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Main and side lobes

The pattern shown on previous figures has radiation concentrated in several lobes. The radiation intensity in one lobe is considerably stronger than in the other. The strongest lobe is called main lobe; the others are side lobes.

Yagi antenna

This antenna form has especially been developed for short wave. It is used also in radar sets.

The director and the reflector in the Yagi antenna are usually welded to a conducting rod or tube at their centres. This support does not interfere with the operation of the antenna.

Half-wave antenna

A half-wave antenna (referred to as a dipole Hertz, or doublet) consists of two lengths of wire rod, or tubing, each 1/4 wavelength long at a certain frequency. It is the basic unit from which many complex antennas are constructed. For a dipole, the current is maximum at the centre and minimum at the ends.

Half-wave antenna

Parabolic antenna

The parabolic dish antenna is the form most frequently used in the radar. A dish antenna consists of one circular parabolic reflector and a point source situated in the focal point of this reflector.

The circular parabolic (paraboloid) reflector is constructed of metal, usually a frame covered by metal mesh at the inner side. The width of the slots of the metal mesh has to be less than λ/10. This metal covering forms the reflector acting as a mirror for the radar energy.

Fan-beam antenna

Truncated parabolic reflector

a – lateral view; b – frontal view

A fan-beam antenna is a directional antenna producing a main beam having a narrow beam width in one dimension and a wider beam width in the other dimension. This pattern will be achieved by a truncated paraboloid reflector or a circular paraboloid reflector. This type of antenna system is generally used in height-finding equipment.

Modulation technique

In communications, modulation is the process of varying a periodic waveform, i.e. a tone, in order to use that signal to convey a message. Normally a high-frequency sinusoid waveform is used as carrier signal. The three key parameters of sine wave are its amplitude (volume), phase (timing) and frequency (pitch), all of which can be modified in accordance with low frequency information signal to obtain the modulated signal.

A device that performs modulation is known as a modulator and a device that performs the inverse operation is known as a demodulator (or detector). A device that can both operations is modem.

The reverse process demodulation is when the useful information is extracted from the unusefulcarrier.

Waveform

Sine-, square-, triangleand sawtoothwaveforms

Modulation techniques

Analogmodulation

•Amplitude modulation (AM, DSB); •Single-sideband modulation (SSB); •Frequency modulation (FM); •Phase modulation (PM);

•Space modulation (SM).

Digitalmodulation

•On-off keying (OOK);

•Frequency-shift keying (FSK), differential frequency-shift keying (DFSK) ; •Amplitude-shift keying (ASK);

•Phase-shift keying (PSK), differential Phase-shift keying (DPSK); •Qadratureamplitude modulation (QAM);

• Pulse-position modulation (PPM);