called a goniometer is used to electrically rotate the antenna. The leads from the antenna are
connected to coils in the goniometer, and another coil is rotated in order to sample each
portion of the circle in turn. Some of our VHF/UHF DF systems use a microprocessor or
minicomputer to electronically measure the time of arrival instead of rotating the antenna.
The Radio Receiver. Your portable radio, whether it be a small Amplitude Modulated (AM)
receiver or the larger, and costlier, transoceanic AM, Frequency Modulated (FM) shortwave
type, is designed to receive broadcasts within certain wavelength imitations. These limits are
called the frequency range of the receiver. Which receiver will be used with your DF system
depends upon the frequency range of the transmitter in which you are interested. Another
consideration is the desired mobility (how fast it can be moved without damage) and
transportability (what does it take to move it) of the DF system. The DF receiver must have
good stability (stays on frequency), sensitivity (you can hear and target), selectivity (you can
tune out the signals you don't want), and a Beat Frequency Oscillator (BFO) which is used to
give a steady tone for ease of direction finding/AM signals.
The input and output circuits of an ordinary communications receiver usually require
modification for use with direction finders. The portable radio, for example, has the correct
frequency range for the targets desired. It is designed to be hand carried, relatively stable,
has selectivity, and is as sensitive as the price allows. It does not have a BFO, therefore, it
does not meet our requirements. If we were to use it with our systems we would have to add
a BFO.
The Method of Indication. By now you are familiar with aural null indication. The most
common system other than aural null is visual indication. This employs a cathode ray tube or
some type of meter which indicates the null or maximum signal for your interpretation. The
indicator is usually associated with some sort of a measuring device so the operator can
record what he/she sees or hears.
A useful direction-finding system must have these four components. The targets, and the
tactical situation will indicate what combination and size of equipment can or should be used.
PART B: DEFINITIONS
Many of the terms used in discussing direction finding are unique to direction finding or are
used in a different sense. In order to understand how these terms relate to the discussion
that follows, you will need to know exactly what is meant by each of them. These terms relate
primarily to a discussion of antenna theory.
(a)
Loop. Originally a loop antenna was a circle of wire. Modern loops come in a
variety of shapes but essentially and electrically they are the same as the original circle. It
might be easier to think of the circle as a square, with vertical and horizontal sides, since the
action of the radio wave on the antenna is discussed in terms of how it affects the horizontal
and vertical members of the loop.
(b)
Conductor. The wire or similar material used to pass current from one part of a
circuit to another.
IT0302
2-4
Previous Page
