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Antenna

Materials and

Accessories

Chapter

This chapter contains information on materials

amateurs use to construct antennas—what types of material

to look for in a particular application, tips on working with

and using various materials. Chapter 21 contains information

on where to purchase these materials.

Basically, antennas for MF, HF, VHF and the

lower UHF range consist simply of one or more

conductors that radiate (or receive) electromagnetic waves.

However, an antenna system must also include some

means to support those conductors and maintain their

relative positions—the boom for a Yagi antenna and the

halyards for a wire dipole, for example. In this chapter we’ll

look at materials for those applications, too. Structural

supports, such as towers, masts and poles, are discussed in

Chapter 22 .

There are two main types of material used for

antenna conductors, wire and tubing. Wire antennas are

generally simple and therefore easier to construct, although

some arrays of wire elements can become rather complex.

When tubing is required, aluminum tubing is used most often

because of its light weight. Aluminum tubing is discussed

in a subsequent section of this chapter.

Wire Antennas

Although wire antennas are relatively simple, they can

constitute a potential hazard unless properly constructed.

Antennas should never be run under or over public utility

(telephone or power) lines. Several amateurs have lost their

lives by failing to observe this precaution.

The National Electric Code ® of the National Fire

Protection Association contains a section on amateur stations

in which a number of recommendations are made concerning

minimum size of antenna wire and the manner of bringing

the transmission line into the station. Chapter 1 contains

more information about this code. The code in itself does

not have the force of law, but it is frequently made a part of

local building regulations, which are enforceable. The

provisions of the code may also be written into, or referred

to, in fire and liability insurance documents.

The RF resistance of copper wire increases as the size

of the wire decreases. However, in most types of antennas

that are commonly constructed of wire (even quite small

wire), the radiation resistance will be much higher than the

RF resistance, and the efficiency of the antenna will still be

adequate. Wire sizes as small as #30, or even smaller, have

been used quite successfully in the construction of

“invisible” antennas in areas where more conventional

antennas cannot be erected. In most cases, the selection of

wire for an antenna will be based primarily on the physical

properties of the wire, since the suspension of wire from

elevated supports places a strain on the wire.

WIRE TYPES

Wire having an enamel coating is preferable to bare

wire, since the coating resists oxidation and corrosion.

Several types of wire having this type of coating are

available, depending on the strength needed. “Soft-drawn”

or annealed copper wire is easiest to handle; unfortunately,

it stretches considerably under stress. Soft-drawn wire should

be avoided, except for applications where the wire will be

under little or no tension, or where some change in length

can be tolerated. (For example, the length of a horizontal

antenna fed at the center with open-wire line is not critical,

although a change in length may require some readjustment

of coupling to the transmitter.)

“Hard-drawn” copper wire or copper-clad steel wire

(also known as Copperweld TM ) is harder to handle, because

it has a tendency to spiral when it is unrolled. These types

of wire are ideal for applications where significant stretch

cannot be tolerated. Care should be exercised in using this

wire to make sure that kinks do not develop—the wire will

have a far greater tendency to break at a kink. After the coil

has been unwound, suspend the wire a few feet above ground

for a day or two before using it. The wire should not be

recoiled before it is installed.

Several factors influence the choice of wire type and

size. Most important to consider are the length of the

unsupported span, the amount of sag that can be tolerated,

the stability of the supports under wind pressure, and whether

Antenna Materials and Accessories

20-1

or not an unsupported transmission line is to be suspended

from the span. Table 1 shows the wire diameter, current-

carrying capacity and resistance of various sizes of copper

wire. Table 2 shows the maximum rated working tensions of

hard-drawn and copper-clad steel wire of various sizes. These

two tables can be used to select the appropriate wire size for

an antenna.

3) Choose an operating tension level (in pounds) consistent

with the values presented in Table 2 (preferably less than

the recommended wire tension).

4) Draw a line from the tension value chosen (plotted on

the tension axis) through the point where the work axis

crosses the original line constructed in step 2, and continue

this new line to the sag axis.

5) Read the sag in feet on the sag axis.

Example:

Weight = 11 pounds/1000 feet

Span = 210 feet

Tension = 50 pounds

Answer: Sag = 4.7 feet

WIRE TENSION

If the tension on a wire can be adjusted to a known

value, the expected sag of the wire ( Fig 1 ) may be determined

before installation using Table 2 and the nomograph of

Fig 2 . Even though there may be no convenient method to

determine the tension in pounds, calculation of the expected

sag for practicable working tensions is often desirable. If

the calculated sag is greater than allowable it may be reduced

by any one or a combination of the following:

1) Providing additional supports, thereby decreasing the span

2) Increasing the tension in the wire if less than

recommended

3) Decreasing the size of the wire

These calculations do not take into account the weight of a

feed line supported by the antenna wire.

Instructions for Using the Nomograph

1) From Table 2 , find the weight (pounds/1000 feet) for

the particular wire size and material to be used.

2) Draw a line from the value obtained above, plotted on

the weight axis, to the desired span (feet) on the span axis,

Fig 2 . Note in Fig 1 that the span is one half the distance

between the supports.

Fig 1—The span and sag of a long-wire antenna.

Table 1

Copper-Wire Table

Turns

Wire

per

Feet

Ohms

Cont.-duty

Wire

per

Feet

Ohms

Cont.-duty

Size

Dia

Linear

per

current 2

Size

Dia

Linear

per

current 2

AWG

Dia

Inch

Pound

1000 ft

Single Wire

AWG

Dia

Inch

Pound

1000 ft

Single Wire

(B&S)

Mils 1

in mm

Enamel

Bare

in Open Air

(B&S)

Mils 1

in mm

Enamel

Bare

in Open Air

289.3

7.348

—

3.947

0.1264

—

25.3

0.644

37.0

514.2

16.46

—

257.6

6.544

—

4.977

0.1593

—

22.6

0.573

41.3

648.4

20.76

—

229.4

5.827

—

6.276

0.2009

—

20.1

0.511

46.3

817.7

26.17

—

204.3

5.189

—

7.914

0.2533

—

17.9

0.455

51.7

1031

33.00

—

181.9

4.621

—

9.980

0.3195

—

15.9

0.405

58.0

1300

41.62

—

162.0

4.115

—

12.58

0.4028

—

14.2

0.361

64.9

1639

52.48

—

144.3

3.665

—

15.87

0.5080

—

12.6

0.321

72.7

2067

66.17

—

128.5

3.264

7.6

20.01

0.6405

11.3

0.286

81.6

2607

83.44

—

114.4

2.906

8.6

25.23

0.8077

—

10.0

0.255

90.5

3287

105.2

—

101.9

2.588

9.6

31.82

1.018

8.9

0.227

101

4145

132.7

—

90.7

2.305

10.7

40.12

1.284

—

8.0

0.202

113

5227

167.3

—

80.8

2.053

12.0

50.59

1.619

7.1

0.180

127

6591

211.0

—

72.0

1.828

13.5

63.80

2.042

—

6.3

0.160

143

8310

266.0

—

64.1

1.628

15.0

80.44

2.575

5.6

0.143

158

10480

335

—

57.1

1.450

16.8

101.4

3.247

—

5.0

0.127

175

13210

423

—

50.8

1.291

18.9

127.9

4.094

4.5

0.113

198

16660

533

—

45.3

1.150

21.2

161.3

5.163

—

4.0

0.101

224

21010

673

—

40.3

1.024

23.6

203.4

6.510

3.5

0.090

248

26500

848

—

35.9

0.912

26.4

256.5

8.210

—

3.1

0.080

282

33410

1070

—

32.0

0.812

29.4

323.4

10.35

1 A mil is 0.001 inch.

2 Max wire temp of 212 ° F and max ambient temp of 135 ° F.

28.5

0.723

33.1

407.8

13.05

—

20-2

Chapter 20

Table 2

Stressed Antenna Wire

American Recommended Tension 1 (pounds) Weight (pounds per 1000 feet)

Wire Gauge Copper-clad Hard-drawn Copper-clad Hard-drawn

steel 2 copper steel 2 copper

4 495 214 115.8 126.0

6 310 130 72.9 79.5

8 195 84 45.5 50.0

10 120 52 28.8 31.4

12 75 32 18.1 19.8

14 50 20 11.4 12.4

16 31 13 7.1 7.8

18 19 8 4.5 4.9

20 12 5 2.8 3.1

1 Approximately one-tenth the breaking load. Might be increased 50% if end supports are firm and there is no danger of ice loading.

2 Copperweld, TM 40% copper.

Wire Splicing

Wire antennas should preferably be made with unbroken

lengths of wire. In instances where this is not feasible, wire

sections should be spliced as shown in Fig 3 . The enamel

insulation should be removed for a distance of about 6 inches

from the end of each section by scraping with a knife or

rubbing with sandpaper until the copper underneath is bright.

The turns of wire should be brought up tight around the

standing part of the wire by twisting with broad-nose pliers.

The crevices formed by the wire should be completely

filled with rosin-core solder. An ordinary soldering iron or

gun may not provide sufficient heat to melt solder outdoors;

a propane torch is desirable. The joint should be heated

sufficiently so the solder flows freely into the joint when the

source of heat is removed momentarily. After the joint has

cooled completely, it should be wiped clean with a cloth, and

then sprayed generously with acrylic to prevent corrosion.

ANTENNA INSULATION

To prevent loss of RF power, the antenna should be

well insulated from ground, unless of course it is a shunt-

fed system. This is particularly important at the outer end or

ends of wire antennas, since these points are always at a

comparatively high RF potential. If an antenna is to be

installed indoors (in an attic, for instance) the antenna may

be suspended directly from the wood rafters without

additional insulation, if the wood is permanently dry. Much

greater care should be given to the selection of proper

insulators when the antenna is located outside where it is

exposed to wet weather.

Fig 2—Nomograph for determining wire sag. (John

Elengo, Jr, K1AFR)

Insulator Leakage

Antenna insulators should be made of material that will

not absorb moisture. The best insulators for antenna use are

made of glass or glazed porcelain. Depending on the type of

material, plastic insulators may be suitable. The length of an

insulator relative to its surface area is indicative of its

comparative insulating ability. A long thin insulator will have

less leakage than a short thick insulator. Some antenna

insulators are deeply ribbed to increase the surface leakage

path without increasing the physical length of the insulator.

Shorter insulators can be used at low-potential points, such

as at the center of a dipole. If such an antenna is to be fed

Antenna Materials and Accessories

20-3

with open-wire line and used on several bands, however,

the center insulator should be the same as those used at the

ends, because high RF potential may exist across the center

insulator on some bands.

stress ratings of antenna insulators are typical:

5 / 8 in. square by 4 in. long—400 lb

1 in. diameter by 7 or 12 in. long—800 lb

1 1 / 2 in. diameter by 8, 12 or 20 in. long, with special

metal end caps—5000 lb

These are rated breaking tensions. The actual working

tensions should be limited to not more than 25% of the

breaking rating.

The antenna wire should be attached to the insulators

as shown in Fig 4 . Care should be taken to avoid sharp

angular bends in the wire when it is looped through the

insulator eye. The loop should be generous enough in size

that it will not bind the end of the insulator tightly. If the

length of the antenna is critical, the length should be

measured to the outward end of the loop, where it passes

through the eye of the insulator. The soldering should be

done as described earlier for the wire splice.

Insulator Stress

As with the antenna wire, the insulator must have

sufficient physical strength to support the stress of the

antenna without danger of breakage. Long elastic bands or

lengths of nylon fishing line provide long leakage paths and

make satisfactory insulators within their limits to resist

mechanical strain. They are often used in antennas of the

“invisible” type mentioned earlier.

For low-power work with short antennas not subject

to appreciable stress, almost any small glass or glazed-

porcelain insulator will do. Homemade insulators of Lucite

rod or sheet will also be satisfactory. More care is required

in the selection of insulators for longer spans and higher

transmitter power.

For a given material, the breaking tension of an insulator

will be proportional to its cross-sectional area. It should be

remembered, however, that the wire hole at the end of the

insulator decreases the effective cross-sectional area. For this

reason, insulators designed to carry heavy strains are fitted

with heavy metal end caps, the eyes being formed in the metal

cap, rather than in the insulating material itself. The following

Strain Insulators

Strain insulators have their holes at right angles, since

they are designed to be connected as shown in Fig 5 . It can

be seen that this arrangement places the insulating material

under compression, rather than tension. An insulator

connected this way can withstand much greater stress.

Furthermore, the wire will not collapse if the insulator breaks,

since the two wire loops are interlocked. Because the wire

is wrapped around the insulator, however, the leakage path

is reduced drastically, and the capacitance between the wire

loops provides an additional leakage path. For this reason,

the use of the strain insulator is usually confined to such

applications as breaking up resonances in guy wires, where

high levels of stress prevail, and where the RF insulation is

of less importance. Such insulators might be suitable for

use at low-potential points on an antenna, such as at the

center of a dipole. These insulators may also be fastened in

the conventional manner if the wire will not be under

sufficient tension to break out the eyes.

Insulators for Ribbon-Line Antennas

Fig 6A shows the sketch of an insulator designed to be

used at the ends of a folded dipole or a multiple dipole made

of ribbon line. It should be made approximately as shown,

out of Lucite or bakelite material about 1 / 4 inch thick. The

advantage of this arrangement is that the strain of the antenna

Fig 3—Correct method of splicing antenna wire. Solder

should be flowed into the wraps after the connection is

completed. After cooling, the joint should be sprayed

with acrylic to prevent oxidation and corrosion.

Fig 4—When fastening antenna wire to an insulator, do

not make the wire loop too snug. After the connection

is complete, flow solder into the turns. Then when the

joint has cooled completely, spray it with acrylic.

Fig 5—Conventional manner of fastening wire to a strain

insulator. This method decreases the leakage path and

increases capacitance, as discussed in the text.

20-4

Chapter 20

is shared by the conductors and the plastic webbing of the

ribbon, which adds considerable strength. After soldering,

the screw should be sprayed with acrylic.

Fig 6B shows a similar arrangement for suspending

one dipole from another in a stagger-tuned dipole system. If

better insulation is desired, these insulators can be wired to

a conventional insulator.

clothesline pulley has a 4-inch diameter plastic wheel with

a 1 / 4 -inch shaft running in bronze bearings. The sheath is

made of cast or forged corrosion-proof alloy. Some look-

alike low-cost pulleys of this type have an aluminum shaft

with no bearings. For antenna work, these cheap pulleys are

of little long-term value.

Marine pulleys have good weather-resisting qualities,

since they are usually made of bronze, but they are

comparatively expensive and are not designed to carry heavy

loads. For extremely long spans, the wood-sheathed pulleys

used in “block and tackle” devices and for sail hoisting

should work well.

PULLEYS AND HALYARDS

Pulleys and halyards commonly used to raise and lower

a wire antenna must also be capable of taking the same strain

as the antenna wire and insulators. Unfortunately, little

specific information on the stress ratings of most pulleys is

available. Several types of pulleys are readily available at

almost any hardware store. Among these are small

galvanized pulleys designed for awnings and several styles

and sizes of clothesline pulleys. Heavier and stronger pulleys

are those used in marine work. The factors that determine

how much stress a pulley will handle include the diameter

of the shaft, how securely the shaft is fitted into the sheath

and the size and material of the frame.

Another important factor to be considered in the

selection of a pulley is its ability to resist corrosion.

Galvanized awning pulleys are probably the most susceptible

to corrosion. While the frame or sheath usually stands up

well, these pulleys usually fail at the shaft. The shaft rusts

out, allowing the grooved wheel to break away under tension.

Most good-quality clothesline pulleys are made of

alloys which do not corrode readily. Since they are designed

to carry at least 50 feet of line loaded with wet clothing in

stiff winds, they should be adequate for normal spans of

100 to 150 feet between stable supports. One type of

Fig 7—This is one type of knot that will hold with

smooth rope, such as nylon. Shown at A, the knot for

splicing two ends. B shows the use of a similar knot in

forming a loop, as might be needed for attaching an

insulator to a halyard. Knot A is first formed loosely

10 or 12 in. from the end of the rope; then the end is

passed through the eye of the insulator and knot A.

Knot B is then formed and both knots pulled tight.

(Richard Carruthers, K7HDB)

Fig 6—At A, an insulator for the ends of folded dipoles, or multiple dipoles made of 300-ohm ribbon. At B, a method

of suspending one ribbon dipole from another in a multiband dipole system.

Antenna Materials and Accessories

20-5

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