20-07-2013, 02:49 PM
DIRECTIVITY & GAIN OF ANTENNAS
AIM:
Measurement of directivity & gain of antennas : Standard dipole (or printed
dipole), microstrip patch antenna and Yagi antenna (printed).
Components Required : Power supply, VCO, 50 ohm transmission line, dipole
antenna, patch antenna, yagi antenna, oscilloscope or VSWR meter.
Theory:
The simplest practical antenna is the half wave dipole. In its original form
it consists of two thin straight wires, each λ/4 in lengths, by a small gap. For this
simple antenna, under fairly realistic approximations, closed form expressions are
available for radiated fields, power, directivity etc.
The important feature of Yagi antenna is that it is an end-fire antenna, ie the direction
of maximum radiation is tangential to the plane formed by the parallel antenna
elements.
The design of a rectangular microstrip patch antenna begins with (a) choice of a
substrate, (b) selecting the feed mechanism, © determining patch length L, (d)
determining patch width w and (e) selecting the feed location.
Procedure:
Setup the system as shown in the figure for a standard dipole antenna.
Keeping the voltage at min, switch on the power supply and keeping the
gain control knob maximum, switch on VCO.
Vary the tuning voltage and check the output for different VCO
frequencies.
Keeping at the resonant frequency calculate and keep the min distance for
field between the transmitting & receiving antennas using the formula