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Chapter

VHF and UHF

Antenna Systems

A good antenna system is one of the most valuable

assets available to the VHF/UHF enthusiast. Compared

to an antenna of lesser quality, an antenna that is well

designed, is built of good quality materials, and is well

maintained, will increase transmitting range, enhance

reception of weak signals, and reduce interference

problems. The work itself is by no means the least

attractive part of the job. Even with high gain antennas,

experimentation is greatly simplified at VHF and UHF

because the antennas are a physically manageable size.

Setting up a home antenna range is within the means of

most amateurs, and much can be learned about the nature

and adjustment of antennas. No large investment in test

equipment is necessary.

The Basics

Selecting the best VHF or UHF antenna for a given

installation involves much more than scanning gain figures

and prices in a manufacturer’s catalog. There is no one “best”

VHF or UHF antenna design for all purposes. The first step

in choosing an antenna is figuring out what you want it to do.

performance over a remarkably wide frequency range,

providing that the boom length is long enough and enough

elements are used to populate the boom. Modern Yagi

designs in fact are competitive with directly driven collinear

arrays of similar size and complexity. The primary

performance parameters of gain, front-to-rear ratio and SWR

can be optimized over all the VHF or UHF amateur bands

readily, with the exception of the full 6-meter band from

50.0 to 54.0 MHz, which has an 8% wide bandwidth. A Yagi

can be easily designed to cover any 2.0-MHz portion of the

6-meter band with superb performance.

Gain

At VHF and UHF, it is possible to build Yagi antennas

with very high gain—15 to 20 dBi—on a physically

manageable boom. Such antennas can be combined in arrays

of two, four, six, eight, or more antennas. These arrays are

attractive for EME, tropospheric scatter or other weak-signal

communication modes.

Height Gain

In general, the higher the better in VHF and UHF antenna

installations. If raising the antenna clears its view over nearby

obstructions, it may make dramatic improvements in

coverage. Within reason, greater height is almost always worth

its cost, but height gain (see Chapter 23 ) must be balanced

against increased transmission-line loss. This loss can be

considerable, and it increases with frequency. The best

available line may not be very good if the run is long in terms

of wavelengths. Line loss considerations (shown in table form

in Chapter 24 ) are important in antenna planning.

Radiation Patterns

Antenna radiation can be made omnidirectional,

bidirectional, practically unidirectional, or anything between

these conditions. A VHF net operator may find an

omnidirectional system almost a necessity, but it may be a

poor choice otherwise. Noise pickup and other interference

problems tend to be greater with such omnidirectional

antennas, and such antennas having some gain are especially

bad in these respects. Maximum gain and low radiation angle

are usually prime interests of the weak signal DX aspirant.

A clean pattern, with lowest possible pickup and radiation

off the sides and back, may be important in high activity

areas, or where the noise level is high.

Physical Size

A given antenna design for 432 MHz has the same gain

as the same design for 144 MHz, but being only one-third

as large intercepts only one-ninth as much energy in

receiving. In other words, the antenna has less pickup

efficiency at 432 MHz. To be equal in communication

effectiveness, the 432-MHz array should be at least equal in

Frequency Response

The ability to work over an entire VHF band may be

important in some types of work. Modern Yagis can achieve

VHF and UHF Antenna Systems

18-1

size to the 144-MHz antenna, which requires roughly three

times as many elements. With all the extra difficulties

involved in using the higher frequencies effectively, it is

best to keep antennas as large as possible for these bands.

used in VHF work if the run is more than a few feet. Lines

of 1 / 2 -inch diameter (RG-8 or RG-11) work fairly well at

50 MHz, and are acceptable for 144-MHz runs of 50 feet or

less. These lines are somewhat better if they employ foam

instead of ordinary PE dielectric material. Aluminum-jacket

“Hardline” coaxial cables with large inner conductors and

foam insulation are well worth their cost, and can sometimes

be obtained for free from local cable TV operators as “end

runs”—pieces at the end of a roll. The most common CATV

cable is 1 / 2 -inch OD 75-

DESIGN FACTORS

With the objectives sorted out in a general way,

decisions on specifics, such as polarization, type of trans-

mission line, matching methods and mechanical design must

be made.

Hardline. Matched-line loss for

this cable is about 1.0 dB/100 feet at 146 MHz and 2.0 dB/

100 feet at 432 MHz. Less commonly available from CATV

companies is the 3 / 4 -inch 75-

Ω

Polarization

Whether to position antenna elements vertically or

horizontally has been widely questioned since early VHF

pioneering. Tests have shown little evidence as to which

polarization sense is most desirable. On long paths, there is

no consistent advantage either way. Shorter paths tend to

yield higher signal levels with horizontally polarized

antennas over some kinds of terrain. Man-made noise,

especially ignition interference, also tends to be lower with

horizontal antennas. These factors make horizontal

polarization somewhat more desirable for weak-signal

communications. On the other hand, vertically polarized

antennas are much simpler to use in omnidirectional systems

and in mobile work.

Vertical polarization was widely used in early VHF

work, but horizontal polarization gained favor when

directional arrays started to become widely used. The major

trend to FM and repeaters, particularly in the 144-MHz band,

has tipped the balance in favor of vertical antennas in mobile

and repeater use. Horizontal polarization predominates in

other communication on 50 MHz and higher frequencies.

Additional loss of 20 dB or more can be expected when

cross-polarized antennas are used.

Hardline, sometimes with a

black self-healing hard plastic covering. This line has 0.8 dB

of loss per 100 feet at 146 MHz, and 1.6 dB loss per 100 feet

at 432 MHz. There will be small additional losses for either

line if 75 to 50-

Ω

transformers are used at each end.

Commercial connectors for Hardline are expensive but

provide reliable connections with full waterproofing.

Enterprising amateurs have “home-brewed” low-cost

connectors. If they are properly waterproofed, connectors

and Hardline can last almost indefinitely. Hardline must not

be bent too sharply, because it will kink.

Beware of any “bargains” in coax for VHF or UHF use.

Feed-line loss can be compensated to some extent by

increasing transmitter power, but once lost, a weak signal can

never be recovered in the receiver. Effects of weather on

transmission lines should not be ignored. Well constructed

open-wire line works optimally in nearly any weather, and it

stands up well. Twin-lead is almost useless in heavy rain, wet

snow or icing. The best grades of coax are completely

impervious to weather; they can be run underground, fastened

to metal towers without insulation, and bent into any

convenient position with no adverse effects on performance.

TRANSMISSION LINES

Transmission line principles are covered in detail in

Chapter 24 . Techniques that apply to VHF and UHF

operation are dealt with in greater detail here. The principles

of carrying RF from one location to another via a feed line

are the same for all radio frequencies. As at HF, RF is carried

principally via open-wire lines and coaxial cables at VHF/

UHF. Certain aspects of these lines characterize them as good

or bad for use above 50 MHz.

Properly built open-wire line can operate with very low

loss in VHF and UHF installations. A total line loss under

2 dB per 100 feet at 432 MHz can easily be obtained. A line

made of #12 wire, spaced 3 / 4 inch or more with Teflon

spreaders and run essentially straight from antenna to station,

can be better than anything but the most expensive coax. Such

line can be home-made or purchased at a fraction of the cost

of coaxial cables, with comparable loss characteristics. Careful

attention must be paid to efficient impedance matching if the

benefits of this system are to be realized. A similar system for

144 MHz can easily provide a line loss under 1 dB.

Small coax such as RG-58 or RG-59 should never be

G-Line

Conventional two-conductor transmission lines and

most coaxial cables are quite lossy in the upper UHF and

microwave ranges. If the station and antenna are separated

by more than 100 feet, common coaxial cables (such as

RG-8) are almost useless for serious work. Unless the very

best rigid coax with the proper fittings can be obtained, it is

worthwhile to explore alternative methods of carrying RF

energy between the station and antenna.

There is a single-conductor transmission line, invented

by Georg Goubau (called “G-Line” in his honor), that can

be effectively used in this frequency range. Papers by the

inventor appeared some years ago, in which seemingly

fantastic claims for line loss were made—under 1 dB per

100 feet in the microwave region, for example. (See the

Bibliography at the end of this chapter.) Especially attractive

was the statement that the matching device was broadband

in nature, making it appear that a single G-Line installation

might be made to serve on, say, 432, 903 and 1296 MHz.

The basic idea is that a single conductor can be an

almost lossless transmission line at UHF, if a suitable

18-2

Chapter 18

Ω

long. The line should be large and

heavily insulated, such as #14, vinyl covered.

Propagation along a G-Line is similar to “ground

wave,” or “surface wave” propagation over perfectly

conducting earth. The dielectric material confines the energy

to the vicinity of the wire, preventing radiation. The major

drawback of G-Line is that it is very sensitive to deviation

from straight lines. If any bends must be made, they should

be in the form of a large radius arc. This is preferable to

even an obtuse angle change in the direction of the run. The

line must be kept several inches away from metal objects

and should be supported with as few insulators as possible.

WAVEGUIDES

Above 2 GHz, coaxial cable is a losing proposition for

communication work. Fortunately, at this frequency the

wavelength is short enough to allow practical, efficient

energy transfer by an entirely different means. A waveguide

is a conducting tube through which energy is transmitted in

the form of electromagnetic waves. The tube is not

considered as carrying a current in the same sense that the

wires of a two-conductor line do, but rather as a boundary

that confines the waves in the enclosed space. Skin effect

prevents any electromagnetic effects from being evident

outside the guide. The energy is injected at one end, either

through capacitive or inductive coupling or by radiation, and

is removed from the other end in a like manner. Waveguide

merely confines the energy of the fields, which are

propagated through it to the receiving end by means of

reflections against its inner walls.

Analysis of waveguide operation is based on the

assumption that the guide material is a perfect conductor of

electricity. Typical distributions of electric and magnetic

fields in a rectangular guide are shown in Fig 1 . The intensity

of the electric field is greatest (as indicated by closer spacing

of the lines of force) at the center along the X dimension

(Fig 1C), diminishing to zero at the end walls. The fields

must diminish in this manner, because the existence of any

electric field parallel to the walls at the surface would cause

an infinite current to flow in a perfect conductor. Waveguides,

of course, cannot carry RF in this fashion.

Fig 1—Field distribution in a rectangular waveguide.

The TE 10 mode of propagation is depicted.

propagation, but has a component of the electric field in

that direction. The other type, designated TE (transverse

electric) has the electric field entirely transverse, but has a

component of magnetic field in the direction of propagation.

TM waves are sometimes called E waves, and TE waves are

sometimes called H waves, but the TM and TE designations

are preferred.

The mode of propagation is identified by the group

letters followed by two subscript numerals. For example,

TE 10 , TM 11 , etc. The number of possible modes increases

with frequency for a given size of guide, and there is only

one possible mode (called the dominant mode ) for the lowest

frequency that can be transmitted. The dominant mode is

the one generally used in amateur work.

Modes of Propagation

Fig 1 represents the most basic distribution of the

electric and magnetic fields in a waveguide. There are an

infinite number of ways in which the fields can arrange

themselves in a waveguide (for frequencies above the low

cutoff frequency of the guide in use). Each of these field

configurations is called a mode .

The modes may be separated into two general groups.

One group, designated TM (transverse magnetic), has the

magnetic field entirely transverse to the direction of

Waveguide Dimensions

In rectangular guide the critical dimension is X in Fig 1.

This dimension must be more than 1 / 2

VHF and UHF Antenna Systems

18-3

“launching device” is used. A similar “launcher” is placed

at the other end. Basically, the launcher is a cone-shaped

device that is a flared extension of the coaxial cable shield.

In effect, the cone begins to carry the RF as the outer

conductor is gradually “removed.” These launch cones

should be at least 3

at the lowest

frequency to be transmitted. In practice, the Y dimension

usually is made about equal to 1 / 2 X to avoid the possibility

of operation in other than the dominant mode.

Cross-sectional shapes other than the rectangle can be

used, the most important being the circular pipe. Much the

same considerations apply as in the rectangular case.

Wavelength dimensions for rectangular and circular

guides are given in Table 1 , where X is the width of a

rectangular guide and r is the radius of a circular guide. All

figures apply to the dominant mode.

on a side. A

horn of this dimension (cutoff) has a unidirectional radiation

pattern with a null toward the waveguide transition. The

gain at the cutoff frequency is 3 dB, increasing 6 dB with

each doubling of frequency. Horns are used extensively in

microwave work, both as primary radiators and as feed

elements for elaborate focusing systems. Details for

constructing 10-GHz horn antennas are given later in this

chapter.

Coupling to Waveguides

Energy may be introduced into or extracted from a

waveguide or resonator by means of either the electric or

magnetic field. The energy transfer frequently is through a

coaxial line. Two methods for coupling to coaxial line are

shown in Fig 2 . The probe shown at A is simply a short

extension of the inner conductor of the coaxial line, oriented

so that it is parallel to the electric lines of force. The loop

shown at B is arranged so that it encloses some of the

magnetic lines of force. The point at which maximum

coupling is obtained depends on the mode of propagation in

the guide or cavity. Coupling is maximum when the coupling

device is in the most intense field.

Coupling can be varied by turning the probe or loop

through a 90

stubs, shorted at

the end opposite the feed line. The open end of such a stub

presents an infinite impedance to the transmission line,

provided the shorted stub is nonreactive. However, the

shorting link has a finite length, and therefore some

inductance. The effect of this inductance can be removed by

making the RF current flow on the surface of a plate rather

than a thin wire. If the plate is large enough, it will prevent

the magnetic lines of force from encircling the RF current.

An infinite number of these 1 / 4 -

angle. When the probe is perpendicular to the

electric lines the coupling is minimum; similarly, when the

plane of the loop is parallel to the magnetic lines the coupling

is minimum.

If a waveguide is left open at one end it will radiate

energy. This radiation can be greatly enhanced by flaring

stubs may be

connected in parallel without affecting the standing waves

of voltage and current. The transmission line may be

supported from the top as well as the bottom, and when an

infinite number of supports are added, they form the walls

of a waveguide at its cutoff frequency. Fig 3 illustrates how

a rectangular waveguide evolves from a two-wire parallel

transmission line as described. This simplified analysis also

shows why the cutoff dimension is 1 / 2

Table 1

Waveguide Dimensions

Rectangular

Circular

Cutoff wavelength

3.41r

While the operation of waveguides is usually described

Longest wavelength trans-

mitted with little attenuation

1.6X

3.2r

Shortest wavelength before

next mode becomes possible 1.1X

2.8r

Fig 2—Coupling coaxial line to waveguide and

resonators.

Fig 3—At its cutoff frequency a rectangular waveguide

can be thought of as a parallel two-conductor trans-

mission line supported from top and bottom by an

infinite number of 1 / 4 -

stubs.

18-4

Chapter 18

the waveguide to form a pyramidal horn antenna. The horn

acts as a transition between the confines of the waveguide

and free space. To effect the proper impedance

transformation the horn must be at least 1 / 2

Evolution of a Waveguide

Suppose an open wire line is used to carry RF energy

from a generator to a load. If the line has any appreciable

length it must be mechanically supported. The line must be

well insulated from the supports if high losses are to be

avoided. Because high quality insulators are difficult to

construct at microwave frequencies, the logical alternative

is to support the transmission line with 1 / 4 -

in terms of fields, current does flow on the inside walls, just

as on the conductors of a two-wire transmission line. At the

waveguide cutoff frequency, the current is concentrated in

the center of the walls, and disperses toward the floor and

ceiling as the frequency increases.

IMPEDANCE MATCHING

Impedance matching is covered in detail in Chapters

25 and 26 , and the theory is the same for frequencies above

50 MHz. Practical aspects are similar, but physical size can

be a major factor in the choice of methods. Only the matching

devices used in practical construction examples later in this

chapter are discussed in detail here. This should not rule

out consideration of other methods, however, and a reading

of relevant portions of both Chapters 25 and 26 is

recommended.

Universal Stub

As its name implies, the double adjustment stub of

Fig 4A is useful for many matching purposes. The stub length

is varied to resonate the system, and the transmission line

attachment point is varied until the transmission line and

stub impedances are equal. In practice this involves moving

both the sliding short and the point of line connection for

zero reflected power, as indicated on an SWR bridge

connected in the line.

The universal stub allows for tuning out any small

reactance present in the driven part of the system. It permits

matching the antenna to the line without knowledge of the

actual impedances involved. The position of the short

yielding the best match gives some indication of the amount

of reactance present. With little or no reactive component to

be tuned out, the stub must be approximately 1 / 2

from

Fig 4—Matching methods commonly used at VHF. The

universal stub, A, combines tuning and matching. The

adjustable short on the stub and the points of

connection of the transmission line are adjusted for

minimum reflected power on the line. In the delta

match, B and C, the line is fanned out and connected to

the dipole at the point of optimum impedance match.

Impedances need not be known in A, B or C. The

gamma match, D, is for direct connection of coax. C1

tunes out inductance in the arm. A folded dipole of

uniform conductor size, E, steps up antenna

impedance by a factor of four. Using a larger conductor

in the unbroken portion of the folded dipole, F, gives

higher orders of impedance transformation.

apart. Preferably it should be mounted

rigidly, on insulators. Once the position of the short is

determined, the center of the short can be grounded, if

desired, and the portion of the stub no longer needed can be

removed.

It is not necessary that the stub be connected directly

to the driven element. It can be made part of an open wire

line, as a device to match coaxial cable to the line. The stub

can be connected to the lower end of a delta match or placed

at the feed point of a phased array. Examples of these uses

are given later.

out reactance, so the universal stub is often used as a

termination for it, to this end.

At one time, the delta match was thought to be inferior

for VHF applications because of its tendency to radiate if

improperly adjusted. The delta has come back into favor

now that accurate methods are available for measuring the

effects of matching. It is very handy for phasing multiple

bay arrays with open wire lines, and its dimensions in this

use are not particularly critical. It should be checked out

carefully in applications like that of Fig 4C, where no tuning

device is used.

antenna at the point of most efficient power

transfer. This is shown in Fig 4B. Both the side length and

the points of connection either side of the center of the

element must be adjusted for minimum reflected power on

the line, but as with the universal stub, the impedances need

not be known. The delta match makes no provision for tuning

Gamma and T Matches

An application of the same principle allowing direct

connection of coax is the gamma match, Fig 4D. Because

VHF and UHF Antenna Systems

18-5

load toward the short.

The stub should be made of stiff bare wire or rod, spaced

no more than 1 / 20

Delta Match

Probably the most basic impedance matching device

is the delta match, fanned ends of an open wire line tapped

onto a 1 / 2 -

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