Radio Direction Finding
By Ike Mowete
Monday, April 11, 2005
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Ever since Marconi and Hertz suggested through
demonstrations carried out in 1900, that directional
antennas may be utilised for—‘focusing’
radio waves in any desired direction, the development of equipment
for use in the location of sources of electromagnetic waves
and radio signals has engaged the attention of inventors and
scientists.
Marconi, in 1912, gave another demonstration, this time of
a ship-mounted system, capable of taking bearings from land-based
transmitters and subsequently obtained a patent for the equipment,
whose capabilities enabled the specification of positions
of ships through triangulation. Military use of Radio Direction
Finding (RDF) began shortly thereafter during the First World
War, when the navy capitalised on the advantage offered by
RDF to locate enemy ships by listening to the radio traffic
to and from the ships. After that war, the development of
RDF equipment continued and it received a boost during the
Second World War when the miniaturisation of electronic components
facilitated the development of sophisticated direction finders,
including those utilised for tracking frequency hopping transmitters,
characteristic of the military communications environment.
Although those techniques were highly ingenious as well, they
were built with the use of the same basic principles that
informed the development of earlier models. In the main, the
foundation of radio direction finding has its basis in the
fundamental properties of electromagnetic waves, namely: that
the field vectors are perpendicular to the direction of propagation
and that phase surfaces are also perpendicular to the direction
of propagation. When regarded in a direction towards the Earth’s
surface, radio waves appear to spread out radially and at
far distances from the transmitter they become nearly flat,
that is plane waves. Consequently, a number of antenna systems
can determine the orientation of the wave fronts, for if a
line is drawn at right angles to the waves, it will point
in the direction of the transmitting antenna.
This observation implies that any direction finding process
utilises one of two basic measurement methods. In the first
of these, favoured by’polarisation direction finders,
it is the direction of either the electric or magnetic field
vectors (or both) that is measured. Phase direction finders,
on the other hand, utilise measurements of the orientation
of the equiphase surfaces or, if elevation is not of interest,
lines of equal phase. Polarisation direction finders, an example
of which is the classical rotating-loop direction finder,
employ dipole or loop antennas. Finders of the variety find
application in situations where only small antennas can be
accommodated. Directional information is obtained by phase
direction finders through an evaluation of the spatial position
of the surfaces (or lines) of equal phase. Use is made in
this case, of one of two methods, the first being that in
which partial waves are coupled at various points of the antenna,
to be combined at one point to form a ‘sum signal’.
This sum signal assumes its maximum value, when the antenna
is that for which phase differences between the partial waves
is a minimum. In the second method, samples taken at various
points in the field are applied as input to sequential or
parallel evaluating circuits, which then determine the bearing
by executing specified mathematical algorithms.
Direction finding, in its simplest form, consists of evaluating
the received voltage (field strength) of a mechanically rotated
directional antenna, with reference to the direction. When
this method is utilised, bearing derives from the characteristic
of the received field strength, described in terms of antenna
rotation angle. Any arriving wave occasions a measured field
strength that furnishes the directional pattern of the radiating
antenna and the pattern position, relative to the receive
antenna’s rotation angle corresponds, to the measured
bearing. By nature, this type of direction finder is a phase
direction finder because the directivity of the receiving
antenna is obtained through a superimposition of partial waves,
whose phase differences are functions of the angle of incidence.
In some cases (spinning-wheel direction finder) the directional
antenna is permanently rotated through of an electric motor
and the received signal strength is displayed graphically,
as a function of the rotation angle.
Although variations of this method of direction finding can
claim a number of advantages, (high sensitivity, simplicity,
use of same antenna for monitoring and direction finding)
drawbacks, which owe to the restriction to the field of view,
inevitably imposed by directivity and speed of rotation, include
the fact that probability of intercept is inversely proportional
to directivity and the failure that occurs in the case of
signals of short duration. It should be apparent from the
foregoing, that every direction finding system will have the
following component parts, as displayed in the illustration
below: antenna system, DF converter, evaluation unit, and
display unit.
Component parts of the Direction Finder (courtesy www.rohde-schwarz.com)
Some configurations make provisions for the inclusion of GPS,
compass, remote-control units and antenna control units, to
facilitate the determination of the finder’s own coordinates
or orientation. Direction finding speed is limited by the
number of receive sections (‘H’ in the illustration),
which specifies the number simultaneously measured antenna
outputs. If maximum speed is to be achieved, then the bearing
must be obtained in a single step and for an unambiguous direction
finding over the extent of the azimuth range, a minimum of
three antenna outputs are required. Carrier frequency antenna
signals are converted by the DF converter to a fixed intermediate
frequency (IF) and because they must be made with equal phase
and amplitude distributions in all receive sections, use must
be made of a common synthesiser. Receive sections in most
direction finders are, prior to actual deployment, calibrated
with the aid of some test generator, to verify the equal amplitude
and phase requirement. Bearing is determined by the evaluation,
which utilises amplitude and phase information supplied by
IF signals.
In spite of the drawbacks of the direction finding methods
that utilise mechanically rotated directional antennas, these
variety of DF equipment are still in present-day use. This
is because the advantages claimed by other methods can be
obtained in part, with a significantly higher outlay. Indeed,
in the microwave regime, the mechanical direction finding
method usually turns out to be the’‘only justifiable
compromise’ in the necessary trade-off between gain,
low noise and outlay.
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