FM Direction Finder | Electronics Seminar Topic
FM Direction Finder
FM Direction Finder has wide applications like navigation of
ships, aircrafts, missiles, radar, tracking satellites and other astronomical
radio sources. In the field of communications, certain requirements could be to
cross check the intensity, direction and positioning of transmitters employed
in a cluster of a GSM network .
It might be used to locate the radio sources
temporarily jamming any communication network. It is also used extensively in
spectrum management for checking the growth of transmitters in specific regions
like radio astronomical observatories, etc.
Several radio direction finders have been built in the past.
Different techniques have been employed in different instruments. These
instruments could be broadly classified under vector-type or scalar-type of
radio direction finders. The vector-types require both amplitude and phase
information of the electromagnetic field at the antenna aperture, while the
scalar-types require only amplitude information. In general, these instruments
employ these measurements at various points in the three dimensional space.
The direction of radio signal is determined by applying some
algorithms on these measured values. The direction finding could be either online or offline or
both. The detected object may be a radio source or a passive device illuminated
by electromagnetic radiations (like in RADAR).
Majority of these instruments
operate with small or medium bandwidths. With the increase of spectrum
allotment to the communication channels and their growing numbers, the
requirement of band coverage in radio direction finding has increased. Attempts
have been made in recent years to broaden the frequency coverage.
In certain instruments based on the principle of radio
interferometer, the intensity of the signal plays an important role in phase
detection. If the signal to noise ratio is weak, the phase information might
not be recovered correctly, especially when the source of the signal is
amplitude modulated.
On the other hand, the scalar-type radio direction finders
might not be significantly accurate in pointing the direction, but might work
at relatively low signal to noise ratios, and could also cover very wide range
of frequencies. With the development of algorithms for categorically
analyzing the terrestrial spectrum, a requirement from the
low frequency radio astronomy community grew for having a portable ultra wide
band radio direction finder. This requirement was for cross verification of the
direction of narrow and broad band radio sources. Based on the requirement, a
scalar type of online radio direction finder was designed
