antenna and microwave laboratory babol noshirvani university of technology [email protected] antenna...

Antenna and Microwave Laboratory Babol Noshirvani University of Technology [email protected]

Antenna and Microwave LaboratoryBabol Noshirvani University of Technology, Iran

In the name of God

Introduction


Research Projects:

Numerical Experiments or Numerical Lab

Laboratory Experiments

On-Site(field) Experiments

Antenna Lab


Electromagnetics Spectrum


Fundamental Parameters of Antenna


1. Radiation Mechanism


2. Radiation Pattern (Directivity)

So, based on what application we are dealing with, an appropriate radiation pattern is selected.

dipole(λ/2) antenna (omnidirectional pattern)

reflector antenna (pencil-beam pattern)


3. Reflection Coefficient (S11)

Vinc

VrefS11= Vref / Vinc

Vref=0 then log(S11)=-inf db Vref=1 then log(S11)=0 db

S11 plot fr-antenna BW-antenna


4. Farfield Criteria


5. Pattern Lobes


6. Half Power Beamwidth

E pattern P pattern 10log (P pattern )


6. Polarization


7. Input Impedance

Zin= RL +Rr + jXA

Zin= input impedance

RL= loss resistance

Rr= radiation

resistance

XA= antenna reactance

At a given frequency all these parameters are controlled by antenna geometry


Antenna Measurements



Many antennas, because of complex configuration & excitation, cannot be investigated analytically.

Experimental results are often needed to validate theoretical data.

It is usually most convenient to perform antenna measurements with test antenna in its receiving mode.

By reciprocity, receiving mode characteristics are identical to TX mode.

Ideal condition for measuring is incidence of a plane waves having uniform amplitude and phase.

Although this ideal condition is not achievable, it can be approximated by separating test antenna from illumination source by a large distance on an outdoor range.

At large radii, curvature of spherical phase front produced by source antenna is small over test antenna aperture.

If separation distance is equal to inner boundary of far-field region, 2D2/λ, then maximum phase error of incident field from an ideal plane wave is about 22.5o, as shown in:

In addition to phase front curvature, reflections from ground and nearby objects are possible sources of degradation of test antenna illumination.



Experimental investigations have a number of drawbacks:

1. For pattern measurements, distance r>2D2/λ is too long even for outside range.

2. It also becomes difficult to keep unwanted reflections from surrounding objects.

3. In many cases, it may be impractical to move antenna from operating environment to measuring site.

4. For some antennas such as phased arrays, time required to measure may be enormous.

5. Outside measuring systems provide an uncontrolled environment, and they do not possess an all-weather capability.

6. Enclosed measuring systems usually cannot accommodate large antenna systems such as ships, aircraft.

7. Measurement techniques are expensive.

Some of above shortcomings can be overcome by using special techniques such as:

Indoor measurements.

Far-field pattern prediction from near-field measurements.

Scale model measurements.

Automated commercial equipment specifically designed for antenna measurements.

Utilizing computer assisted techniques.



Because of accelerated progress made in aerospace/defense related systems, more accurate measurement methods were necessary.

Accurate measurement methods are:

Tapered anechoic chambers.

Compact and extrapolation ranges.

Near-field probing techniques.

Improved polarization techniques.

Swept-frequency measurements.

Automated test systems.

A more extensive and exhaustive treatment of these and other topics can be found in:

IEEE Standard Test Procedures for Antennas [7].

A summarized journal paper [8].

A book on microwave antenna measurements [6].



Reflection Ranges:

There are two basic types of antenna ranges:

The reflection ranges.

The free-space ranges.

Reflection ranges for a judiciously design, can create a constructive interference in region of test antenna which is referred to as the “quiet zone.”

This is accomplished by designing ranges so that specular reflections from ground as shown:

Usually it is desirable for illuminating field to have a small and symmetric amplitude taper.

This can be achieved by adjusting transmitting antenna height while maintaining constant that of receiving antenna.

They are used for systems operating in the UHF to 16GHz frequency region.

Free-space ranges, designing to suppress contributions from surrounding environment, include as:

Elevated ranges.

Slant ranges.

Anechoic chambers.

Compact ranges.

Near-field ranges.


RectangularTapered


Anechoic Chambers:

Anechoic chamber is an alternative to outdoor testing.

To provide a controlled environment, indoor anechoic chambers have been developed.

In general, as operating frequency is lowered, thickness of RF absorbing material must be increased to maintain a given level of reflectivity performance.

There are two basic types of anechoic chamber designs:

Rectangular chamber.

Tapered chamber.

Design of each is based on geometrical optics techniques.

Each attempts to reduce or to minimize specular reflections.

The rectangular chamber is designed to simulate free-space conditions and maximize volume of quiet zone.

Its design takes into account pattern and location of source, frequency and isotropic antenna.

Reflected energy is minimized by the use of high quality RF absorbers.

Despite use of RF absorbing material, significant specular reflections can occur.



Tapered anechoic chambers take form of a pyramidal horn.

They begin with a tapered chamber which leads to a rectangular configuration at test region.

Source is usually placed near apex so that reflections from side walls occur near source antenna.

For such paths, phase difference between paths are very small by properly locating source antenna near apex.

Thus direct and reflected rays near test antenna provide a relatively smooth amplitude illumination taper.

This can be illustrated by ray-tracing techniques.

By increasing f0, it becomes increasingly difficult to place source sufficiently close to apex that phase difference between direct and secularly reflected rays can be maintained below an acceptable level.

For such applications, reflections from walls of chamber are suppressed by using high gain source antennas whose radiation toward walls is minimal.

In addition, source is moved away from apex, and it is placed closer to end of tapering section so as to simulate a rectangular chamber.



Compact Range antennas:

Microwave antenna measurements require a uniform plane wave.

Requirement of an ideal plane wave illumination can be achieved by utilizing a compact range.

A Compact Antenna Test Range (CATR) is a collimating device which generates nearly planar wave fronts in a very short distance.

Distance is typically 10–20m compared to the 2D2/λ.

Some attempts have been made to use dielectric lenses as collimators [15].

Name CATR refers to one or more curved metal reflectors which perform collimating function.

CATR are very large reflector antennas designed to optimize planar characteristics.

CATR are designated according to their analogous reflector antenna configurations: parabolic, Cassegrain, Gregorian.

Major drawbacks of compact ranges are :

Aperture blockage.

Direct radiation from source to test antenna.

Diffractions from edges of reflector.

Feed support.

Depolarization coupling between two antennas.

Wall reflections



CATR Performance:

Amplitude and phase ripple in quiet-zone fields produced by a CATR caused by phasor sum of reflected and diffracted rays from reflector



Two common CATR reflector edge treatments that are used to reduce diffracted fields in quiet zone.

CATR Performance:


Antenna Room


1) Chamber Geometry

a) Rectangular Chamber


b) Taperd Chamber


) جاذب دی الکتریک:1 ) جاذب مغناطیسی: 2

دی الکتریک می باشدPermittivity قسمت تلفاتی در جذب موج الکترومغناطیسی به خاطر وجود-

استریزهای کربن و خرده urethaneپلیمر ساختار اصلی این نوع جاذب -

فرم فیزیکی آن ها ابر مانند بوده، پس سبک می باشند، اما پایداری کمتری دارند-

-Tuneترند، اما پهنای باند کمتری دارند

ساختار مغناطیسی می Permeability قسمت تلفاتی در جذب موج الکترومغناطیسی به خاطر وجود-باشد

می باشدمواد مغناطیسی موسوم به فریت ساختار اصلی این نوع جاذب -

فرم فیزیکی آن ها بصورت الیه ای و ورقه ای بوده، سنگین ترند، اما پایداری بیشتری دارند-

اند، اما پهنای باند بیشتری دارندTuneکمتر -

2) Radar Absorbing Material


شرکت های ارائه کننده جاذب مایکروویوی

Telemeter Electronicشرکت 1.

FRD EMI Shielding Materialsشرکت 2.

Laird Technologyشرکت 3.

Emerson and Cumingشرکت 4.Microwave

MAJR Product Corporationشرکت 5.

.Eeonyx Corpشرکت 6.

antenna and microwave laboratory babol noshirvani university of technology [email protected] antenna...

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