Antennas Notes

8/12/2019 Antennas Notes

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AntennasAntenna- a device used to efficiently transmit and/or receive electromagnetic waves.

Antenna Types Wire antennas (monopoles, dipoles, loop, etc.) Aperture antennas (sectoral horn, pyramidal horn, slots, etc.) Reflector antennas (parabolic dish, corner reflector, etc.) Lens antennas Microstrip antennas Antenna arrays

Antenna Performance Parameters

Radiation pattern- angular plot of the radiation.

Omni directional pattern - uniform radiation in one plane

Directive patterns - narrow beam(s) of high radiation

Directivity - ratio of antenna power density at a distant point relative to that of an

isotropic radiator [isotropic radiator- an antenna that radiates uniformly in all directions(point source radiator)].

Gain- directivity reduced by losses.

Polarization- trace of the radiated electric field vector (linear, circular, elliptical).

Impedance - antenna input impedance at its terminals.

Bandwidth - range of frequencies over which performance is acceptable (resonant

antennas, broadband antennas).

Beam scanning - movement in the direction of maximum radiation by mechanical or

electrical means.

Other system design constraints- size, weight, cost, power handling, etc.

Antenna Radiation Patterns

Antenna Pattern is a graphical representation of the antenna radiation properties as afunction of position (spherical coordinates).

Common Types of Antenna Pattern

Power Pattern- normalized power vs. spherical coordinate position. Field Pattern- normalized E-Field or H-Field vs. spherical coordinate position.


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Antenna Field Types

Reactive field - the portion of the antenna field characterized by standing(stationary) waves which represent stored energy.

Radiation field - the portion of the antenna field characterized by radiating(propagating) waves which represent transmitted energy.Antenna Field Regions

Reactive Near Field Region - the region immediately surrounding the antennawhere the reactive field (stored energy standing waves) is dominant.

Near-Field (Fresnel) Region- the region between the reactive nearfield and thefar-field where the radiation fields are dominant and the field distribution is

dependent on the distance from the antenna.

Far-Field (Fraunhofer) Region - the region farthest away from the antennawhere the field distribution is essentially independent of the distance from the

antenna (propagating waves).

Antenna Pattern Definitions Isotropic Pattern - an antenna pattern defined by uniform radiation in all

directions, produced by an isotropic radiator (point source, a non-physical antenna

which is the only non-directional antenna).

Directional Pattern - a pattern characterized by more efficient radiation in onedirection than another (all physically realizable antennas are directional antennas).

Omnidirectional Pattern- a pattern which is uniform in a given plane.


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Principal Plane Patterns - the E-plane and H-plane patterns of a linearly polarized

antenna.

E-plane - the plane containing the electric field vector and the direction ofmaximum radiation.

H-plane - the plane containing the magnetic field vector and the direction ofmaximum radiation.

Antenna Pattern Parameters

Radiation Lobe- a clear peak in the radiation intensity surrounded by regions ofweaker radiation intensity.

Main Lobe(major lobe, main beam) - radiation lobe in the direction of maximumradiation.

Minor Lobe- any radiation lobe other than the main lobe. Side Lobe - a radiation lobe in any direction other than the direction(s) of

intended radiation.

Back Lobe- the radiation lobe opposite to the main lobe. Half-Power Beamwidth (HPBW) - the angular width of the main beam at the

half-power points.

First Null Beamwidth(FNBW) - angular width between the first nulls on eitherside of the main beam.

Directivity (D)- the ratio of the radiation intensity in a given direction from the antenna

to the radiation intensity averaged over all directions. The directivity of an isotropicradiator is equal to 1.


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Antenna Efficiency

When an antenna is driven by a voltage source (generator), the total power radiated by

the antenna will not be the total power available from the generator. The loss factors

which affect the antenna efficiency can be identified by considering the common example

of a generator connected to a transmitting antenna via a transmission line as shownbelow.

Zg - source impedance

ZA - antenna impedanceZo - transmission line characteristic impedance

Pin - total power delivered to the antenna terminals

Pohmic - antenna ohmic (I2R) losses[Conduction loss + dielectric loss]

Prad - total power radiated by the antenna

The total power delivered to the antenna terminals is less than that available from the

generator given the effects of mismatch at the source/T-line connection, losses in the T-

line, and mismatch at the T-line/antenna connection. The total power delivered to theantenna terminals must equal that lost to I2R (ohmic) losses plus that radiated by the

antenna.

Pin= Prad+ Pohmic

Antenna Radiation Efficiency (Are) is thus defined as: PradPin

This is usually less than unitybecause of the conduction, dielectric and reflection losses.

Antenna Gain

The definitions of antenna directivityand antenna gainare essentially the same except forthe power terms used in the definitions.

Directivity [D] - ratio of the antenna radiated power density at a distant point to the total

antenna radiated power (Prad) radiated isotropically.

Gain [G] - ratio of the antenna radiated power density at a distant point to the total

antenna input power (Pin) radiated isotropically.


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Thus, the antenna gain, being dependent on the total power delivered to the antenna inputterminals, accounts for the ohmic losses in the antenna while the antenna directivity,

being dependent on the total radiated power, does not include the effect of ohmic losses.

Antenna Impedance

The antenna impedance is a complex term defined in terms of resistive (real) and reactive(imaginary) components.

ZA = RA+jXARA- Antenna resistance dissipation or ohmic losses + radiation

XA - Antenna reactance - energy storage in the antenna near field

The antenna resistance (RA) is defined as the sum of two resistances which separatelyrepresent the ohmic losses and the radiation.

RA= Rr+RL

Rr- Antenna radiation resistance (radiation)

RL- Antenna loss resistance (ohmic loss)

The typical transmitting system can be defined by a generator, transmission line and

transmitting antenna as shown below.

The generator is modeled by a complex source voltage Vg and complex sourceimpedanceZg.

Antenna Loss ResistanceThe antenna loss resistance (conductor and dielectric losses) for many antennas is

typically difficult to calculate. In these cases, the loss resistance is normally measured

experimentally. However, the loss resistance of wire antennas can be calculated easilyand accurately.

Assuming a conductor of length l and cross-sectional area A which carries a uniform

current density, the DC resistance is

RDC= l/A

where is the conductivity of the conductor.


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At high frequencies, the current tends to crowd toward the outer surface of the conductor(skin effect). The HF resistance can be defined in terms of the skin depth .

= l ;

(f)

where is the permeability of the material andf is the frequency in Hz.

The skin depth for copper (= 5.8107s/m, = 0= 410

-7H/m) may be written as:

Antenna PolarizationThe polarization of a plane wave is defined by the figure traced by the instantaneous

electric field at a fixed observation point. The following are the most commonly

encountered polarizations assuming the wave is approaching.

The polarization of the antenna in a given direction is defined as the polarization of thewave radiated in that direction by the antenna. Note that any of the previous polarizationfigures may be rotated by some arbitrary angle.


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ApertureA useful parameter calculating the receive power of an antenna is the effective areaor

effective aperture. Assume that a plane wave with the same polarization as the receive

antenna is incident upon the antenna. Further assume that the wave is traveling towardsthe antenna in the antenna's direction of maximum radiation (the direction from which the

most power would be received).

Then the effective aperture parameter describes how much power is captured from agiven plane wave. Let p be the power density of the plane wave (in W/m^2). If Pt

represents the power (in Watts) at the antennas terminals available to the antenna's

receiver, then:

Hence, the effective area simply represents how much power is captured from the plane

wave and delivered by the antenna. This area factors in the losses intrinsic to the antenna

(ohmic losses, dielectric losses, etc.).


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A general relation for the effective aperture in terms of the peak antenna gain (G) of any

antenna is given by:

Effective aperture or effective area can be measured on actual antennas by comparison

with a known antenna with a given effective aperture, or by calculation using the

measured gain and the above equation.

Effective aperture will be a useful concept for calculating received power from a planewave. To see this in action, go to the next section on the Friis transmission formula.

Antenna TemperatureThis is a parameter that describes how much noise an antenna produces in a given

environment. This temperature is not the physical temperature of the antenna. Moreover,

an antenna does not have an intrinsic "antenna temperature" associated with it; rather the

temperature depends on its gain pattern and the thermal environment that it is placed in.Antenna temperature is also sometimes referred to as Antenna Noise Temperature.

Antenna ArraysAn antenna array (often called a 'phased array') is a set of 2 or more antennas. Thesignals from the antennas are combined or processed in order to achieve improved

performance over that of a single antenna. The antenna array can be used to:

increase the overall gain provide diversity reception cancel out interference from a particular set of directions "steer" the array so that it is most sensitive in a particular direction determine the direction of arrival of the incoming signals to maximize the Signal to Interference Plus Noise Ratio (SINR)

Demerits of the Antenna arrays Cost Size Complexity

Array Design Variables General array shape (linear, circular, planar, etc.). Element spacing. Element excitation amplitude. Element excitation phase. Patterns of array elements.


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Types of Arrays

Active and Passive Arrays

An active array is one in which an active element (oscillator ,amplifier ,or mixer) isconnected to the path of each radiator. These elements, along with the radiator, form the

array module. Active antenna arrays are categorized as receiving, transmitting, and

transceiving. Active antenna array advantages include the capability to increase radiatedpower, decrease thermal losses, increase reliability, and reduce the length of the paths

between radiators and transceiving circuits.

A passive array is one in which all elements are excited from a common oscillator or

connected to a common receiver. Therefore, an immutable part of a passive array is the

feed network connecting the elements. Passive antenna arrays are categorized as

receiving, transmitting, and transceiving. They are finding wide use in variable purposeradars.

Linear ArraysA linear array is one consisting of a group of identical elements placed in one dimension

along a given direction. Linear arrays may have equidistant or nonequidistant element

spacings. They are used in the analysis of the directional properties of arrays in antennatheory, and as building blocks for forming an array of arrays.

Planar Arrays

A planar array has all elements located in a single plane occupying a definite area. Planararrays have different configurations of elements. rectangular, triangular, or hexagonal, in

which the elements are positioned at the vertices of the rectangles, right triangles,

regular hexagons and also at the center of the hexagon .

Adaptive Arrays

An adaptive array consists of an N-element array (usually in the receiving mode), wherethe useful signal is maximized based on an analysis of the signal to interference ratio. Animportant aspect of adaptive arrays is the appropriate choice of weighting coefficients

W(t), which are placed between the antenna elements and a combining network. In the

general case, the vector W(t) must have the capability of changing the amplitude and

phase of the received signal from each element. The rates of change must correspond tothe rates of change of the signal to interference ratio, and the range of change must

correspond to the dynamic range of the levels of signal and interference, and the range ofphase relationships between the different array elements.

The criteria for array performance in suppression of interference may be:-

The maximum ratio of signal to interference at the output of the array.


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The minimum mean square deviation of the received signal from the givenreference level at the output of the array

Minimum interference power at the array output The maximum probability of detection of the desired target signal

Cylindrical ArraysA cylindrical array is one whose radiators are positioned on a cylindrical surface. Theradiators used are wire and slot dipoles, open ended waveguides and horns, and spiral and

dielectric rod antennas. The selection of the type of radiators depends on the wavelength

and the required bandwidth, on the application and the operating conditions, and on theconstruction requirements of the array. To obtain a narrow beamwidth, the number of

elements must be close to 104. Cylindrical arrays are used in cases where azimuthal

scanning with a constant beam shape and gain is required.

Antenna Size Electrical Size of an Antenna- the physical dimensions of the antenna defined

relative to wavelength.

Electrically small antenna- the dimensions of the antenna are small relative towavelength.

Electrically large antenna - the dimensions of the antenna are large relative towavelength.

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Antennas Notes