antenna_project.pdf

Antennas & Projects

ANTENNA BASICS

very ham needs at least one antenna, and most hams have built one. This chapter, 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.

The amount of available space should be high on the list of factors to consider when selecting an

antenna. Those who live in urban areas often must accept a compromise antenna for the HF bands

because a city lot won’t accommodate full-size wire dipoles, end-fed systems or high supporting struc-

tures. Other limitations are imposed by the amount of money available for an antenna system (including

supporting hardware), the number of amateur bands to be worked and local zoning ordinances.

Operation objectives also come into play. Do you want to dedicate yourself to serious contesting and

DXing? Are you looking for general-purpose operation that will yield short- and long-haul QSOs during

periods of good propagation? Your answers should result in selecting an antenna that will meet your

needs. You might want to erect the biggest and best collection of antennas that space and finances will

allow. If a modest system is the order of the day, then use whatever is practical and accept the perfor-

mance that follows. Practically any radiator works well under some propagation conditions, assuming

the radiator is able to accept power and radiate it at some useful angle. Any antenna is a good one if it

meets your needs!

In general, the height of the antenna above ground is the most critical factor at the higher end of the

HF spectrum, that is from roughly 14 through 30 MHz. This is because the antenna should be clear of

conductive objects such as power lines, phone wires, gutters and the like, plus high enough to have a low

radiation angle. Lower frequency antennas, operating between 2 and 10 MHz, should also be kept well

away from conductive objects and as high above ground as possible if you want good performance.

Antenna Polarization

Most HF-band antennas are either vertically or horizontally polarized, although circular polarization

is possible, just as it is at VHF and UHF. Polarization is determined by the position of the radiating

element or wire with respect to the earth. Thus a radiator that is parallel to the earth radiates horizontally,

while an antenna at a right angle to the earth (vertical) radiates a vertical wave. If a wire antenna is slanted

above earth, it radiates waves that have both a vertical and a horizontal component.

Antennas & Projects 20 . 1

For best results in line-of-sight communications, antennas at both ends of the circuit should have the

same polarization; cross polarization results in many decibels of signal reduction. 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 communications path the wave may be horizontal, vertical or somewhere in between at any

given instant. On multihop transmissions, in which the signal is refracted more than once from the

ionosophere, and subsequently reflected from the Earth’s surface during its travel, considerable polar-

ization shift will occur. 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 generally to the range of frequencies over which the antenna can

be used to obtain good performance. The bandwidth is often referenced to some SWR value, such as,

“The 2:1 SWR bandwidth is 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 operating frequency of a given antenna design, the narrower is the

bandwidth. This follows the rule that the bandwidth of a resonant 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 narrow 2:1 SWR bandwidth can still radiate

a good signal at each end of the band, provided that an antenna tuner is used to allow the transmitter to

load properly. Broadbanding techniques, such as fanning the far ends of a dipole to simulate a conical

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 maximum (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

current loop (maximum). The loss resistance of a half-wave antenna is ordinarily small, compared with

the radiation resistance, and can usually be neglected for practical purposes.

Antenna impedance may be either resistive or complex (that is, containing resistance 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, however small, is a serious matter. This is not true. The importance of a matched line is

Ω

20 . 2 Chapter 20

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

described in detail in the Transmission Lines chapter of this book. The significance 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 input impedance at the feedpoint close to that of certain trans-

mission lines. For example, a 0.5-

Ω

. In such a case it is practical to use a

75-

Ω

coaxial or balanced line to feed the antenna. But few amateur half-wave dipoles actually exhibit

impedance. This is because at the lower end of the high-frequency spectrum the typical height

above ground is rarely more than 1 / 4

Ω

feed-point impedance is most likely to be realized in

a practical installation when the horizontal dipole is approximately 1 / 2 , 3 / 4 or 1 wavelength above

ground. Coax cable having a 50-

. The 75-

Ω

characteristic impedance is the most common transmission line used

in amateur work.

Fig 20.1 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 horizontal half-wave an-

tenna is not drastic so long as the height of the antenna is greater

than 0.2

, and thereafter increases

as height decreases further. The reason for the increasing resis-

tance is that more and more of the induction field of the antenna

is absorbed by the earth as the height drops below 1 / 4

Conductor Size

The impedance of the antenna also depends on the diameter of

the conductor in relation to the wavelength, as indicated in

Fig 20.2 . If the diameter of the conductor is increased, the capaci-

tance 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 de-

crease 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 frequencies

where the wavelength is small.

Fig 20.1—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.

Directivity and Gain

All antennas, even the simplest types, exhibit directive effects

in that the intensity of radiation is not the same in all directions

from the antenna. This property of radiating more strongly in

some directions than in others is called the directivity of the an-

tenna.

The gain of an antenna is closely related to its directivity. Be-

cause directivity is based solely on the shape of the directive pat-

tern, it does not take into account any power losses that may occur

in an actual antenna system. Gain takes into account those losses.

Gain is usually expressed in decibels, and is based on a compari-

son with a “standard” antenna—usually a dipole or an isotropic

radiator . An isotropic radiator is a theoretical antenna that would,

Fig 20.2—Effect of antenna

diameter on length for half-

wavelength resonance, shown as

a multiplying factor, K, to be

applied to the free-sp ace, h alf-

wavelength equation.

Antennas & Projects 20 . 3

(or half-wave) center-fed dipole, placed at a correct height above

ground, will have a feedpoint impedance of approximately 75

a 75-

. Below this height, while decreasing rapidly to zero

over perfectly conducting ground, the resistance decreases less

rapidly with height over actual ground. At lower heights the resis-

tance stops decreasing at around 0.15

if placed in the center of an imaginary sphere, evenly illuminate that sphere with radiation. The isotropic

radiator is an unambiguous standard, and so is 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 compressed—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 determined by the size and shape

of the antenna, and on its position and orientation relative to the Earth.

Elevation Angle

For HF communication, the

vertical (elevation) angle of

maximum radiation is of consid-

erable importance. You will want

to erect your antenna so that it

radiates at desirable angles.

Tables 20.1 , 20.2 and 20.3 show

optimum elevation angles from

locations in the continental US.

These figures are based on statis-

tical averages over all portions of

the solar sunspot cycle.

Since low angles usually are

most effective, this generally

means that horizontal antennas

should be high—higher is usually

better. Experience shows that sat-

isfactory results can be attained

on the bands above 14 MHz with

antenna heights between 40 and

70 ft. Fig 20.3 shows this effect

at work in horizontal dipole an-

tennas.

Table 20.1

Optimum Elevation Angles to Europe

Upper Lower West

Band Northeast Southeast Midwest Midwest Coast

10 m

5°

3°

7°

3°

12 m

5°

6°

4°

6°

5°

15 m

5°

7°

8°

5°

6°

17 m

4°

8°

7°

5°

20 m

11°

9°

8°

5°

6°

30 m

11°

9°

8°

40 m

15°

14°

12°

75 m

20°

15°

11°

Table 20.2

Optimum Elevation Angles to Far East

Upper Lower West

Band Northeast Southeast Midwest Midwest Coast

10 m 4°

5°

6°

12 m 4°

8°

5°

12°

6°

15 m 7°

10°

8°

17 m 7°

10°

9°

10°

5°

20 m 4°

10°

9°

10°

9°

30 m 7°

13°

11°

12°

9°

40 m 11°

12°

13°

75 m 12°

14°

12°

15°

Imperfect Ground

Earth conducts, but is far from

being a perfect conductor. This

influences the radiation pattern

of the antennas that we use. The

effect is most pronounced at high

vertical angles (the ones that

we’re least interested in for long-

distance communications) for

horizontal antennas. The conse-

quences for vertical antennas are

greatest at low angles, and are

quite dramatic as can be clearly

seen in Fig 20.4 , where the eleva-

Table 20.3

Optimum Elevation Angles to South America

Upper Lower West

Band Northeast Southeast Midwest Midwest Coast

10 m 5°

4°

7°

12 m 5°

5°

6°

3°

8°

15 m 5°

5°

7°

4°

8°

17 m 4°

5°

3°

7°

20 m 8°

8°

6°

8°

30 m 8°

11°

9°

40 m 10°

11°

9°

10°

75 m 15°

15°

13°

14°

20 . 4 Chapter 20

(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.

(33 ft) and 1 / 2

high.

The low-angle response is

greatly degraded over average

ground compared to sea water,

which is virtually a perfect

ground.

tion pattern for a 40-m vertical

half-wave dipole located over

average ground is compared to

one located over saltwater. At

10° elevation, the saltwater an-

tenna has about 7 dB more gain

than its landlocked counterpart.

A vertical antenna may work

well at HF 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 conductivity. 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 verti-

cal HF antenna.

Antennas & Projects 20 . 5

Fig 20.3—Elevation patterns for

two 40-m dipoles over average

ground (conductivity of 5 mS/m

and dielectric constant of 13) at

1 / 4

Fig 20.4—Elevation patterns for

a vertical dipole over sea water

compared to average ground. In

each case the center of the

dipole is just over 1 / 4

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