Antenna Fundamentals

Antenna Theory and Measurements

with the

Model 8092 System

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FEATURES• Low cost complete antenna measurement system.

• No anechoic chamber required to obtain good results.

• Covers several antenna technologies.

• 1GHz and 10GHz operation.

• Usable from 1GHz to 30GHz with separate generators.

• Software can be used in stand-alone mode.

• Unique options: phasing system, Rotman lens based multi-beam array antenna.

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Background

• Maxwell (1831-79) Fundamental equations. (Scottish)

• Hertz (1857-94) First aerial propagation (German)

• Marconi (1874-1937) Transatlantic transmission (Italian)

• DeForest (Triode tube 1920) Signal generators (American)

• World War II (1939-45) Intense war-driven development

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Definitions

• An antenna is a device for radiating or receiving radio waves. Antennas act as the transition between waveguides or transmission lines and free space.

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Definitions• An isotropic source is a hypothetical

antenna which is non(all)directional, that is, which has equal radiation intensity in all directions

• The dipole antenna is a simple type of antenna consisting of two rods or wires. The length of this antenna is L. The dipole is connected at the center to the transmitter through a transmission line.

6Note balanced transmission line

The Dipole Antenna

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Polar Coordinate System

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Definitions

• The current distribution, that is the magnitude of the alternating current along the length of a dipole antenna, is not necessarily uniform. Instead, it is zero at the ends, and may be highest at the center or at other points.

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Current Distribution in a λ/2 Dipole

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Definitions

• An ideal dipole is another hypothetical antenna which is useful in the study of antennas. It can be considered to be a dipole of infinitesimal length with a uniform current distribution. The theoretical characteristics of an ideal dipole approximate those of electrically small dipole antennas.

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Current Distribution in an Ideal Dipole (L <<λ)

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Definitions

• A radiation pattern is a three-dimensional, graphical representation of the far-field radiation properties of an antenna as a function of space coordinates. The far-field region is a region far enough for the radiation pattern to be independent of the distance from the antenna. The radiation pattern of a particular antenna can be measured by experiment or can be calculated, if the current distribution is known.

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Definitions

• Although the term "radiation" pattern is used, it applies just as well to receiving antennas. The reception pattern of an antenna is identical to its radiation (transmission) pattern. This is a general rule, known as the reciprocity theorem.

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Definitions

• Although the complete radiation pattern is a three-dimensional function, a pair of two-dimensional patterns are usually sufficient to characterize the directional properties of an antenna. In most cases, the two radiation patterns are measured in planes which are perpendicular to each other. A plane parallel to the electric field is chosen as one plane and the plane parallel to the magnetic field as the other. The two planes are called the E-plane and the H-plane, respectively.

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E-plane (y-z or θ) and H-plane (x-y or φ) of a Dipole

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Definitions

• The half-power beamwidth (HPBW) of an antenna is the angular separation of the points in the main beam where the power equals one-half (-3 dB) the power radiated in the direction of maximum power

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Theoretical E-plane Radiation Pattern of an Ideal Dipole Showing the Half-power Beamwidth

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Radiation Pattern Characteristics

• 3 dB beamwidth

• Sidelobes

• Nulls

• Front-to-back ratio

• Gain (approximate)

• Maximum signal position

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Far Field Conditions

• How far is far enough?

• If D >2.5λ then

• If D<λ/3 then

• If λ/3 < D < 2.5λ then rff>5D

22Drff

6.1ffr

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Antenna Field Regions

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Antenna Input Impedance

• Input Impedance (resistance + reactance)

• Radiation Resistance (corresponds to energy that is transmitted)

• Loss Resistance

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Input resistance (red line) and reactance (green line) of a dipole antenna as a function of antenna length

Antenna Input Impedance

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Baluns

• BALanced to UNbalanced device

• Similar to transformers

• Used at RF

• Usually band-limited

• Improve matching and prevent unwanted currents on coaxial cable shields

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Balun for connecting a center-fed dipole to a coaxial cable

Baluns

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Transition from a 50Ώ coaxial cable to a 300 Ώ half-wave folded dipole through a four-to-one impedance transformation balun

Baluns As Impedance Transformers

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Yagi (Parasitic) Antennas

• Remember the near-field equations• The near-fields can be used to induce currents in

adjacent antenna elements• With the proper delay (spacing and lengths),

phases can be obtained that will add in certain directions and cancel in others

• Reflector elements and director elements are thus used to give directivity to an antenna

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Six-element Yagi antenna

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Graph of gain versus the total number of elements of a typical Yagi antenna

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Elements Gain

3 8.7

4 9.9

5 10.5

6 11.1

Gain of a Yagi antenna for different numbers of elements (spacing = 0.5 λ)

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Other Antennas

• Other dipoles

• Monopole

• Helical

• Loop (electrically small & large)

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Radiation patterns of the λ dipole (a) and the 3 λ /2 dipole (b)

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(a) Dipole over a perfectly conducting plane(b) Equivalent model with image theory

Image Theory

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λ /4 monopole over perfectly conducting ground plane

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Coaxially-fed Monopole and Ground Plane

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Drooping monopole

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« Large » loop antenna (L=λ)

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« Small » loop antenna (L<<λ)

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Axial-mode helical antenna : (a) geometry, (b) pencil-beam radiation pattern

Helical Antenna (d~λ)

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Normal-mode helical antenna: (a) geometry, (b) radiation pattern

Helical Antenna (d<<λ)

42Microstrip patch with microstrip transmission-line feed

Patch Antenna

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Single Patch Antenna in the Antenna Training and Measuring System

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Microstrip Patch Array

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Overview of Lab-Volt Antenna Measurement System

• Complete system to build antennas and to measure antenna patterns

• 2 ranges of operation (1 GHz and 10GHz)• With separate generator, can be used from 1GHz

to over 30GHz• Contains wire antennas & aperture antennas

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Wire Antennas versus Aperture Antennas

• Wire antennas are usually operated at frequencies where the wavelength is larger than or approximately equal to the antenna length. Ex: dipoles, monopoles, folded dipoles…(λ≥L)

• Aperture antennas are usually operated at frequencies where the wavelength is smaller than the antenna dimension. Ex: Horn antennas, parabolic reflectors, patch antennas…(λ≤L)

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RF Generator

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Antenna Positioner

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Data Acquisition Interface

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Power Supply

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Set-up of the System

Tx AeRx (test) Ae

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System Connections

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Equipment Required

RF GeneratorAntenna PositionerData Acquisition InterfacePower Supply

Antennas 1 GHzDipolesFolded Dipoleλ /4 MonopoleLoopYagi-Uda

Antennas 10 GHzHornsOpen-Ended WaveguideHelicalSlot ArraySingle Rectangular PatchParallel-Fed Patch ArraySeries-Fed Patch Array

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Overview of Lab-Volt Multi-Beam Array Antenna (Model 9556)

• 16 element array antenna

• Uses Rotman lens to form beam

• Allows easy beam-steering

• Demonstrates array theory

57Rotman Lens realized in Microstrip

TSA s

58Ray Optics Model of the Rotman Lens

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Antenna Elements of the MBAA

• Note that antenna elements must be as omni-directional as possible to ensure as uniform a beam as possible.

• Otherwise the beam will narrow as it scans further and further out.

• (see Multibeam Array Antenna manual, pp 10-13)

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The Major Components and Important Dimensions that make up a TSA

61Different Taper Profiles used for a TSA

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Overview of Lab-Volt Log-Periodic Antenna (Model 9562)

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1 2

C

Overview of Lab-Volt Two-Element Phasing Kit (Model 9563)

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Table of Contents

• Unit 1 Basic Operation• Ex. 1-1 Basic Principles, Operation

and Adjustments• When you have completed this exercise, you will

be able to set up and operate the PAA with the Digital Radar System.

• Ex. 1-2 The True-Time Delay Rotman Lens

• When you have completed this exercise, you will understand the operation principles of the Rotman lens.

• Ex. 1-3 The Switching Matrix• When you have completed this exercise, you will

understand the operation principles of the RF Switching Matrix.

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Sample experiments

• Demonstrate software, set-up and operation.

• Ex. 1-1

• Ex. 1-2

• Ex. 2-3

• Ex. 2-4

• Ex. 3-1

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Appendix

• Other definitions & equations

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Po Power fed into a transmitting antenna (watts)

Prad Power radiated by the transmitting antenna (watts)

Radiation efficiency

For many antennas, the radiation efficiency is close to 1 (100%). However, for some antennas, such as short-wire antennas (for example, the ideal dipole), the radiation efficiency is quite small.

Radiation intensity (watts per steradian)

A steradian (sr) is a unit solid angle, and there are 4 steradians in a sphere. We can therefore define the average radiation intensity as

P

( )r ad

oPu n itless

av g

ad

W srP

/r

4

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D Directivity (unitless)

Directivity is the maximum radiation intensity in a given direction relative to the average radiation intensity (i.e. relative to the radiation intensity of an isotropic antenna transmitting the same total power).

G Antenna gain or Directive gain (unitless)

For a lossless antenna, the antenna gain or directive gain are the same as the directivity. However, for an antenna whose radiation efficiency is less than 1 (100%), there is a difference.

Dav g ad

m ax m ax

rP / 4

G D

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a Antenna beam solid angle (steradians)

a corresponds to the solid angle which would be required to radiate all the power Prad at the maximum radiation intensity level max:

An alternate definition of directivity is then:

Ae Effective area or Effective aperture (square meters)

The effective area corresponds to the effective absorbance area presented by an antenna to an incident plane wave. For an aperture antenna, it is equal to or smaller than the physical aperture. The relationship between the gain and the wavelength is (with and Ae in the same units):

P r m axad a

Da

4

G A e4

2

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ap Aperture efficiency or Antenna efficiency of an aperture antenna (unitless)

ap is the ratio between the effective area Ae and the physical area of the aperture in an aperture antenna. 50% is often a convenient approximate value to use for the aperture efficiency.

F/B Front-to-back ratio

A ratio comparing the signal strength in the desired direction of transmission or reception to the signal strength in the opposite direction. One use of this ratio is to describe the antenna's ability to discriminate between the signal coming from the front and the interfering signals coming from the rear when the antenna is used for reception.

ap

e

p

A

A

F B M ain L o b e d B B ack L o b e d B/ ( ) ( )

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37722

cff

H

E

Calculation of Intrinsic Impedance