ELEC4604. RF Electronics. Experiment 1

Transcription

1 ELEC464 RF Electronics Experiment ANTENNA RADATO N PATTERNS. ntroduction The performance of RF communication systems depend critically on the radiation characteristics of the antennae it employs. These antennae are designed to suit particular applications. n satellite communications, signals transmitted from the ground must be focussed into a very narrow beam directed towards the satellite. To achieve this end the radiation pattern of a ground terminal antenna must have a very narrow main beam, i.e. its pattern must be highly directional. As a rule, the beamwidth of an antenna is inversely proportional to its dimension relative to the wavelength at the operating frequency. Thus, a very narrow beam normally means a physically very large antenna. n practise the most compact and cost effective antenna with a narrow beam is achieved by using a combination of a large reflector and a small feed horn (generally known as a radio telescope or simply a reflector antenna). To maximise the gain of a reflective antenna it is more cost effective to concentrate on the feed horn design. The wide beam width of the feed horn must efficiently and uniformly illuminate the whole dish any irregularities will compromise the performance of the reflector antenna. By contrast, the antenna of a mobile terminal must be designed so that it can receive signals coming from any direction because the orientation of the antenna relative to its base station is changing with time in a random manner. n this application the radiation pattern should not have a flat beam structure, and be as omni directional as possible. A simple and small antenna is generally used. Finally a high performance surveillance radar antenna will need to have its beam scanned very rapidly with time in order to track very fast moving targets. Similarly an antenna on board a lowearth orbit (LEO) satellite (or a 3G mobile base station antenna, for that matter) may also need a narrow beam that can be scanned in different directions. For these applications an antenna array should be used. n this experiment we will investigate the radiation pattern of a horn antenna, a monopole antenna, and an antenna array.

2 . Aim n this experiment the radiation pattern of different types of antennas will be measured. The main aim is to study the various methods of shaping the radiation pattern of an antenna, as well as to study the effects on the radiation pattern of changing the antenna size and the electric field distribution at the antenna aperture. 3. Measurement Principle Since the antenna radiation pattern is the same whether it used a receiving or transmitting antenna we can limit ourselves to measuring the receive pattern only. This can be done by using a fixed radiation source (another antenna) placed at a large distance from the antenna under test (acting as the receiving antenna). The antenna under test is then rotated about its phase centre with the aid of a turn table, and the received signal is plotted to give the sensitivity vs. direction, equivalent to its radiation pattern. 4. Procedure 4. Ensure that you are not exposed to the microwave radiation used in this experiment, as it may be harmful to humans. Do not have the signal generator turned on while a person is setting up the receiving antenna on the turn table, etc. 4. Record in detail the name of each piece of equipment used in the experiment 4.3 Measure the E and H plane radiation patterns of the following antennae: a. Rectangular horn at both 9.GHz and.5ghz. b. Microstrip antenna array at 9.GHz only. Note that the E place is the plane parallel to the electric field at the antenna aperture, while the H plane is the plane parallel to the magnetic field at the aperture. n measuring the radiation pattern you must ensure that the electric fields of the receiving and transmitting antennae are parallel to one another. 5. Discussions 5. Explain the function or role of each piece of equipment. 5. Discuss the similarities and differences between the E plane and H plane radiation patterns. 5.3 Discuss the effect of antenna size and frequency on the beamwidth of the radiation pattern. f your results do not agree with predicted values, give explanations for any discrepancies.

3 3 6. Schematic Diagram of the Experimental Set up Modulator Frequency Meter Microwave Absorber Receiving Antenna Signal Generator Variable Attenuator Directional Coupler Transmitting Coupler Turn-table Plotter 7. Calculations Figure Set up for Radiation pattern measurement The directivity of an antenna can be calculated from its radiation pattern by the following formulae: The directivity is defined at the point of maximum intensity in the radiation pattern D max 4,, max av P rad where the total power radiated over all angles is: P rad, sin d d n measuring the radiation patterns you have effectively measured the radiated intensity as a function of direction. Recall, E, H, For both the antenna whose radiation patterns you measured, calculate their directivity, expressing your answer in dbi (decibels relative to an isotropic radiator). You may wish to use a computational package like MATLAB to help you with your numerical calculations.

4 4 8. Antenna Arrays As shown in lecture notes, the radiation pattern of an antenna array formed by M equally spaced elements can be expressed as: F, E where kd cos represents the phase difference between adjacent a elements at direction θ. α is the electronic phase shift inserted between the elements, and d is the spacing between the t linear elements, as shown in Figure. E iss the radiation pattern produced by each constituent element in the array. M sin M sin Figure Antenna Array geometry Some important remarks:.. The unit pattern E is fixed once we choose the antenna elements of the array in your case, microstrip antennae. The radiation pattern is the product of E with a term dependent on the array geometry, which is called the array factor (AF). For the linear M element array, the array factor is:

5 5 AF, AF( ) M sin M sin 3. The main beam direction will occur when the waves coming from each constituent element are in phase, so ψ =. This means: cos m kd Note that the main beam direction can be controlled by changing the electronic phase shift provided to each element, α. This is the principle of electronic scanning. 4. The characteristics of the main lobe depend on the number of elements within the array. n particular: Beamwidth: Md Side lobe level: Side - lobe Amplitude Main lobe Ampliude 3 For the microstrip antenna array you are given, determine the array spacing, the number of elements, the wavelength, the direction of the main beam, the electronic phase shift, the mainbeamwidth, and the side lobe level. Compare the properties you measure to the theoretical results presented above.