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.
FM broadcast radio sends music and voice with higher fidelity than AM radio. In frequency modulation, amplitude variation at the microphone causes the transmitter frequency to fluctuate. Because the audio signal modulates the frequency and not the amplitude, an FM signal is not subject to static and interference in the same way as AM signals. Due to its need for a wider bandwidth, FM is transmitted in the Very High Frequency (VHF, 30 MHz to 300 MHz) radio spectrum. VHF radio waves act more like light, traveling in straight lines; hence the reception range is generally limited to about 50-100 miles. During unusual upper atmospheric conditions, FM signals are occasionally reflected back towards the Earth by the ionosphere, resulting in long distance FM reception. FM receivers are subject to the capture effect, which causes the radio to only receive the strongest signal when multiple signals appear on the same frequency. FM receivers are relatively immune to lightning and spark interference.
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