There are certain navigational aids which require to transmit their signals only in a particular direction. For example, Instrument Landing System (ILS) need to transmit in the direction of runway only. Directional antennas serve this purpose by focusing radio waves in a particular direction.
Elements of a Directional Antenna.
If you look at a Radar Antenna, you would see a semicircular cup shaped reflector attached behind the transmitter. These reflectors are called as parasitic elements which help in achieving directivity. Metallic reflectors are 5% longer and placed behind the ariel. In general terms, there are certain principles to be followed for constructing reflectors. The distance between ariel and reflector must be equal to quarter of the wavelength of wave.
Characteristics of a Yagi Antenna
In order to improve directivity, at times parasitic elements are placed ahead of the transmitter also. Such antennas are called as Yagi antenna. These antennas use additional parasitic elements in front of the transmitter also for directivity. The flip side of Yagi antenna is creation of unwanted side lobes. Side lobes are unwanted signals transmitted by Yagi antennas. In order to make use of these signals in radio navigation we need to understand the nature of side-lobes and avoid using them. You would learn about these in the later lessons.
Loop Antenna in Navigational Aids
We are aware that navigational aids help us in direction finding during bad weather. Loop antenna is a rotating loop shaped antenna used for direction finding in certain navigational aids like NonDirectional Beacons. Loop antenna has a rectangular ariel in which only the vertical elements transmit and receive the signals. It is ensured that the horizontal elements do not receive or transmit.
Rotating Loop Antenna
When the loop antenna is used in a receiver, the antenna is rotated around 360 degrees to sense the direction. Depending on the direction from which the signal is received, the phase of the wave is different between the two vertical elements. Rotating loop antenna senses and calculates the phase difference between its vertical elements and convert it to direction.
Direction Finding using Phase Comparison
The Phase difference is zero only when loop is absolutely perpendicular to direction from which the wave is received. In all other positions the phase is different. Receiver circuits generate an error signal by measurement of phase difference. This error signal produces a current in the circuit indicating directions. Some navigational aids like Automatic Direction Finder (ADF) utilises loop ariel principle for direction measurement.