ANTENNA MEASUREMENTS Measurement of Radiation Pattern Gain Polarization VSWR Test Ranges

Antenna Measurement Antenna measurements are part of the analysis of antenna parameters. Antenna measurements are useful for the application oriented specific designs of the antennas. Accurate measurements are necessary to establish the actual performance of antennas.

Antenna Measurements are required for the following purpose To verify design. To control quality. To analyze different parameters. To find analytical and statistical errors. To indicate actual performance of antennas. To adjust critical components and dimensions. To calibrate and store data for different types of antennas.

Drawbacks in Measurements of Antenna Parameters Measurement are time consuming. Measuring equipment is expensive. Open site measurement are not accurate. It is difficult to bring large antennas to the measuring site. It is sometimes difficult to provide far-field distance. For large antennas, at higher frequencies the pattern measurement becomes very difficult because distance to the far field region becomes too large for even outside ranges.

Methods to overcome Drawbacks in Measurements Determining of far-field patterns from near field measurement. Making scale model measurements. Using automated measuring equipment. Using computerized techniques.

Accurate Measurements The following methods are used for a accuracy, Near field techniques. Polarization techniques. Computer controlled test systems. Compact ranges.

Radiation Pattern Measurement We need two antennas for measurement of radiation pattern one will transmit and other will receive. One is the origin other at some distance away from the first antenna. There are 2 procedure for Radiation Pattern Measurement. First procedure is suitable for low frequency antennas and Second procedure is suitable for high frequency antennas.

Radiation Pattern Measuring setup Antenna Rotator Mechanism Secondary Antenna Primary Antenna RF Cable Antenna Rotator Mechanism Transmitter Receiver Indicator

First procedure is suitable for low frequency antennas Primary antenna is not moving, secondary is moved along a circular path at a constant radius. Secondary is directional antenna, usually the primary antenna is transmitting. The field strength readings and direction of the secondary antenna with respect primary antenna are recorded along the circle at different points. The plot of radiation pattern is made either polar form or rectangular form.

Second procedure is suitable for high frequency antennas Both the antennas are kept in fixed positions having a suitable spacing between them. Now, Primary antenna is rotated and secondary antenna is transmitting. So that the field strength reading and direction of the primary antenna with respect to secondary antenna can be made. The readings are taken at number of points, by stopping the rotation of primary antenna for recording the readings or continuous reading can also be taken if pattern recorder is available.

Experimental setup for Antenna Test Direct Rays Primary Antenna Secondary Antenna d – Maximum linear direction of either antenna Indirect Rays Tower Tower Ground

Measurement of Antenna Gain Radiation Intensity is defined as the power radiated from an antenna per unit solid angle. Basically there are two standard methods used for the measurement of gain of an antenna such as, Gain comparison method or direct comparison method. Absolute gain method.

1. Measurement of gain by Direct Comparison Method This method is commonly used at HF. The measurement of gain is carried out by comparing the signal strengths transmitted or received with the test antenna and a Standard Gain Antenna (SGA). A standard gain antenna is that antenna whose gain is accurately known so that it can be used in measurement of other antenna. HORN antenna at microwave frequencies (300MHz and 300GHz) is mostly used as SGA.

The secondary antenna may be an arbitrary transmitting antenna and it is not necessary to know its gain. In place of primary antenna, there will be two antennas, one is test antenna and another one is standard antenna at a considerable distance. The distance between primary and secondary antennas must satisfy the condition and reflections between them should be minimized. The attenuator pad is inserted to the input of receiver, in order that AUT & SGA work into a matched load.

Measurement of gain by Direct Comparison Method

Procedure At first, standard antenna is connected to the receiver with the help of switch “ S ”. The input to the transmitting antenna (secondary antenna) is adjusted to a convenient level and corresponding reading at the receiver (primary antenna circuit) is recorded. The attenuator dial setting and the power bridge reading are also recorded. Say it is W1 and P1 respectively. Now connect the test antenna whose gain is to be measured in place of standard gain antenna. The attenuator dial is adjusted such that receiver indicates the same previous reading as was with SGA.

Power gain (G) = GP * (P1 / P2) Let the attenuator dial setting reading W2 and power bridge reading P2. Case I. When P1 = P2 If P1 = P2, then no correction need to be applied and the gain of the test antenna measurement with respect to standard gain antenna is given by Power gain (GP) = W2 / W1 Case II. When P1 ‡ P2 The power level is changed during the measurements and P1 ‡ P2, then actual power gain of test antenna can be obtained by multiplying Gp with ratio (P1 / P2) = P. Power gain (G) = GP * (P1 / P2)

2. Absolute gain Method There are two methods with which the calibration of standard gain antenna can be done. Two Identical Antennas Three Arbitrary Antennas i). Two Identical Antennas The input to the transmitting antenna is adjusted to an appropriate level and corresponding receiver reading level is recorded. The attenuator dial setting and the power bridge reading are recorded. Say it is Wt and Pt1 respectively.

Absolute gain of Identical Antenna Measuring setup Receiving Antenna (AUT) Transmitting Antenna Precision Calibrated Variable Attenuator Signal Source Attenuator Padd. Modulator Detector Receiver Detector Power Bridge Indicator

Now the transmitter is disconnected from the antenna and is connected to the receiver through pads. The attenuation dial is adjusted until the receiver reads the same previous levels. The attenuator dial setting and power bridge reading are noted, say Wr and Pt2. If Pt1 = Pt2 Then no correction would be needed. If drifting of power is involved, then correction would be needed. The second method is needed only when two identical antennas are not available.

ii). Gain Measurement by 3 Antenna Method This method consists of Three unknown antennas. Using antenna 1 as transmitter and antenna 2 as receiver, the received power W1 is measured. Let the transmitter power be P1.

Gain Measurement by 3 Antenna Method Replacing antenna 2 by antenna 3, the received power W2 is measured for the same transmitted power (P2 = P1). When antenna 2 is used as transmitter and antenna 3 is used as receiver, receiver power W3 is measured. Let the transmitter power be P3.

Measurement of Polarization Polarization is a property of waves that can oscillate with more than one orientation.

Types of Polarization There are three types of polarization Polarization is a general property of waves that describes the orientation of their oscillations. For transverse waves such as many electromagnetic waves, it describes the orientation of the oscillations in the plane perpendicular to the wave's direction of travel. The oscillations may be oriented in a single direction (linear polarization), or the oscillation direction may rotate as the wave travels (circular or elliptical polarization). Circularly polarized waves can rotate rightward or leftward in the direction of travel, and which of those two rotations is present in a wave is called the wave's chirality. linear circular elliptic 24

The basic setup for Polarization Measurements

Output of Measurement when the Test Antenna is Vertically Linearly Polarized Left: Rectangular Plot & Right. Polar Plot

Output measurement of Linearly Polarized Test Antenna (Horizontal Polarization)

Output of Measurement when the Test Antenna is Circularly Polarized

Output of Measurement when the Test Antenna is Elliptically Polarized

Measurement of VSWR VSWR stands for Voltage Standing Wave Ratio, and is also referred to as Standing Wave Ratio (SWR). VSWR is a function of the reflection coefficient, which describes the power reflected from the antenna. If the reflection coefficient is given by Г, then the VSWR is defined by the following formula

VSWR is the ratio of the peak amplitude of a standing wave to the minimum amplitude of a standing wave.

The VSWR is always a real and positive number for antennas. The smaller the VSWR is, the better the antenna is matched to the transmission line and the more power is delivered to the antenna. The minimum VSWR is 1.0. In this case, no power is reflected from the antenna.

TEST RANGES Free Space Ranges The most popular free space ranges are 1.Anechoic Chambers, ("an-echoic" meaning non-reflective, non-echoing or echo-free). 2. Elevated Ranges. 3. Compact Ranges.

1. Anechoic Chambers An anechoic chamber is a room designed to completely absorb either sound or electromagnetic waves. They are also insulated from exterior sources of noise. Anechoic chambers are indoor antenna ranges. Indoor ranges are desirable because the test conditions can be much more tightly controlled than that of outdoor ranges. Anechoic Chambers are most often used for frequencies above 300 MHz.

Anechoic Chamber is shown in the following picture, along with some test equipment.

2. Elevated Ranges Elevated Ranges are outdoor ranges. Placed higher than the surrounding area. In this setup, the source and antenna under test are mounted above the ground. These antennas can be on mountains, towers, buildings. For elevated ranges, once a source and test antenna location are determined, the test operators then determine where the significant reflections will occur, and attempt to minimize the reflections from these surfaces.

The basic diagram of an elevated range is shown in Figure.

3. Compact Ranges The source antenna must be placed in the far field of the test antenna. The reason is that the wave received by the test antenna should be a plane wave for maximum accuracy. The compact range operates over the 400 MHz to 100 GHz band.

The parabolic antenna changes spherical waves into plane waves towards AUT. This involves small distances, and hence it is called compact range.

Differences between indoor and outdoor ranges S. No. Indoor Ranges Outdoor Ranges 1. Space is limited Space is unlimited 2. Only small antennas can be tested Antennas of any size can be tested 3. Ground and other reflections are controlled Ground and other reflections cannot be controlled fully 4. They have controlled environment They have uncontrolled environment 5. They have all weather capability They do not have all weather capability

The selection range for antenna measurement depends on the following factor: Frequency, Cost, Accuracy required, Size of the antenna and so on.

Measurement of Antenna Efficiency Antenna radiation efficiency is the ratio of power radiated to the power supplied to the antenna. Measurement of Directivity Directivity is defined in terms of radiation intensity.

Problems 1.A parabolic reflector antenna is designed for operation at 3000MHz. Its largest aperture dimensions is 20 ft. for measurement of radiation pattern, what should be minimum distance between primary and secondary antenna (one feet = 0.3048 meter) Given f = 3000 MHz, d = 20 ft 20 * 0.3048 = 6.0960 meters To Find r

Solution λ = c / f = ( 3*10Λ8 ) / 3000MHz = 3 / 30 λ = 1 / 10 r ≥ ( 2 * (6.0960) Λ 2 ) * 10 r ≥ 743.22 meters

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