The importance to antenna in a radio navigation cannot be ignored. Just to remind you, we had seen earlier that the polar diagram of transmission varies depending on the shape of the antenna. We had also seen that the shape of an antenna plays an important role in both transmission and reception of signals. For example, to transmit and receive horizontally polarised waves, we need both the transmitter and receiver antenna to be horizontal.
Transmitter and Receiver Ariel
There are two types of antenna. The transmitter and the receiver. Transmitter Antenna is used to transmit radio waves which are carrier waves modulated to carry the intelligence. Radio waves carry intelligence in the form of a modulated signal. On the other hand, Receiver Antennas are used to receive the radio waves. On receipt of modulated radio wave, the intelligence has to be separated using electronic circuits. Intelligence is obtained by demodulating radio waves in the receiver.
Characteristics of a good antenna
Firstly, Antenna or Ariels should be made of good conductors like aluminium. This would enable the power available in alternating current to be transmitted without loss of energy. Secondly, the length of antenna must be proportional to the wave length transmitted. That means to say that if we have to use a particular frequency like Low Frequency Band, the antenna must be 10 to 1 KM. Have a look at the approximate length required for some of the bands of radio waves.
- Low frequency: 10 to 1 KM
- Medium frequency: 100 to 10 M
- Very high frequency: 10 to 1 M
- Super high frequency: 10 to 1 Cm
Types of Antenna
Ideal length of an antenna has to be proportional to the wave length. By now you would have realised that the ideal length of an ariel is quite large and unwieldy. Like for using LF band we need an antenna of 10 KM length. Therefore we use less than ideal lengths of antenna which is half of the wavelength or quarter of the wave length.
Half wave and Quarter wave dipole antennas
When we use an antenna with a size equal to half the wave length of the wave it is called as a Half wave dipole antenna. Such antennas are used in televisions and is considered to be sufficient size. Folded half wave dipoles are used to accept a broader band of frequencies. On the same lines, when we use an antenna with a size equal to quarter of the wave length of the wave it is called as a Quarter wave dipole antenna.
Quarter wave antenna is also called as an Marconi antenna. The ideal size for quarter wave dipole antenna is 95% of quarter of wavelength. You would have realised that it is difficult to carry even a quarter wave dipole on board aircraft. Therefore, Marconi antennas are fixed to metal surfaces of aircraft to increases the antenna size. In fact, the whole aircraft becomes an antenna.
Antenna Loading Unit in Aircraft
In an aircraft we have a wide range of navigational equipments such as NDB, VOR, ILS, DME, GPS. Each one of them work on different frequencies and wavelength. We have to find a way to have ariels which can accommodate a wide range of frequencies in a small space in the aircraft. Ariel Loading Unit enables utilisation of the same antenna for a range of signals