ANTENNA GAIN MEASUREMENT
Objective: To measure the gain of wave guide horn.
List of Equipment:
1. Microwave source with square wave modulation
3. Variable attenuator
4. Two identical waveguide horns
5. Antenna test bench
6. Tunable Detector
7. VSWR meter
The ability of an antenna to concentrate the radiated power in a given direction, or conversely to absorb effectively incident power form that direction, is specified variously in terms of its gain, power gain, directive gain, or directivity. The precise significance of each of these terms is most readily stated by first defining a quantity known as radiation intensity.
Radiation intensity is defined a power radiated per unit solid angle. Thus,
Φ (θ,φ) = (E2/η)r2 watts per unit solid angle (1)
Where Φ (θ,φ) radiation intensity in the direction
E = far electric field intensity
= intrinsic impedance of the medium
r = distance from the antenna
The average radiation intensity is defined as
where w,r is the total power radiated. Φav may be considered to be the radiation intensity of an isotropic radiator, radiating same total power. The directive gain gd, in a given direction is defined as the ratio or radiation intensity in that direction to the average radiation intensity,
Often the directive gain is expressed in decibels as
Clearly, gd or Gd is a function of direction (θ,φ). Maximum directive gain is called directivity. In practice, it is power gain gp, defined as
which comes into play. Here, wr= wt+wl is the total power including losses, fed to the antenna.
Although these definitions have been framed by considering a transmitting antenna, these are applicable to receiving antennas too. Of course, the gain thus defines can be realized on a receiving antenna only when it is properly matched and an approximately polarized field is present. For receiving antennas, one defines the effective area A as
where G is gd for antennas with an efficiency of 100%, and G is gp for lossy antennas.
Power density at a distance R from a point source fed with a power is
If however, the radiator is an antenna with gain G, the power density increases to
If an antenna with gain Gz is used to receive the signal, its effective aperture is
and the power received Pr is given by
Pr = power density × effective aperture
Measuring Pt/Pr by interesting an attenuator to maintain constant power detected with and without antenna system allows G^2 to be calculated. As several readings must be taken, it is best to plot Pt/Pr against R2 and evaluate G from the slope of the graph.
Theoretical calculation of gain of waveguide horn antennas is given in  and . For a well constructed horn, measured power gain values are quite close to the calculated values.
Fig 1. Experimental arrangement for Gain measurement
1. Calculate 2D2/λ where D is the largest dimension of the aperture of the horn and λ is the free space wavelength corresponding to 10 GHz. During the experiment, the horns should not get any closer to each other than this distance.
2. Set up apparatus as shown in fig.1. Set the source at 10 GHz by given slider. Fix the amplitude (maximum to get maximum power) and do square modulation using the modulator.
3. Isolator must be in “forward direction” and attenuation should be minimum, so that we get maximum power in output.
4. We can vary the distance between two horns by using “Sample” button. But the minimum distance should be where D is the largest dimension of the aperture of the horn and is the free space wavelength corresponding to 10 GHz.
5. A graph is plotted between the square of the distance between horns along y-axis and the power ratio along x-axis in GAIN Measurement window. Where m shows the tangent of the slope and G is the gain.
6. We can verify the result by calculating the slope form the graph. Knowing, G can be easily calculated.
1. What do you understand by antenna gain?
2. How do you know that antenna is tuned at the correct frequency?
3. Why the distance between both the horns should be more than?
4. What will happen if both antennas are not aligned horizontally or vertically?
1. E.C. Jordan and K.G.Balmain, ‘Electromagnetic wave Radiating Systems’ 1968.
2. R.E. Collin, ‘Antennas and Radio Wave Propagation’, 1985.
3. C.A. Balanis, ‘Antenna Theory: Analysis and Design, 1982.
4. A.W. Cross, ‘Experimental Microwaves’, 1977.