Awp Unit Iv Antenna Measurements 496f5u

ANTENNA MEASUREMENTS Introduction: Antenna measurements are necessary to know actual performance of antennas: their gain, radiation pattern, bandwidth, efficiency etc.

Radiation Patterns

There are several regions of radiated field in the vicinity of antenna like reactive near field region (radian sphere), radiating near field or Fresnel region and far-field or Fraunhofer region.

Measurement of antenna parameters is usually done in the far-field region. The advantages of far-field measurement are: 1.

2. 3. 4.

The measured field pattern is valid for any distance in the farfield region; only simple transformation of the field strength according to 1/r is required. If a power pattern is required, only power (amplitude) measurement in needed. Coupling & multiple reflections between the antennas are not significant. In practical environment, generally the receiver is in the farfield region.

The main disadvantage of far-field measurement is the required large distance between the antennas leading to large antenna ranges. The far-field region of transmitting antenna is the region beyond the distance rff given by the well known Rayleigh distance:

rff



2D 2 

(meters)

where D: largest dimension of the physical aperture of the antenna

Measurement Ranges The various measurement ranges used for the measurement of antenna parameters are: 1. Elevated Ranges 2. Ground-Reflection Ranges 3. Anechoic Chambers 4. Compact Antenna Test Ranges (CATRs) 5. Near Field Ranges Which range type suits best for the measurement of a certain antenna depends mainly on the physical size and frequency of the antenna.

Elevated Ranges is the basic far-field range.

Antennas are placed high on towers, buildings or hills to reduce effects of

• Generally, AUT (Antenna Under Test) is operated as receiving antenna. • If source antenna is near ground and AUT on a tower, range is called slant range. • Range length is determined by the far-field criterion. • Range width should be sufficient to keep the main beam of the source antenna within it. • The antenna heights and source antenna pattern should be such that the main beam of source antenna does not illuminate the ground between the antennas.

The lower limit of the source diameter DT mus 1.5R DT  (meters) HR

The upper limit of the source antenna diameter m

R DT  3D

(meters)

Combining the two requirements for the source antenna diameter gives that AUT must be mounted at least 5 times as high as its diameter D.

Ground-Reflection Ranges It is used at VHF and lower frequencies as a directional source antenna is very large and ground reflections are difficult to avoid.

Kraus: Fig. 21-7

• Antennas are placed above a flat reflecting surface with (or without) buried ground screen. • Reflection appears to come from the image source. • Real and image source form together an interference pattern. • AUT is placed in the first lobe of the interference pattern as it has the flattest amplitude distribution.

Assuming a negative image, i.e. the surface has reflection coefficient of -1, the peak of the first lobe is at height λR HR  (m) 4HT

Above equation indicates that the ideal height is a function of the wavelength. So, the height of the source antenna or AUT may need to be

Height of AUT must be at least 3.3D. The surface of a ground-reflection range should be smooth, which requires that the rms height variation of the surface, Δh is small enough:

 h  M sin

λ : wavelength, m M : smoothness factor (ex. 16) ψ : grazing angle

(m)

Anechoic Chambers • Walls, ceiling and floor of an anechoic chamber are completely covered with absorbing material. • Such chamber simulates a reflection-less free space and allows all-weather measurements in a controlled laboratory environment. • In this chamber, test area is isolated from interfering signals much better than at outdoor ranges. • Anechoic chambers can be used for the far-field measurements of small antennas. • Absorbing materials are used for reducing sidelodes and back-lobes. • An ideal absorber provides an impedance match for incoming waves at all frequencies & all angles of incidence.

• Broadband absorbers are generally made of carbon-loaded polyurethene foam. • Shapes of absorbers can be: Pyramids or Wedges

Kraus: Fig. 218

• Pyramids work best at normal incidence as they behave like tapered transition.

• Wedges work well at large incidence angles with wedge direction along plane of incidence.

• Absorbers can also be made with magnetic materials with losses – Salisbury screens and Jauman absorbers. • Salisbury screen: It has a resistive sheet placed λ0/4 from a reflecting plate. If sheet has surface impedance of 377 Ω per square, a normally incident wave is completely absorbed.

Fig. With normalized impedances

Kraus: Fig. 21-9 (a) Transmission line equivalent of Salisbury sc

• Jauman absorber: It has a broader bandwidth than Salisbury screen. It is made by stacking resistive sheets.

Kraus: Fig. 21-9 (b) Transmission line equivalent of Jauman abs

Types of anechoic chambers: Rectangula r

Tapere d

Kraus: Fig. 21-10

Compact Antenna Test Ranges (CATRs) • CATRs simulate an infinite range length by producing a flat phase front with reflector, lens, horn, array or hologram. • A CATR is usually installed indoors in an anechoic chamber but very large CATRs are outdoor ranges.

• As the beam is collimated, the required power is less than on a farfield range. Kraus: Fig. 21-11

• To reduce amplitude taper, feed antenna must have a proper pattern. • Corrugated horns have symmetric patterns, low side lobes & low cross-polarization levels and so are widely used as feeds. • Diffraction can be reduced by edge treatment; for which three methods are in use: serrating the edge, rolling the edge & resistive tapering.

Dual reflector CATRs: They provide advantages, over the basic single reflector CATRs, like: •a larger quiet zone for a given reflector size and

Kraus: Fig. 21-13. Dual-chamber Gregorian-fed CATR

Near Field Ranges • In these ranges, fields near the antenna are measured. From near-field measurements, the far-field patterns can be computed. • Near-field ranges are very compact and many kinds of antenna parameters can be computed from data. • Near-field measurements are done by scanning the field close to AUT with a known probe antenna, small in size & having a broad beam, e.g. open-ended waveguide.

Basic measurements systems are planar, cylindrical and spherical coordinate systems.

Plan ar

Cylindric al

Spheric al

Kraus: Fig. 21-16. Near field scanning geometries

Planar system: • In this the AUT is fixed and the probe moves. • It is suited for high gain antennas. Cylindrical system: • For this, the AUT rotates and the probe moves on a linear track. • Antennas having dipole-type patterns can be measured with this system. Spherical system: • In this AUT rotates in different planes and the probe is fixed. • With this system, omnidirectional antennas can be measured.

Gain and Directivity Measurement Two basic gain measurement methods are: 1.Absolute Method: This method can be done in either of the two ways a. Two-antenna method b. Three-antenna method 2.Direct Comparison Method (Gain-Transfer Method )

Absolute Method This method is based on Friis transmission formula.   2    PR  PTGTGR 







4R  

• Two antenna method

(Watts)

Direct Comparison Method (Gain-Transfer Method)

In this, gain measurement is done by comparing powers received with the AUT and with a known reference antenna. This measurement can be performed on either a free-space or a ground-reflection range.

Radiation Pattern Measurement

• Power pattern or amplitude pattern is measured. • Generally, patterns are measured in the Eplane and H-plane (for a linearly polarized antenna). • Beamwidth, pattern shape, side-lobe levels with their directions and null directions are parameters which are obtained from measured pattern.

Spectrum Analyzer A spectrum analyzer produces a visible display of the frequency content of an input signal. Most spectrum analyzers are of heterodyne type, alsosystems, called its scanning spectrum In commn. is required to extract analyzers. frequency contents from a time varying voltage signal x(t). Methods for this are: • A Digital Storage Oscilloscope can provide solution by calculating FFT of the signal from stored samples. • To x(t) through a long series of very narrow BPFs with adjacent bands and then plot amplitudes of filter outputs. • Using a simple property of Fourier Transform according to which:

If a signal in time domain is multiplied by a sinusoid, the spectrum of signal is shifted in frequency by an amount equal to the frequency of the sinusoid. Mathematically, x(t ) cos(2f 0t )  Fourier

Transform

1 1  X ( f  f0 )  X ( f  f0 ) 2 2

Spectrum Analyzer with Analog Display

Kraus: Fig. 21-34

Spectrum Analyzer with Digital Display

Spectrum Analyzer

Spectrum Analyzer display