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Chapter 22

Antennas

Every ham needs at least one antenna,

and most hams have built one. This chap-

ter, by Chuck Hutchinson, K8CH, covers

theory and construction of antennas for

most radio amateurs. Here you’ll find

simple verticals and dipoles, as well as

quad and Yagi projects and other antennas

that you can build and use.

ANTENNA POLARIZATION

Most HF-band antennas are either verti-

cally or horizontally polarized. Although

circular polarization is possible, just as it

is at VHF and UHF, it is seldom used at

HF.

Polarization

is determined by the po-

sition of the radiating element or wire with

respect to the earth. Thus a radiator that is

parallel to the earth radiates horizontally,

while a vertical antenna radiates a vertical

wave. If a wire antenna is slanted above

earth, it radiates waves that have both a

vertical and a horizontal component.

For best results in line-of-sight com-

munications, antennas at both ends of the

circuit should have the same polarization;

cross polarization results in many deci-

bels of signal reduction. However, it is

not essential for both stations to use the

same antenna polarity for ionospheric

propagation (sky wave). This is because

the radiated wave is bent and it tumbles

considerably during its travel through the

ionosphere. At the far end of the commu-

nications path the wave may be horizon-

tal, vertical or somewhere in between at

any given instant. For that reason, the

main consideration for a good DX antenna

is a low angle of radiation rather than the

polarization.

ANTENNA BANDWIDTH

The

bandwidth

of an antenna refers gen-

erally to the range of frequencies over

which the antenna can be used to obtain a

specified level of performance. The band-

width is often referenced to some SWR

value, such as, “The 2:1

SWR bandwidth

3.5 to 3.8 MHz.” Popular amateur usage

of the term bandwidth most often refers to

the 2:1 SWR bandwidth. Other specific

bandwidth terms are also used, such as the

gain bandwidth

and the

front-to-back

ratio bandwidth.

For the most part, the lower the operat-

ing frequency of a given antenna design,

the narrower is the bandwidth. This fol-

lows the rule that the bandwidth of a reso-

nant circuit doubles as the frequency of

operation is doubled, assuming the Q is

the same for each case. Therefore, it is

often difficult to cover all of the 160 or

80-m band for a particular level of SWR

with a dipole antenna. It is important to

recognize that SWR bandwidth does not

always relate directly to gain bandwidth.

Depending on the amount of feed-line

loss, an 80-m dipole with a relatively nar-

row 2:1 SWR bandwidth can still radiate a

good signal at each end of the band, pro-

vided that an antenna tuner is used to al-

low the transmitter to load properly.

Broadbanding techniques, such as fanning

the far ends of a dipole to simulate a coni-

cal type of dipole, can help broaden the

SWR response curve.

CURRENT AND VOLTAGE

DISTRIBUTION

When power is fed to an antenna, the

current and voltage vary along its length.

The current is nearly zero (a current

node)

at the ends. The current does not actually

reach zero at the current nodes, because of

capacitance at the antenna ends. Insulators,

loops at the antenna ends, and support wires

all contribute to this capacitance, which is

also called the

end effect.

In the case of a

half-wave antenna there is a current maxi-

mum (a current

loop)

at the center.

The opposite is true of the RF voltage.

That is, there is a voltage loop at the ends,

and in the case of a half-wave antenna

there is a voltage minimum (node) at the

center. The voltage is not zero at its node

because of the resistance of the antenna,

which consists of both the RF resistance

of the wire (ohmic loss resistance) and

the

radiation resistance.

The radiation

resistance is the equivalent resistance

that would dissipate the power the

antenna radiates, with a current flowing

in it equal to the antenna current at a cur-

rent loop (maximum). The loss resistance

of a half-wave antenna is ordinarily

small, compared with the radiation resis-

tance, and can usually be neglected for

practical purposes.

IMPEDANCE

The

impedance

at a given point in the

antenna is determined by the ratio of the

voltage to the current at that point. For

example, if there were 100 V and 1.4 A of

RF current at a specified point in an

antenna and if they were in phase, the

impedance would be approximately 71

Ω.

Antenna impedance may be either re-

sistive or complex (that is, containing re-

sistance and reactance). This will depend

on whether or not the antenna is

resonant

at the operating frequency. You need to

know the impedance in order to match the

feeder to the feedpoint. Some operators

mistakenly believe that a mismatch, how-

ever small, is a serious matter. This is not

true. The importance of a matched line is

described in detail in the

Transmission

Lines

chapter of this book. The signifi-

cance of a perfect match becomes more

pronounced only at VHF and higher,

where feed-line losses are a major factor.

Some antennas possess a theoretical in-

put impedance at the feedpoint close to that

of certain transmission lines. For example,

a 0.5-λ (or half-wave) center-fed dipole,

Antennas

22.1

Table 22.1

Optimum Elevation Angles to Europe

Band

10 m

12 m

15 m

17 m

20 m

30 m

40 m

75 m

Northeast

5°

4°

11°

15°

20°

Southeast

3°

6°

7°

8°

9°

11°

15°

Upper

Midwest

3°

4°

8°

7°

8°

11°

14°

15°

Lower

Midwest

7°

6°

5°

9°

14°

11°

West

Coast

3°

5°

6°

5°

6°

8°

12°

11°

Fig 22.1 — Curves showing the

radiation resistance of vertical and

horizontal half-wavelength dipoles at

various heights above ground. The

broken-line portion of the curve for a

horizontal dipole shows the resistance

over

average

real earth, the solid line

for perfectly conducting ground.

Table 22.2

Optimum Elevation Angles to Far East

Band

Northeast

4°

7°

4°

7°

11°

12°

Southeast

5°

8°

10°

13°

12°

14°

Upper

Midwest

5°

10°

9°

11°

12°

14°

Lower

Midwest

5°

12°

10°

12°

West

Coast

6°

8°

5°

9°

13°

15°

Table 22.3

Optimum Elevation Angles to South America

Fig 22.2 — Effect of antenna diameter on

length for half-wavelength resonance,

shown as a multiplying factor, K, to be

applied to the free-space, half-

wavelength equation.

Band

10 m

12 m

15 m

17 m

20 m

30 m

40 m

75 m

Northeast

5°

4°

8°

10°

15°

Southeast

4°

5°

8°

11°

15°

Upper

Midwest

4°

6°

7°

5°

8°

9°

13°

Lower

Midwest

4°

3°

4°

3°

6°

9°

14°

West

Coast

7°

8°

7°

8°

9°

10°

14°

placed at a correct height above ground,

will have a feedpoint impedance of ap-

proximately 75

Ω.

In such a case it is prac-

tical to use a 75-Ω coaxial or balanced line

to feed the antenna. But few amateur half-

wave dipoles actually exhibit a 75-Ω im-

pedance. This is because at the lower end of

the high-frequency spectrum the typical

height above ground is rarely more than

λ.

The 75-Ω feed-point impedance is

most likely to be realized in a practical in-

stallation when the horizontal dipole is

approximately

or 1 wavelength above

ground. Coax cable having a 50-Ω charac-

teristic impedance is the most common

transmission line used in amateur work.

Fig 22.1

shows the difference between

the effects of perfect ground and typical

earth at low antenna heights. The effect of

height on the radiation resistance of a hori-

zontal half-wave antenna is not drastic so

long as the height of the antenna is greater

22.2

Chapter 22

than 0.2

λ.

Below this height, while de-

creasing rapidly to zero over perfectly con-

ducting ground, the resistance decreases

less rapidly with height over actual ground.

At lower heights the resistance stops de-

creasing at around 0.15

λ,

and thereafter

increases as height decreases further. The

reason for the increasing resistance is

that more and more of the induction field

of the antenna is absorbed by the earth

as the height drops below

λ.

CONDUCTOR SIZE

The impedance of the antenna also de-

pends on the diameter of the conductor in

relation to the wavelength, as indicated in

Fig 22.2.

If the diameter of the conductor

is increased, the capacitance per unit

length increases and the inductance per

unit length decreases. Since the radiation

resistance is affected relatively little, the

decreased L/C ratio causes the Q of the

antenna to decrease so that the resonance

curve becomes less sharp with change in

frequency. This effect is greater as the

diameter is increased, and is a property of

some importance at the very high frequen-

cies where the wavelength is small.

DIRECTIVITY AND GAIN

All antennas, even the simplest types,

exhibit directive effects in that the inten-

sity of radiation is not the same in all di-

rections from the antenna. This property

of radiating more strongly in some direc-

tions than in others is called the

directivity

of the antenna.

The

gain

of an antenna is closely related

to its directivity. Because directivity is

based solely on the shape of the directive

pattern, it does not take into account any

power losses that may occur in an actual

antenna system. Gain takes those losses

into account.

Gain is usually expressed in decibels,

and is based on a comparison with a

stan-

dard

antenna—usually a dipole or an

iso-

tropic radiator.

An isotropic radiator is a

theoretical antenna that would, if placed in

the center of an imaginary sphere, evenly

illuminate that sphere with radiation. The

isotropic radiator is an unambiguous stan-

dard, and for that reason frequently used as

the comparison for gain measurements.

When the standard is the isotropic radiator

in free space, gain is expressed in dBi.

When the standard is a dipole,

also located

in free space,

gain is expressed in dBd.

The more the directive pattern is com-

pressed—or focused—the greater the

power gain of the antenna. This is a result

of power being concentrated in some

directions at the expense of others. The

directive pattern, and therefore the gain,

of an antenna at a given frequency is de-

termined by the size and shape of the

antenna, and on its position and orienta-

tion relative to the Earth.

ELEVATION ANGLE

For long-distance HF communication,

the (vertical)

elevation angle

of maximum

radiation is of considerable importance.

You will want to erect your antenna so that

it radiates at desirable angles.

Tables 22.1,

22.2

and

22.3

show optimum elevation

angles from locations in the continental

US. These figures are based on statistical

averages over all portions of the solar sun-

spot cycle.

Since low angles usually are most

Fig 22.3 — Elevation patterns for two

40-m dipoles over average ground

(conductivity of 5 mS/m and dielectric

constant of 13) at

(33 ft) and

(66 ft) heights. The higher dipole has a

peak gain of 7.1 dBi at an elevation

angle of about 26°, while the lower

dipole has more response at high

elevation angles.

effective for long distance communica-

tions, this generally means that horizontal

antennas should be high—higher is usually

better. Experience shows that satisfactory

results can be attained on the bands above

14 MHz with antenna heights between 40

and 70 ft.

Fig 22.3

shows this effect at work

in horizontal dipole antennas.

The higher angles can be useful for

medium to short-range communications.

Dean Straw, N6BV, illustrates this in

The

ARRL Antenna Book.

Straw shows that

elevation angles between 20 and 65° are

useful on the 40 and 80-m bands over the

roughly 550-mile path between Cleveland

and Boston. Even higher angles may be

useful on shorter paths when using these

lower HF frequencies. A 75-m dipole be-

tween 30 and 70 ft high works well for

ranges out to several hundred miles. See

the

Propagation of RF Signals

chapter.

IMPERFECT GROUND

Earth conducts, but is far from being a

perfect conductor. This influences the radi-

ation pattern of the antennas that we use.

The effect is most pronounced at high verti-

cal angles (the ones that we’re least inter-

ested in for long-distance communications)

for horizontal antennas. The consequences

for vertical antennas are greatest at low

angles, and are quite dramatic as can be

clearly seen in

Fig 22.4,

where the elevation

pattern for a 40-m vertical half-wave dipole

located over average ground is compared

to one located over saltwater. At 10° eleva-

tion, the saltwater antenna has about 7 dB

more gain than its landlocked counterpart.

An HF vertical antenna may work very

well for a ham living in the area between

Dallas, Texas and Lincoln, Nebraska. This

area is pastoral, has low hills, and rich soil.

Ground of this type has very good conduc-

tivity. By contrast, a ham living in New

Hampshire, where the soil is rocky and a

poor conductor, may not be satisfied with

the performance of a vertical HF antenna.

Fig 22.4 — Elevation patterns for a

vertical dipole over sea water

compared to average ground. In each

case the center of the dipole is just

over

high. The low-angle

response is greatly degraded over

average ground compared to sea

water, which is virtually a perfect

ground.

Antennas

22.3

Dipoles and the Half-Wave Antenna

A fundamental form of antenna is a wire

whose length is half the transmitting

wavelength. It is the unit from which many

more complex forms of antennas are con-

structed and is known as a

dipole antenna.

The length of a half-wave in free space is

From Fig 22.2, K = 0.945 for this ratio.

The length of the antenna, from equation 3

492 0.945

50.1

9.28 ft

Length (ft)

492

f (MHz)

(1)

or 9 ft 3

inches. The answer is obtained

directly in inches by substitution in equa-

tion 4

5904 0.945

111.4 in

50.1

The actual length of a resonant

-λ

antenna will not be exactly equal to the

half wavelength in space, but depends on

the thickness of the conductor in relation

to the wavelength. The relationship is

shown in Fig 22.2, where K is a factor that

must be multiplied by the half wavelength

in free space to obtain the resonant antenna

length. An additional shortening effect

occurs with wire antennas supported by

insulators at the ends because of the ca-

pacitance added to the system by the insu-

lators. This shortening is called end effect.

The following formula is sufficiently ac-

curate for wire antennas for frequencies

up to 30 MHz.

Length of half-wave antenna (ft)

dipole is

or greater above earth and

is not degraded by nearby conductive

objects. This assumption is based also on

a symmetrical feed system. In practice, a

coaxial feed line may distort this pattern

slightly, as shown in Fig 22.5. Minimum

horizontal radiation occurs off the ends of

the dipole if the antenna is parallel to the

earth.

As a horizontal antenna is brought

closer to ground, the elevation pattern

peaks at a higher elevation angle as shown

in Fig 22.3.

Fig 22.6

illustrates what hap-

492 0.95

f (MHz)

468

f (MHz)

(2)

Example: A half-wave antenna for

7150 kHz (7.15 MHz) is 468/7.15 = 65.45

ft, or 65 ft 5 inches.

Above 30 MHz use the following for-

mulas, particularly for antennas con-

structed from rod or tubing. K is taken

from Fig 22.2.

The length of a half-wave antenna is also

affected by the proximity of the dipole ends

to nearby conductive and semiconductive

objects. In practice, it is often necessary to

do some experimental

pruning

of the wire

after cutting the antenna to the computed

length, lengthening or shortening it in

increments to obtain a low SWR. When the

lowest SWR is obtained for the desired part

of an amateur band, the antenna is resonant

at that frequency. The value of the SWR

indicates the quality of the match between

the antenna and the feed line. If the lowest

SWR obtainable is too high for use with

solid-state rigs, a Transmatch or line-input

matching network may be used, as de-

scribed in the

Transmission Lines

chap-

ter.

RADIATION CHARACTERISTICS

The radiation pattern of a dipole

antenna in free space is strongest at right

angles to the wire (Fig

22.5).

This figure-

8 pattern appears in the real world if the

Length (ft)

492 K

f (MHz)

5904 K

f (MHz)

(3)

Length (in)

(4)

Example: Find the length of a half-wave

antenna at 50.1 MHz, if the antenna is

made of

-inch-diameter tubing. At

50.1 MHz, a half wavelength in space is

492

9.82 ft

50.1

The ratio of half wavelength to conduc-

tor diameter (changing wavelength to

inches) is

(9.82 ft 12 in )

0.5 in

22.4

235.7

Fig 22.5 — Response of a dipole

antenna in free space, where the

conductor is along 90° to 270° axis,

solid line. If the currents in the halves of

the dipole are not in phase, slight

distortion of the pattern will occur,

broken line. This illustrates the case

where a balun is not used on a balanced

antenna fed with unbalanced line.

Fig 22.6 — At A, elevation response

pattern of a dipole antenna placed

above a perfectly conducting ground.

At B, the pattern for the same antenna

when raised to one wavelength. For

both A and B, the conductor is coming

out of the paper at right angle. C shows

the azimuth patterns of the dipole for

the two heights at the most-favored

elevation angle, the solid-line plot for

the

-λ height at an elevation angle of

30°, and the broken-line plot for the 1-λ

height at an elevation angle of 15°. The

conductor in C lies along 90° to 270°

axis.

Chapter 22

pens to the directional pattern as antenna

height changes. Fig 22.6C shows that

there is significant radiation off the ends

of a low horizontal dipole. For the

-λ

height (solid line), the radiation off the

ends is only 7.6 dB lower than that in the

broadside direction.

FEED METHODS

Most amateurs use either

coax

open-

wire

transmission line. Coax is the com-

mon choice because it is readily available,

its characteristic impedance is close to that

of the antenna and it may be easily routed

through or along walls and among other

cables. The disadvantages of coax are in-

creased RF loss and low working voltage

(compared to that of open-wire line). Both

disadvantages make coax a poor choice for

high-SWR systems.

Take care when choosing coax. Use

-inch foam-dielectric cables only for

low power (25 W or less) HF transmis-

sions. Solid-dielectric

-inch cables are

okay for 300 W if the SWR is low. For

high-power installations, use

-inch or

larger cables.

The most common two-wire transmis-

sion lines are

ladder line

and

twin lead.

Since the conductors are not shielded,

two-wire lines are affected by their envi-

ronment. Use standoffs and insulators to

keep the line several inches from struc-

tures or other conductors. Ladder line has

very low loss (twin lead has a little more),

and it can stand very high voltages (high

SWR) as long as the insulators are clean.

Two-wire lines are usually used in bal-

anced systems, so they should have a balun

at the transition to an unbalanced trans-

mitter or coax. A Transmatch will be

needed to match the line input impedance

to the transmitter.

BALUNS

A balun is a device for feeding a bal-

anced load with an unbalanced line, or vice

versa (see the Transmission Lines chapter

of this book). Because dipoles are balanced

(electrically symmetrical about their feed-

points), a balun is often used at the feed-

point when a dipole is fed with coax. When

coax feeds a dipole directly (as in

Fig 22.7),

current flows on the outside of the cable

shield. The shield can conduct RF onto the

transmitter chassis and induce RF onto

metal objects near the system. Shield cur-

rents can impair the function of instruments

connected to the line (such as SWR meters

and SWR-protection circuits in the trans-

mitter). The shield current also produces

some feed-line radiation, which changes

the antenna radiation pattern, and allows

objects near the cable to affect the antenna-

system performance.

Fig 22.7 — Method of affixing feed line

to the center of a dipole antenna. A

plastic block is used as a center

insulator. The coax is held in place by a

clamp. A balun is often used to feed

dipoles or other balanced antennas to

ensure that the radiation pattern is not

distorted. See text for explanation.

Fig 22.8 — Center-fed multiband

Zepp

antenna at A and an end-fed Zepp at B.

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