ANTENNA RADIATION PATTERN MEASUREMENT

OBJECTIVE:

To measure the radiation pattern of a waveguide horn antenna, (Microwave range).

Equipments and devices required:

1. Gunn oscillator PM 7015X2. Modulator PM 7016X3. Ferrite isolator PM 7045X4. Frequency meter PM 7070X5. Rotary vane attenuator PM 7101X6. Detector PM 7195X7. Horn antennas - 2 No’s PM 7320X/018. Waveguide E-bend - 2 No’s PM 7345X9. Straight waveguide section PM 7366X10. Waveguide carrier - 2 No’s PM 770011. Waveguide clamp- 2 No’s PM 7702X12. Gunn power supply PM 781513. SWR meter PM 783214. Rotary joint PM 7888X15. Antenna stand SL 1970516. Work bench SL 80300

17.Measuring tape SL 80599

THEORY:

An antenna is a connecting device between two systems and a medium, i.e. an antenna transfer’s energy between a transmitter and a receiver through the medium. One of the more interesting features of all antenna design is the radiation pattern, and from this the gain. If an end of a transmission line propagating energy is left open at one end there will be radiation from this end. In the case of rectangular waveguide this “antenna” presents a mismatch of about 2:1 (SWR=2) and it radiates in many directions. The match will be improved if the open waveguide is given a “horn” shape.

A point source, i.e. an isotropic radiator, will radiate power evenly into space. This however is an ideal situation. A real antenna favors certain directions rather than others. The radiation diagram is a representation of the field strength, or more often, the power intensity Vs the antenna angle at a constant distance from the radiating antenna. The antenna pattern of a receiving antenna is identical to that of transmitting antenna, with the difference that the

receiving sensitivity in various directions is shown. Often the diagram is in polar form with relative power measured in dB.

An antenna pattern is of course 3D, but for practical reasons it is normally presented as a 2D pattern in one or several planes. For an rectangular horn antenna these planes are E-plane and H-plane. Normally the E-pattern and H-pattern of antenna will differ from each other.

An antenna pattern consists of several lobes. The major power is normally in main lobe. The power of side lobes and back lobe are kept as low as possible. The 3dB beamwidth is the angle between the two points on a main lobe where the power intensity is half of the maximum power intensity.

Radiation from an open wave guide Radiation from a waveguide horn

PROCEDURE:

1. Set up the equipment as shown in figure.2. Set the rotary vane attenuator at approximately 5dB.3. Connect the SWR-meter to the BNC connector of the frequency meter.4. Press 60dB button on the instrument.5. Connect the Gunn oscillator to the BNC connector “Gunn Osc” and connect the

modulator to the BNC connector “modulator” on the front panel of the power supply.6. Set the frequency meter at 9000MHz.7. Switch on the power supply and increase the voltage to 9V. Put the modulator switch in

the “ON” position.8. Tune the Gunn Oscillator until a maximum deflection can be seen in the SWR meter. (If

necessary adjust the rotary vane attenuator. The frequency of the Gunn Oscillator should indicate 9GHz).

9. Adjust the Gunn voltage until maximum deflection on the SWR- meter is achieved.10. Tune the SWR-meter for the maximum deflection by adjusting the 1 KHz, bandwidth-20

Hz.11. Detune the frequency meter by 150 MHz

Detector

Rotary joint

Gunn power supply

Isolator

Frequency meter

Rotary vane attenuator

Gunn oscillator

SWR meter

Tx Horn Rx Horn

SETUP DETAIL:

Fig 2 Test bench for the radiation pattern measurement

RADIATION PATTERN:

1. Separate the antennas by atleast 0.5m (> 2d2/λ). The scale on the rotary joint should indicate 90º when the horns are “in line”. (Reduce the reflections from the surrounding).

2. Connect the SWR-meter to the detector and obtain full scale deflection by adjusting the gain control. If necessary adjust the rotary vane attenuator.

3. Turn the transmitting antenna to the left in 10º steps and note the SWR-meter deflection. When necessary press the next higher gain button and add 10dB to the observed value. Continue to the 90º from the initial position and record in a table.

4. Repeat step 3 but turn the HORN in the right.5. Use the obtained values to draw a relative power pattern in the antenna diagram.6. With the horn in the initial position (90º; SWR-meter=0dB) turn the horn to the left until

the SWR-meter shows 3dB. Record the observed angle in the polar plot.7. Repeat step 6, but turn the HORN in the right.

8. Keeping the receiving antenna in the “in line” position as a reference, move the HORN front and back, in steps of few centimeters and record the values in another table. Draw distance Vs attenuation plot.

9. Keeping the receiving antenna in the “in line” position as a reference, move the HORN up and down, in steps of few centimeters and record the values in another table. Draw height Vs attenuation plot.

MODEL GRAPH:

TABULATION:

Degree 0º 10º 20º 30º 40º 50º 60º 70º 80º 90º

Left sideRightside

OBSERVATION:

RESULT: