximum signal, we would use aural null indication to locate the
transmitter. Of course the problem for a portable radio is locating the best reception rather
than the least, but the principle is the same.
1-13
IT0302
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