antennas 11

8/2/2019 Antennas 11

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ANTENNAS: PART II

For the DX University

Presented by

Pete Rimmel N8PR


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Antenna types:Wire antennas-- Dipoles

-- G5RV-- Zepp-- Off center fed

Horizontal arrays+Yagi+Quad+Wires+Commercial

Verticals- wave-Arrays

-Commercial


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Almost all antennas are based on a variation of aHalf Wave Dipole. Most antennas are compared

to the radiation characteristics of a dipole.

A half wave dipole 8 Dipole has a radiationpattern like a figure 8.

DIPOLES


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In this figure the antenna is in the vertical axis andradiation is maximal in the plane of the wire, andminimal off the ends of the antenna.
http://en.wikipedia.org/wiki/File:RadPatt-lin.png


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For a 8/2 Dipole:Length (ft.) = 468

f(MHz)

Calculating the length of a Half Wave Dipole (bare wire)(for insulated wire deduct 2-3% for Vf)
http://en.wikipedia.org/wiki/File:Dipole_antenna_in_meters.png


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Example: A dipole resonant at 14.250 MHz

468 = 32.84 ft. = 32 ft. 10 inches14.250


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VOLTAGE ON THE WIRE

CURRENT ON THE WIRE

On a Half Wave Dipolethe voltage at the feed point

is at a minimum, and ata maximum but 180

degrees out of phase atthe ends of the wire.

On the same wire, thecurrent is at a maximumat the feed point and aminimum at the ends.
http://en.wikipedia.org/wiki/File:Dipole_Antenna.jpghttp://en.wikipedia.org/wiki/File:Lambdaover2-antenna.jpghttp://en.wikipedia.org/wiki/File:Lambdaover2-antenna.jpghttp://en.wikipedia.org/wiki/File:Dipole_Antenna.jpg


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Here is why we donot try to feed fullwave antennas atthe middle, or halfwave verticals at thebottom.

We cannot feed an

antenna (generally)at a high voltagepoint. We must feed it at a voltage null which is alsoa high current point A or B in the top part of the drawing.


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Until we got privileges on the WARC bands it wasrelatively simple to build harmonic antennas whichwould work reasonably well on the original hambands.

1.8 MHz x2 = 3.6 MHz \3.6 MHz x2 = 7.2 MHz \7.1 MHz x2 = 14.2 MHz |- all harmonically7.1 MHz x3 = 21.3 MHz / related14.2 MHz x2 = 28.4 MHz /

Unfortunately, the WARC bands do not fit into thatnice, neat mathematical relationship.


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VERTICALS

Lets take that dipole and turn one of the horizontalwires 90 degrees and make it into a vertical element.


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We still have a half waveantenna, but one element is in thevertical plane, and the other one is

in the horizontal plane.

This is a basic 8/4 vertical.

Currents and voltages are the same as they are

in a half wave dipole.

Now we can replace the horizontal element withA counterpoise, radial system or ground.


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Add three more 8/4 radials and you have madea ground plane antenna. It can be a few feet abovethe ground or elevated high above ground.

The feed point impedance of this antenna isapproximately 37 Ohms. By tipping the fourradials down about 45 degrees you can get a good

50 Ohm match to coax cable


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Here the 8/4 radials of the

antenna have been replacedby a ground. The groundreflection creates an imagethat becomes the other

part of the half wave antenna.

The antenna can be fed against acounterpoise laid on the ground. Thiscan be a few wires of random length.Ideally, 120 8/4 to 8/2 radials will

create an efficient counterpoise
http://en.wikipedia.org/wiki/File:A6-3EN.jpghttp://en.wikipedia.org/wiki/File:A6-3EN.jpghttp://en.wikipedia.org/wiki/File:A6-3EN.jpghttp://en.wikipedia.org/wiki/File:A6-3EN.jpghttp://en.wikipedia.org/wiki/File:A6-3EN.jpghttp://en.wikipedia.org/wiki/File:A6-3EN.jpghttp://en.wikipedia.org/wiki/File:A6-3EN.jpghttp://en.wikipedia.org/wiki/File:A6-3EN.jpghttp://en.wikipedia.org/wiki/File:A6-3EN.jpghttp://en.wikipedia.org/wiki/File:A6-3EN.jpghttp://en.wikipedia.org/wiki/File:A6-3EN.jpghttp://en.wikipedia.org/wiki/File:A6-3EN.jpg


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Loop antennas

Loop antennas share one common factor. The endsof a dipole antenna are connected together toform a closed antenna. This antenna has more gain

broadside to it than a dipole, if in the vertical plane.It is usually 18 +5%long on the desired band.

Loop antennas are usually quieter than long wire

antennas. ie. They are less susceptible to man madenoise and static.


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The large loop antenna is similar to a dipole, except that

the ends of the dipole are connected to form a circle,triangle or square. Typically such a loop is some multiple ofa half or full wavelength in circumference.Good results can be had with a 18 loop.

A loop has a pattern similar to a dipole with the maximumradiation broadside to the plane of the loop. The minimumis in the plane of the loop.

A single full wave loop has about 3 dB gain over a dipole


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The Folded Dipole is a special case ofa closed loop antenna.

It is 1/28 long and only a few inches high.


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A horizontal loop held up equally high at 4 corners andfed with ladder line or a balun will perform well on anumber of bands. Its height above ground will

determine its vertical take off pattern. Usually, on thelow bands, it is used as a cloud warmer to talk to near instations.

It can be 18 or longer on desired bands. It must be at

least 18 long on the lowest band used.


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A round or square loop that is fed at the top orcenter of the bottom leg will be horizontally polarized.

A round or square loop that is fed in the middle ofthe side will be vertically polarized.

HORIZONTALLY POLARIZEDQUAD LOOP


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A delta loop that is fed 8/4 down from the top will bevertically polarized. If it is fed at the top or the middle of thebottom, or bottom corner, it will be horizontally polarized


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Before we talk about parasitic antennas and arrays,lets talk about various wire antennas that we canbuild and some general characteristics of some antennas:

Dipoles-- flat, sloping, inverted Vee

G5RV a special multiband dipole

Zepp end fed wire

Windom off center fed wire

Loops Square, Delta, Rectangle


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Dipole antennas work best when horizontal. They alsowork well when operated on odd multiples of a halfwavelength.


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The take off angle ofa dipole or any horizontal

antenna is dependent onits height above ground.

To the right are patterns

of a half wave dipolefrom 1/8 wave aboveground to 2 wavelengthsabove ground. This holdsfor all HF Frequencies.

These patterns occur andvary due to the groundreinforcement and

reflection of the RF.


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You can feed more than one dipole with the samefeed line, but you must keep the wires somewhatseparated to be effective, otherwise they interactand detune each other and nothing works.


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Sloping Dipoles must be resonant, and can be heldup by supporting one end higher than the other.

The feed line should be led away at right angles tothe wires. It will show a small gain in the directionthat the wire slopes.

Gain


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Inverted Vee antennas are easy to erect because theycan be held up by only one support.

It is imperative that the included angle between thetwo wires is greater than 90 degrees, otherwisecanceling will occur and the antenna will not radiate.

The inverted vee has horizontal polarization broadsideto the antenna and vertical components off the ends.


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The G5RV antenna is a special dipole that by design (oraccident of properties) allows a dipole that is fed with a

special length of ladder line, a balun and coax cable toradiate fairly well on the HF bands.

This antenna is usually 102 feet long. The ladder line

to it is 34 feet long. The two lengths added togetherin one instance create two ~8/4 wires on 80 Meters,

and other resonant lengths for the higher bands from

40 through 10 meters. It must be fed with a Balun,

to match a coax feed line.


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Connect 4:1 Balun and Coax fromHere to the Shack

G5RV Multiband Antenna


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A variant of this antenna can be fed with ladder line

all the way to a tuner in the ham shack.

It is best that this antenna be as horizontal aspossible, but sloping the ends down a bit will not

affect the antenna radiation patterns too much.

G5RV onVariousBands


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The Zepp antenna derives from the end fed wiresthat trailed the zeppelin airship. It was end fed,and unless it is an odd multiple of half wavelengths

on the band desired, it will be difficult to feed.

The longer this antenna is in wavelengths, the morethe pattern is skewed away from broadside radiation

toward a more end fire pattern.


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Question:

How do you use an antenna tuner to tune an antenna?

? ? ?


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Question:

How do you use an antenna tuner to tune an antenna?

Answer: You dont tune the antenna with a tuner.

You create a match between the transmitter and the

transmission line with a tuner. This allows the transmitterto put out its maximum power. If there is a poor match,the protection circuits for the solid state finals will cut backon the output power of your rig.

NOTE: A pi-network final in a tube rig is a built in tuner.

Only by altering the antenna do you tune it or makeit resonant.


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A Windom antenna is a wire antenna that must be

resonant. It is off center fed, which allows multi-bandoperation if fed with ladder line. Ideally, the feed pointis placed at a 8/4 away from one end on the favoriteband to be used. Hopefully, the other bands will stillfind an acceptable match relative to that feed point. It canbe fed with ladder line or a single wire feed.


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Harmonically fed dipole antennas

We know the typicalpattern of a half wavedipole a figure 8

Here is the radiationpattern when that same

wire is fed as a fullwave antenna thepattern is now like a4 leaf clover


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At Left is a 3/28

Antenna

These are highermultiple patterns.

You can see howthe patterns skewtoward the ends asthe wire gets longer.


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ARRAYS

Take the antennas we have just discussed and wecan create arrays of them to force the RF to befocused in one or two directions. This createsgain in those antennas relative to a dipole, loop

or vertical antenna.

These antennas are Yagis, Quads and phased verticalarrays. The more elements, the more gain in a given

direction. This can be good and bad.

Good: We have more ERP in a given direction.Bad: We cant hear or be heard in other directions.


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The Yagi Uda Antenna

The Yagi antenna consistsof two or more elements.

The driven element is adipole and the directorsand reflector are calledparasitic elements.They are resonant elements

and will cause the RF tobe reflected or directed ina specific direction giving theantenna gain in that direction.


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ADDITIONAL DIRECTOR

- - - - - - - - - - - - - - - - -

The yagi antenna


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A 4 element Yagi polar plot shows that the Half PowerBeamwidth (3 dB down points) is about 50 degrees.

The antenna has about 8 dB gain over a dipole (dottedline) and a front-to-back ratio of about 18 dB.

Since the pattern is broad, precise aiming is not necessary.


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A 3 element Yagi, at various heights is compared

to a dipole at similar heights. The same amountof radiated energy is seen by the area of bothcurves being equal. Only the Yagi concentratesthe energy more in one direction.


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As the number of elements increase, so does theforward gain of the Yagi. Once it is 18 aboveground, the ground effects are lessened and theantenna gain is as if it were in free space. Subtract

2.3 dB for reference to a dipole antenna.


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In the early daysof Ham Radio, whenaluminum tubingwas not readily

available, Yagis weremade of wire andsuspended betweenbamboo poles.

You could still make avery effective antennathis way, but rotatingit would be tough !!


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The Lazy H antenna (left) andthe W8JK antenna (above) areexamples of wire arrays that

were widely used on the lowbands where size makes it quitedifficult to put up an aluminum

Yagi antenna.


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The Hy Gain TH-11 covers 20, 17, 15, 12, and 10 Meters

With good results. It weighs 88 pounds. Forward gain is inThe area of 7.5 to 9.2 dBi depending on the band.

$1039.00


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The Force 12 XR-5 antenna covers 5 bands 10/20MIt weighs 56 # and has gain figures approximately4.5 dBd (6.7 dBi) on each band. It has two

active elements on each band. $1664.00


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The Cushcraft MA5B 5 band trap yagi shows gain in the3.5 to 5 dBd range for 10, 15, and 20M and unity with

the resonant dipoles on 12 and 17 M. It weighs 26 #and will handle 1200 W PEP. $489.95

Th t ib d th t 2 t 6 l t


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There are many tribanders that are 2 to 6 elementtrapped antennas. They cover the 10-15-20 MeterBands but not the WARC bands 12 & 17 M.

The reason that there are so many is that they havebeen around much longer, since they were designedbefore the WARC bands were created.

Also, they are in demand by contesters where only thenon-WARC bands are used.

To compliment them, there are duo-band trap yagis

available for 12 & 17 meters that can be added to a stack.

Also, Single band yagi antennas are available for thosewho prefer only one band, or want to stack them in aChristmas tree array, or on several different towers.


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The SteppIR series of antennas are the new generationof technology. Each element in the Yagi or verticalantenna slides inside a hollow fiberglass housing. Each

element length is continuously adjustable using a steppermotor (thus the name) to make each element the properlength for the operating frequency chosen. The adjustmentscan be made manually or automatically. You can have your

rig or logging program tell the antenna the frequency.
http://www.steppir.com/files/Vertical%20FACT%20Sheet.pdfhttp://www.steppir.com/files/DB36%20FACT%20Sheet.pdfhttp://www.steppir.com/files/3E%20FACT%20Sheet.pdf


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Quad antennas

Quad antennas are similarto the Yagi antenna in thatthey use a driven elementand a reflector and/or

directors to focus the RF ina desired direction.

The only difference is that

the elements are loops andnot dipoles or linear elements


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Being a loop antenna, the quad is a quieter receiveantenna. It has a gain figure per element a bit higherthan a Yagi. Remember a full wave loop has a gainfigure of 3 dB over a dipole to start with.

The quad is easily


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placed on multiplebands by interlacingresonant elements

on each set ofspreaders. Thedriven elements canbe driven in parallel

or individually.

Sometimes smallerspreaders are usedto mount intermediate

elements for thehigher frequencies.This creates a betterspacing between those

elements and adds gain.


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The Delta loop quad is easy to build, but not

really suited to windy South Florida. It radiatesequivalent to a diamond or square shaped 2element quad.


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Vertical ArraysJust like Yagi or Quad arrays of elements, thevertical antenna is suited to creating gain in agiven direction. By arranging the elements ina specific orientation and phasing how the RF

is delivered to the antennas, all elements areusually active and radiating.

When the signals from the various elementsmeet, they reinforce or cancel similar to waves

in a pond when several stones are thrown intoit simultaneously or in succession.

The radiation patterns are determined by element

spacing and phase lag or lead of the RF


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Except for the ground plane verticals discussed earlier,vertical antennas are usually best suited for the low bands,160, 80 and sometimes 40 Meters.

They can be 8/4 tall or trap/coil loaded to shorten them.

If you have a large area, the verticals can be put up invarious configurations to give gain, just like Yagi antennas.

2, 3, 4 or more verticals can be fed in or partially out ofphase to produce gain in a desired direction. This is atopic that could consume several nights.

1/8 1/4 3/8


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The plots shown hereare for a pair of phasedverticals fed with equal

current each, with

spacing and phase lagbetween the twoverticals as shown

1/88 1/48 3/8

8

0

45

90

135

180


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4 - wave verticals phased:

All 4 verticals are fed diagonally with two in phaseand the leading and lagging corners fed 90 degreesleading or lagging to produce gain as shown above.

A=+90deg.; B&D= 0deg.; C= -90 degrees phase.

AB

C

D

D C

A B


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Commercial, trapped verticalscan be made to perform well onmany of the HF bands. Theymust be fed against acounterpoise or ground system.

(Right) Butternut HF9V 80-6M

Vertical. $449.00


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Exceptions to the need for radials ora counterpoise are the R5, R6, R8

and MFJ verticals.

The R series are end fed half waveantennas with a high impedancematching system for a feed.

(Right) Cushcraft R8 40-6M Vertical $529.00


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The MFJ is an off-center fedvertical dipole with the trappedlower resonant parts of the

antenna rotated 90 degrees.

(Right) MFJ 1798 80-2M Vertical

$279.00

h d bl


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This extendablevertical can be usedat home or on an RV

or at field day.

It contains a flexiblewire and extends to

32 feet. Whennested it is 4 feet long.

Here an antenna tunerfeeds the vertical

against the chassis ofthe RV as a counterpoise

Under $100.00


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antennas 11

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