special purpose antenna 3

8/4/2019 Special Purpose Antenna 3

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Folded Dipole Yagi-Uda

Log-PeriodicTurnstile

Helical


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Folded Dipole

Antenna


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Design L = 143/f

where:

L is in meters

f is in MHz

L = 468/f

where:L is in feet

f is in MHz


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Design folded dipole uses a 300 ohm balanced feeder line

elements can be any conductor from simple wires to copper oraluminum tubing

tips of the antenna are folded back until they almost meet at thefeedpoint, such that the antenna comprises one entire wavelength


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Where:d1 is the conductor diameter for the feed arm of the dipoled2 is the conductor diameter for the non-fed arm of the dipoleS is the distance between the conductorsratio is the step up ratio

Design


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Voltage and Current


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Description single antenna made up of two elements

a dipole antenna whose outer ends are folded back and

joined together at the center; the impedance is about 300 ohms, as compared to 70

ohms for a single-wire dipole;


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Advantages

wider bandwidthcompare to the basic half-wave dipole the bandwidth can be increased further by making the

dipole elements larger in diameter

well matched to 300-Ohm balanced transmission lines


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Application used in the simple FM dipole antennas that can be bought

to use as temporary FM broadcast antennas

used within other larger antennas such as the Yagi.

widely used with television and frequency-modulationreceivers.


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The Elements of a Yagi

The Driven Element

where feed point is locatedand where the feed line isattached from the transmitterto the Yagi to perform thetransfer of power from thetransmitter to the antenna

resonant when the electricallength is 1/2 of thewavelength, for the usedfrequency, applied to the feedpoint


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Parasitic Elements

- element that is not directly

connected to the feed line

- elements used for the purpose ofobtaining directional power gain

- reradiate power which theyreceive from the driven element

Directors

Reflectors


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Parasitic Elements

Directors length is 5% shorter than

the driven element

used to provide the antennawith directional pattern andgain

the spacing is generallybetween .1 and .2wavelength


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Parasitic Elements

Reflector

length is 5% longer thanthe driven element

the spacing is generallybetween .1 and .2wavelength

Prevents antenna fromsending backward

protect the antennaagainst the parasiticsignals coming from therear


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Design Guidelines typical spacing between elements vary from about 0.1 to 0.2

of a wavelength, depending on the specific design

elements are usually parallel in one plane, supported on asingle crossbar known as a boom.

the elements must be made of some kind of conducting material:copper, aluminum, coaxial cable are best suited.

with coaxial cable, we can use only the shield or connect the shieldwith the inner conductor.

the horizontal stick (boom) can be made of anything.

if it is made of conducting material, the elements must be isolated fromthe stick.


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Design Guidelines


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Design Guidelines


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Design Guidelines

Driven Element

typically a /2dipole or foldeddipole

electrically

connected tothe feedline

Director/s

length is 5% shorterprogressively than thedriven element

Reflector

length is5% longerthan thedrivenelement


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The Elements


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A simple three element Yagi


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Design Equal spaced, equal length directors may give higher gain at a

particular frequency, but the bandwidth is more narrow andlarger sidelobe levels are created.

Wide spacing will increase the bandwidth, but the sidelobesbecome large.

By varying both the spacing and director lengths the patternand the pattern bandwidth may be more controlled.

More directors within a given boom length won't increase thegain by any great amount, but will give better control of theantenna's pattern over a wider range of frequencies in the bandof design.

If the length of each succeeding director is reduced by a set

factor (%), and increase the spacing of each succeedingdirector by another factor, a very clean pattern with goodpattern bandwidth can be obtained. The trade off will be a smallloss in the optimum forward gain (10% to 15%).

http://www.hamuniverse.com/yagibasics.html


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Description a linear array consisting of a dipole and two or more parasitic

elements: one reflector and one or more directors

a directional antenna designed to maximize reception over longdistances

the typical directivity is between 7dB and 9dB named after two Japanese scientists who invented it


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Advantages bandwidth can be increased by using more than one folded dipole,

each cut to a slightly different length

a directional antenna designed to maximize reception over longdistances

the typical directivity is between 7dB and 9dB


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Application commonly used for VHF TV reception because of its wide

bandwidth used for amateur radio used during World War II for airborne radar sets


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Log Periodic Antenna


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Design

Typically,

Typically,


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Design Bandwidth:

Number of Dipoles:


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Design the length and spacing of the elements of a

log-periodic antenna increase logarithmicallyfrom one end to the other

diminish in size from the back towards thefront

element at the back of the array is a halfwavelength at the lowest frequency ofoperation

element spacing decrease towards the front ofthe array

the feed phase is reversed from one elementto the next to ensure that the phasing of thedifferent elements is correct

the direction of maximum radiation is towardsthe feed point


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Description Consist of several dipoles of different length and spacing that are

fed from a single source at the small end. The transmission line iscrisscrossed between the feedpoints of adjacent pairs of dipoles.

Physical structure is repetitive, making electrical characteristics

repetitive as well the radiation pattern of this RF antenna design stays broadly the

same over the whole of the operating band as do parameters likethe radiation resistance and the standing wave ratio

it offers less gain for its size than does the more conventional Yagi


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Description the antenna input

impedance variesrepetitively when

plotted as a function offrequency andperiodically whenplotted against the logof the frequency


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Advantages independent of radiation resistance and radiation pattern to

frequency able to provide directivity and gain while being able to operate

over a wide bandwidth


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Application used where a wide range of frequencies is needed while still having

moderate gain and directionality

used on the HF portion of the spectrum where operation is requiredon a number of frequencies to enable communication to be

maintained used at VHF and UHF for a variety of applications, including some

uses as a television antenna


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Turnstile Antenna


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Design consists of two horizontal half-

wave antennas mounted atright angles to each other in

the same horizontal plane


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Design Radiation Pattern

The radiation pattern shown is the sum of radiation patterns from the

two dipoles, which produces a nearly omnidirectional pattern


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Description a set of two dipole antennas aligned at right angles to each other and

fed 90 degrees out-of-phase

When mounted horizontally the antenna is nearly omnidirectional onthe horizontal plane.

When mounted vertically the antenna is directional to a right angle toits plane and is circularly polarized.


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Application developed primarily for omnidirectional vhf communications

often used for communication satellites because, being circularlypolarized, the polarization of the signal doesn't rotate when thesatellite rotates


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Helical Antenna


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Design The dimensions of the helix are determined

by the wavelength of the radio wavesused, which depends on the frequency.

The length of the coil determineshow directional the antenna will be as wellas its gain; longer antennas will be moresensitive in the direction in which theypoint.

a ground plane at the driven end makesthe radiation unidirectional from the far

(open) end


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Design


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Design pitch angle :

where:

s is the spacing from turn to turn

r is the radius of the helix


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Design The ground plane diameter is

typically 0.94l in diameter at thecenter frequency

the radiating element is a helix ofwire, driven at one end andradiating along the axis of the helix


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Power Gain

where:Ap(dB) = antenna power gain (dB)

D = helix diameter (meters)

N = number of turns (any positive integer)

S = pitch(meters)

= wavelength (meters per cycle)


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3-dB Beamwidth

where:

= beamwidth (degrees)

D = helix diameter (meters)

N = number of turns (any positive integer)

S = pitch(meters) = wavelength (meters per cycle)


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Description An antenna that has the form of a helix A helical antenna is a specialized antenna that emits and responds

to electromagnetic fields with rotating (circular)polarization


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Application typically used for applications where reduced size is a critical

operational factor

most suitable for Mobile and PortableHigh-frequency(HF) communications inthe 1MHz to 30MHz operating range

used extensively for 27MHzCB radio

special purpose antenna 3

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