Antennas

Simple Antennas

• Isotropic radiator is the simplest antenna mathematically

• Radiates all the power supplied to it, equally in all directions

• Theoretical only, can’t be built

• Useful as a reference: other antennas are often compared with it

Half-Wave Dipole

• Simplest practical antenna

• Actual length is typically about 95% of a half wavelength in free space

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Radiation Resistance

• Signal radiated into space appears as loss from the antenna

• Electrically this translates into a resistance• For a half-wave dipole fed in the center the

radiation resistance is approximately 70 ohms

• Antennas also have actual resistance due to their conductors

Antenna Efficiency

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Directional Characteristics

• All real antennas transmit more power in some directions than in others

• Two, two-dimensional diagrams are generally used to show radiation patterns

• Distance from the center represents radiation in different directions

• Calibration may be in dB relative to max. for that antenna, or relative to isotropic (dBi) or half wave dipole (dBd)

Antenna Gain Specifications

• dBi means decibels with respect to an isotropic radiator

• dBd means decibels with respect to an ideal half-wave dipole in its direction of maximum radiation

• The gain of a dipole is 2.14 dBi

dBd/dBi Conversion

• Gain (dBi) = Gain (dBd) + 2.14 dB

• Use dBi in Friis’s Formula

• Use dBi when it is necessary to find gain as a power ratio compared with isotropic:

Gain (ratio) = antilog (dBi/10)

• Antennas may be specified either way in catalogs, etc. (check!)

Gain and Directivity

• Directivity is a theoretical value ignoring losses

• Gain includes losses

• As a ratio,

gain = directivity efficiency

• Specifications give gain, but computer models often find directivity

EIRP and ERP

• EIRP = effective isotropic radiated power– Equal to the amount of power that would have

to be applied to an isotropic radiator to give the same power density at a given point

• ERP = effective radiated power– Equal to the amount of power that would have

to be applied to a half-wave dipole, oriented in direction of maximum gain, to give the same power density at a given point

EIRP/ERP Conversion

• EIRP = ERP + 2.14 dB

• EIRP is used in all our equations

• Sometimes government regulations specify ERP for transmitting installations

• Conversion is easy (see above)

Dipole Impedance

• At resonance, Z = 70 resistive if fed in center

• Above resonant frequency: inductive

• Below resonant frequency: capacitive

• Impedance can be raised by moving feedpoint out towards ends (delta match)

Dipole Polarization

• Polarization is same as axis of wire:– Vertical dipole is vertically polarized– Horizontal dipole is horizontally polarized

Ground Effects

• Effect of ground near antenna is important when antenna is within a few wavelengths of ground

• Very important up to and including HF, usually less important for VHF and up

• Effect of ground depends on ground characteristics and distance of antenna from ground

Reflection from Ground

• Phase shift at ground of 180 degrees• Perfectly conductive ground would reflect

all the power that hits it• Real ground is not perfectly conductive

– conductivity depends largely on moisture content

• Effect of combinining reflected and direct signals depends on distance from ground

Folded Dipole Antenna

• Same length as half wave dipole

• Uses 2 conductors

• Impedance 4 times that of normal dipole– Approximately 300 ohms at resonance

• Bandwidth is greater than single-conductor dipole

Monopole Antenna

• Vertical • Half the length of a dipole (one-quarter wave

approximately)• Ground supplies the other half• If installed above ground, a ground plane can be

used instead• For a car antenna, the car is the ground plane• Input impedance half that of a dipole, about 35

ohms

1/4 wave monopole with ground plane for 144 MHz

AM Transmitter Tower (The tower is the antenna)

Loop Antennas

• Usually small in comparison with wavelength

• Used in AM receivers and direction finders

• May be air-wound or wound on a ferrite rod

• Bidirectional as shown on next slide

5/8 Wavelength Antenna

• Lower radiation angle and higher impedance than 1/4 wave antenna

• Can be used without an efficient ground because of the high impedance

Discone Antenna

• Very wide bandwidth

• Often used for wideband receiving applications such as scanners

Discone antenna for 25-1300 MHz with whip antenna for transmitting on ham bands

Helical Antenna

• Used to produce circular polarization

• Several turns of tubing, usually with a reflector

• A variant is used for FM broadcasting

Antenna Matching

• Antennas usually are resistive at only one frequency

• Even then, resistance may not match feedline impedance

• Any of the matching schemes discussed previously can be used

Antenna Loading Coil

• When an antenna is too short an inductance can be added to increase its electrical length

• Loading coils often used at base or center of a vertical monopole

• The whole antenna can also be wound into a coil– This is often done with handheld transceivers

Loading Coil

Antenna Arrays

• Simple antennas can be combined to achieve desired directional effects

• Individual antennas are called elements and the combination is an array

Types of Arrays

• Broadside: maximum radiation at right angles to main axis of antenna

• End-fire: maximum radiation along the main axis of antenna

• Phased: all elements connected to source• Parasitic: some elements not connected to

source– They re-radiate power from other elements

Yagi-Uda Array

• Often called Yagi array

• Parasitic, end-fire, unidirectional

• One driven element: dipole or folded dipole

• One reflector behind driven element and slightly longer

• One or more directors in front of driveh element and slightly shorter

Yagi for 14, 21, 28 MHz Amateur Bands

UHF-TV Antenna: Yagi with Corner Reflector

Log-Periodic Dipole Array

• Multiple driven elements (dipoles) of varying lengths

• Phased array

• Unidirectional end-fire

• Noted for wide bandwidth

• Often used for TV antennas

VHF/UHF TV Antenna

VHF LPDA

UHF Yagi with reflector

Turnstile Antenna

• 2 dipoles

• 90 degrees between them

• fed 90 degrees out of phase

• mounted horizontally

• Gives an omnidirectional pattern in horizontal plane with horizontal polarization

Turnstile Antenna for FM Broadcast Band

Monopole Array

• Vertical monopoles can be combined to achieve a variety of horizontal patterns

• Patterns can be changed by adjusting amplitude and phase of signal applied to each element

• Not necessary to move elements– Useful for AM broadcasting

Collinear Array

• All elements along same axis

• Used to provide an omnidirectional horizontal pattern from a vertical antenna

• Concentrates radiation in horizontal plane

Broadside Array

• Bidirectional Array

• Uses Dipoles fed in phase and separated by 1/2 wavelength

End-Fire Array

• Similar to broadside array except dipoles are fed 180 degrees out of phase

• Radiation max. off the ends

Stacked Yagis

• Stacking in-phase Yagis with half-wavelength vertical spacing

• Reduces radiation above and below horizon

• Increases gain in plane of the antenna

Plane Reflector

• Mount antenna 1/4 wavelength from flat metallic surface

• Reflected wave and direct wave are in phase along normal to survace

• Increases radiation in that direction

Corner Reflector

• More focused radiation than plane reflector

• Often used with UHF TV antennas

UHF-TV Antenna: Yagi with Corner Reflector

Parabolic Reflector

• All radiation emitted at focus emerges in a beam parallel to the axis

• Gives a narrow beam

• Suitable mainly at microwave frequencies because it must be large compared with the wavelength

Parabolic Reflector Beamwidth

• Beamwidth at half-power points

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Parabolic Reflector Gain

• As a power ratio (not dB)

• With respect to isotropic

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