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Contents
21.1 Antenna Basics
21.1.1 Directivity and Gain
21.1.2 Antenna Polarization
21.1.3 Current and Voltage Distribution
21.1.4 Impedance
21.1.5 Impedance and Height Above Ground
21.1.6 Antenna Bandwidth
21.1.7 Effects of Conductor Diameter
21.1.8 Radiation Patterns
21.1.9 Elevation Angle
21.1.10 Imperfect Ground
21.2 Dipoles and the Half-Wave Antenna
21.2.1 Radiation Characteristics
21.2.2 Feed Methods
21.2.3 Baluns
21.2.4 Building Dipoles and Other Wire
Antennas
21.2.5 Dipole Orientation
21.2.6 Inverted-V Dipole
21.2.7 Sloping Dipole
21.2.8 Shortened Dipoles
21.2.9 Half-Wave Vertical Dipole (HVD)
21.2.10 Folded Dipoles
21.2.11 Multiband Dipole Systems
21.2.12 NVIS Antennas
Project
: Multiband Center-Fed Dipole
Project
: 40-15 Meter Dual-Band Dipole
Project
: W4RNL Inverted-U Antenna
Project
: Two W8NX Multiband, Coax-Trap
Dipoles
Project
: Extended Double-Zepp for
17 Meters
21.3 Vertical (Ground-Plane) Antennas
21.3.1 Ground Systems
21.3.2 Full-Size Vertical Antennas
21.3.3 Physically Short Verticals
Project
: Top-Loaded Low-Band Antenna
21.3.4 Cables and Control Wires on Towers
21.3.5 Multiband Trap Verticals
21.4 T and Inverted-L Antennas
21.5 Slopers and Vertical Dipoles
21.5.1 The Half-Sloper Antenna
Project
: Half-Wave Vertical Dipole (HVD)
Project
: Compact Vertical Dipole (CVD)
Project
: All-Wire 30 Meter CVD
21.6 Yagi Antennas
21.6.3 Two-Element Beams
21.6.4 Three-Element Beams
21.6.5 Construction of Yagi Antennas
Project
: Family of Computer-Optimized HF
Yagis
21.7 Quad and Loop Antennas
Project
: Five-Band, Two-Element HF Quad
Project
: Wire Quad for 40 Meters
21.7.1 Loop Antennas
Project
: Vertical Loop Antenna for 28 MHz
Project
: Multiband Horizontal Loop Antenna
21.8 HF Mobile Antennas
21.8.1 Simple Whips
21.8.2 Coil-Loaded Whips
21.8.3 Base vs Center vs Continuous Loading
21.8.4 Remotely Controlled HF Mobile
Antennas
21.8.5 Ground Losses
21.8.6 Antenna Mounting
21.8.7 Mobile HF Antenna Matching
21.8.8 Remotely-Tuned Antenna Controllers
21.8.9 Efficiency
Project
: Mounts for Remotely-Tuned Antennas
Project
: Retuning a CB Whip Antenna
21.9 VHF/UHF Mobile Antennas
21.9.1 VHF/UHF Antenna Mounts
21.10 VHF/UHF Antennas
21.10.1 Gain
21.10.2 Radiation Pattern
21.10.3 Height Gain
21.10.4 Physical Size
21.10.5 Polarization
21.10.6 Circular Polarization
21.10.7 Transmission Lines
21.10.7 Impedance Matching
21.10.8 Baluns and Impedance transformers
Project
: Simple, Portable Ground-Plane
Antenna
Project
: Dual-Band Antenna for 146/446 MHz
21.11 VHF/UHF Yagis
21.11.1 Stacking Yagis
Project
: Three and Five-Element Yagis for
6 Meters
Project
: Medium-Gain 2 Meter Yagi
Project
: Cheap Yagis by WA5VJB
21.12 Glossary
21.13 References and Further Reading
21.6.1 Parasitic Excitation
21.6.2 Yagi Gain, Front-to-Back Ratio, and
SWR
Chapter
21
Antennas
21.1 Antenna Basics
This section covers a range of topics that are fundamental to understanding how antennas
work and defines several key terms. (A glossary is included at the end of the chapter.) While
the discussion in this section uses the dipole as the primary example, the concepts apply to
all antennas.
In the world of radio, the antenna
is “where the rubber meets the
road!” With antennas so funda-
mental to communication, it is
important that the amateur have a
basic understanding of their func-
tion. That understanding enables
effective selection and applica-
tion of basic designs to whatever
communications task is at hand.
In addition, the amateur is then
equipped to engage in one of
the most active areas of amateur
experimentation, antenna design.
The goal of this chapter is to define
and illustrate the fundamentals of
antennas and provide a selection of
basic designs; simple verticals and
dipoles, quads and Yagi beams,
and other antennas. The reader will
find additional in-depth coverage of
these and other topics in the
ARRL
Antenna Book
and other references
provided. This chapter was origi-
nally written by Chuck Hutchinson,
K8CH, and has been updated by
Ward Silver, NØAX. Alan Applegate,
KØBG, contributed the sections on
mobile antennas.
21.1.1 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 antenna. Directivity
is the same for receiving as transmitting.
The directive pattern 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 and any other
reflecting or absorbing surfaces.
The more an antenna’s directivity is enhanced in a particular direction, the greater the
gain
of the antenna. This is a result of the radiated energy being concentrated in some directions
at the expense of others. Similarly, gain describes the ability of the antenna to receive signals
preferentially from certain directions. Gain does not create additional power beyond that
delivered by the feed line — it only focuses that energy.
Gain is usually expressed in decibels, and is always stated with reference to a
standard
antenna — usually a dipole or an
isotropic 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 standard, and for that reason
frequently used as the comparison for gain measurements.
When the reference for gain 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. Because the
dipole has some gain (2.1 dB) in its favorite direction with respect to the isotropic antenna (see
the next section on the dipole antenna), the dipole’s gain can be expressed as 2.1 dBi. Gain in
dBi can be converted to dBd by subtracting 2.1 dB and from dBd to dBi by adding 2.1 dB.
Gain also takes losses in the antenna or surrounding environment into account. For ex-
ample, if a practical dipole antenna’s wire element dissipated 0.5 dB of the transmitter power
as heat, that specific dipole’s gain with respect to an isotropic antenna would be 2.1 – 0.5
= 1.6 dBi.
21.1.2 Antenna Polarization
An electromagnetic wave has two components: an electric field and a magnetic field at
right angles to each other. For most antennas, the field of primary interest is the electric, or
E-field
. The magnetic field is called the
H-field
. (The abbreviations E- and H- come from
Maxwell’s equations that describe electromagnetic waves.) By convention, the orientation
of the E-field is the reference for determining the electromagnetic wave’s
polarization
. The
E-field of an electromagnetic wave can be oriented in any direction, so orientation with
Antennas
21.1
respect to the Earth’s surface is the usual
frame of reference. The wave’s polarization
can be vertical, horizontal, some intermediate
angle, or even circular.
Antennas are considered to have polariza-
tion, too, determined by the orientation of the
E-field of the electromagnetic field radiated
by the antenna. Because the E-field of the
radiated wave is parallel to the direction of
current flow in the antenna’s elements, the
polarization of the wave and the orientation of
the antenna elements is usually the same. For
example, the E-field radiated by an antenna
with linear elements is parallel to those ele-
ments, so that the polarization of the radiated
wave is the same as the orientation of the
elements. (This is somewhat over-simplified
and additional considerations apply for ele-
ments that are not linear.) Thus a radiator that
is parallel to the earth radiates a horizontally
polarized wave, while a vertical antenna ra-
diates a vertically polarized wave. If a wire
antenna is slanted, it radiates waves with an
E-field that has both vertical and horizontal
components.
Antennas function symmetrically — a re-
ceived signal will create the strongest anten-
na current when the antenna’s elements are
parallel to the E-field of the incoming wave
just as the radiated wave’s E-field will be
strongest parallel to current in the antenna’s
radiating elements. This also means that for
the strongest received signal, the antenna
elements should have the same polarization
as that of the incoming wave. Misalignment
of the receiving antenna’s elements with the
passing wave’s E-field reduces the amount of
signal received. This is called
cross-polariza-
tion
. When the polarizations of antenna and
wave are at right angles, very little antenna
current is created by the incoming signal.
For best results in line-of-sight communi-
cations, antennas at both ends of the circuit
should have the same polarization. However,
it is not essential for both stations to use the
same antenna polarity for ionospheric prop-
agation or sky wave (see the
Propagation
chapter). This is because the radiated wave
is bent and rotated considerably during its
travel through the ionosphere. At the far end
of the communications path the wave may
be horizontal, vertical or somewhere in be-
tween 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.
Most HF-band antennas are either vertical-
ly or horizontally polarized. Although circu-
lar polarization is possible, just as it is at VHF
and UHF, it is seldom used at HF. While most
amateur antenna installations use the Earth’s
surface as their frame of reference, in cases
such as satellite communication or EME the
terms “vertical” and “horizontal” have no
meaning with respect to polarization.
21.1.3 Current and Voltage
Distribution
When power is fed to an antenna, the cur-
rent and voltage vary along its length. The
current is minimum (a
node
) at the ends, re-
gardless of the antenna’s length. 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 ca-
pacitance, which is also called the
end effect
.
The opposite is true of the RF voltage. That
is, there is a voltage minimum (a
loop
) at
the ends.
In the case of a half-wave antenna there
is a current maximum at the center and a
voltage minimum at the center as illustrat-
ed in
Fig 21.1
. The voltage and current in
this case are 90° out of phase. The pattern
of alternating current and voltage nodes a
quarter-wavelength apart repeats every half-
wavelength along a linear antenna as shown
in Fig 21.1B. The phase of the current and
voltage are inverted in each successive half-
wavelength section.
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 re-
sistance that would dissipate the power the
antenna radiates, with a current flowing in it
equal to the antenna current at a current loop
(maximum). Radiation resistance represents
the work done by the electrons in the antenna
in transferring the energy from the signal
source to the radiated electromagnetic wave.
The loss resistance of a half-wave antenna is
ordinarily small, compared with the radiation
resistance, and can usually be neglected for
practical purposes except in electrically small
antennas, such as mobile HF antennas.
21.1.4 Impedance
The
impedance
at a given point in the an-
tenna 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 ap-
proximately 71 Ω. The antenna’s
feed point
impedance
is the impedance at the point
where the feed line is attached. If the feed
point location changes, so does the feed point
impedance.
Antenna impedance may be either resis-
tive or complex (that is, containing resistance
and reactance). The impedance of a
resonant
antenna is purely resistive anywhere on the
antenna, no matter what value that impedance
may be. For example, the impedance of a
resonant half-wave dipole may be low at the
center of the antenna and high at the ends, but
it is purely resistive in all cases, even though
its magnitude changes.
The feed point impedance is important
in determining the appropriate method of
matching the impedance of the antenna and
the transmission line. The effects of mis-
matched antenna and feed line impedances
are described in detail in the
Transmission
Lines
chapter of this book. Some mistakenly
believe that a mismatch, however small, is
a serious matter. This is not true. The sig-
Fig 21.1 — The current and voltage distribution along a half-wave dipole (A) and for an
antenna made from a series of half-wave dipoles (B).
21.2
Chapter 21
nificance of a perfect match becomes more
pronounced only at VHF and higher, where
feed line losses are a major factor. Minor
mismatches at HF are rarely significant.
increasing resistance is that more and more
energy from the antenna is absorbed by the
earth as the height drops below ¼ λ, seen as
an increase in feed point impedance.
21.1.5 Impedance and Height
Above Ground
The feed point impedance of an antenna
varies with height above ground because of
the effects of energy reflected from and ab-
sorbed by the ground. For example, a ½-λ
(or half-wave) center-fed dipole will have a
feed point impedance of approximately 75 Ω
in free space
far from ground, but
Fig 21.2
shows that only at certain electrical heights
above ground will the feed point impedance
be 75 Ω. The feed point impedance will vary
from very low when the antenna is close
to the ground to a maximum of nearly
100 Ω at 0.34-λ above ground, varying
around ±5 Ω as the antenna is raised farther.
The 75-Ω feed point impedance is most likely
to be realized in a practical installation when
the horizontal dipole is approximately ½, ¾
or 1 wavelength above ground. This is why
few amateur λ/2-dipoles exhibit a center-fed
feed point impedance of 75 Ω, even though
they may be resonant.
Fig 21.2 compares the effects of perfect
ground and typical soil at low antenna heights.
The effect of height on the radiation resis-
tance of a horizontal half-wave antenna is not
drastic so long as the height of the antenna is
greater than 0.2 λ. Below this height, while
decreasing rapidly to zero over perfectly con-
ducting ground, the resistance decreases less
rapidly with height over actual lossy ground.
At lower heights the resistance stops decreas-
ing at around 0.15 λ, and thereafter increases
as height decreases further. The reason for the
21.1.6 Antenna Bandwidth
The
bandwidth
of an antenna refers gener-
ally to the range of frequencies over which
the antenna can be used to obtain a specified
level of performance. The bandwidth can be
specified in units of frequency (MHz or kHz)
or as a percentage of the antenna’s design
frequency.
Popular amateur usage of the term antenna
bandwidth most often refers to the 2:1 SWR
bandwidth, such as, “The 2:1
SWR bandwidth
is 3.5 to 3.8 MHz” or “The antenna has a
10% SWR bandwidth” or “On 20 meters, the
antenna has an SWR bandwidth of 200 kHz.”
Other specific bandwidth terms are also used,
such as the
gain bandwidth
(the bandwidth
over which gain is greater than a specified
level) and the
front-to-back ratio bandwidth
(the bandwidth over which front-to-back ratio
is greater than a specified level).
As operating frequency is lowered, an
equivalent bandwidth in percentage becomes
narrower in terms of frequency range in kHz
or MHz. For example, a 5% bandwidth at
21 MHz is 1.05 MHz (more than wide
enough to cover the whole band) but at
3.75 MHz only 187.5 kHz! Because of the
wide percentage bandwidth of the lower fre-
quency bands 160 meters is 10.5% wide, 80
meters is 3.4% wide) it is difficult to design
an antenna with a bandwidth sufficient to
include the whole band.
It is important to recognize that SWR band-
width does not always relate directly to gain
bandwidth. Depending on the amount of feed
line loss, an 80 meter 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 bandwidth.
Fig 21.3 — 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.
which a specific length of that conductor is
1
⁄
2
-wavelength long electrically.
/ 2
λ
= =
300
l/d
×
(1)
d
2f
d
where f is in MHz and d is in meters.
For example, a
1
⁄
2
-wavelength dipole
for 7.2 MHz made from #12 AWG wire
(0.081- inch dia) has an l/d ratio of
300
l/d
=
=
2f
d
300
×
=
10,126
0.081in
2
××
7.2
39.37 in / m
The effect of l/d is accounted for by the fac-
tor K which is based on l/d. From Fig 21.3 an
l/d ratio of 10,126 corresponds to K ≈ 0.975,
so the resonant length of that ½-wave dipole
would be 0.975 × (300 / 2f) = 20.31 meters
instead of the free-space 20.83 meters.
Most wire antennas at HF have l/d ratios in
the range of 2500 to 25,000 with K = 0.97 to
0.98. The value of K is taken into account in
the classic formula for ½-wave dipole length,
468/f (in MHz). If K = 1, the formula would
be 492/f (in MHz). (This is discussed further
in the following section on Dipoles and the
Half-Wave Antenna.)
For single-wire HF antennas, the effects
of ground and antenna construction make
a precise accounting for K unnecessary in
practice. At and above VHF, the effects of
l/d ratio can be of some importance, since
the wavelength is small.
Since the radiation resistance is affected
relatively little by l/d ratio, the decreased L/C
ratio causes the Q of the antenna to decrease.
This means that the change in antenna imped-
ance with frequency will be less, increasing
the antenna’s SWR bandwidth. This is often
used to advantage on the lower HF bands by
using multiple conductors in a cage or fan to
decrease the l/d ratio.
21.1.7 Effects of Conductor
Diameter
The impedance and resonant frequency
of an antenna also depend on the diameter
of the conductors that make up its elements
in relation to the wavelength. As diameter
of a conductor increases, its capacitance per
unit length increases and inductance per unit
length decreases. This has the net effect of
lowering the frequency at which the antenna
element is resonant, as illustrated in
Fig 21.3
.
The larger the conductor diameter in terms of
wavelength, the smaller its
length-to-diame-
ter ratio (l/d)
and the lower the frequency at
Fig 21.2 — 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.
Antennas
21.3
21.1.8 Radiation Patterns
Radiation patterns
are graphic representa-
tions of an antenna’s directivity. Two exam-
ples are given in
Figs 21.4
and
21.5
. Shown
in polar coordinates (see the math references
in the
Electrical Fundamentals
chapter for
information about polar coordinates), the
angular scale shows direction and the scale
from the center of the plot to the outer ring,
calibrated in dB, shows the relative strength
of the antenna’s radiated signal (gain) at each
angle. A line is plotted showing the antenna’s
relative gain (transmitting and receiving) at
each angle. The antenna is located at the exact
center of the plot with its orientation speci-
fied separately.
The pattern is composed of
nulls
(angles
at which a gain minimum occurs) and
lobes
(a range of angles in which a gain maximum
occurs). The
main lobe
is the lobe with the
highest amplitude unless noted otherwise and
unless several plots are being compared, the
peak amplitude of the main lobe is placed at
the outer ring as a 0 dB reference point. The
peak of the main lobe can be located at any
angle. All other lobes are
side lobes
which
can be at any angle, including to the rear of
the antenna.
Fig 21.4 is an
azimuthal
or
azimuth pattern
that shows the antenna’s gain in all horizon-
Fig 21.6 — Rectangular azimuthal pattern of an 8-element 2 meter Yagi beam antenna
by itself and with another identical antenna stacked two feet above it. This example
shows how a rectangular plot allows easier comparison of antenna patterns away from
the main lobe.
amplitude of the antenna’s strongest lobe as
0 dB. All other points on the pattern represent
relative gain
to the peak gain. The antenna’s
absolute gain
with respect to an isotropic
(dBi) antenna or dipole (dBd) is printed as a
label somewhere near the pattern. If several
antenna radiation patterns are shown on the
same plot for comparison, the pattern with
the largest gain value is usually assigned the
role of 0 dB reference.
The gain amplitude scale is usually divided
in one of two ways. One common division
is to have rings at 0, –3, –6, –12, –18, and
–24 dB. This makes it easy to see where the
gain has fallen to one-half of the reference
or peak value (–3 dB), one-quarter (–6 dB),
one-sixteenth (–12 dB), and so on. Another
popular division of the amplitude scale is 0,
–10, –20, –30, and –40 dB with intermediate
rings or tick marks to show the –2, –4, –6, and
–8 dB levels. You will encounter a number of
variations on these basic scales.
tal directions (azimuths) around the antenna.
As with a map, 0° is at the top and bearing
angle increases clockwise. (This is different
from polar plots generated for mathematical
functions in which 0° is at the right and angle
increases counter-clockwise.)
Fig 21.5 is an
elevation pattern
that shows
the antenna’s gain at all vertical angles. In
this case, the horizon at 0° is located to both
sides of the antenna and the zenith (directly
overhead) at 90°. The plot shown in Fig 21.5
assumes a ground plane (drawn from 0° to
0°) but in free-space, the plot would include
the missing semicircle with –90° at the bot-
tom. Without the ground reference, the term
“elevation” has little meaning, however.
You’ll also encounter E-plane and H-plane
radiation patterns. These show the antenna’s
radiation pattern in the plane parallel to the
E-field or H-field of the antenna. It’s im-
portant to remember that the E-plane and
H-plane do not have a fixed relationship to the
Earth’s surface. For example, the E-plane pat-
tern from a horizontal dipole is an azimuthal
pattern, but if the same dipole is oriented
vertically, the E-plane pattern becomes an
elevation pattern.
Antenna radiation patterns can also be
plotted on rectangular coordinates with gain
on the vertical axis in dB and angle on the
horizontal axis as shown in
Fig 21.6
. This is
particularly useful when several antennas are
being compared. Multiple patterns in polar
coordinates can be difficult to read, particu-
larly close to the center of the plot.
The amplitude scale of antenna patterns is
almost always in dB. The scale rings can be
calibrated in several ways. The most common
is for the outer ring to represent the peak
Fig 21.4 — Azimuthal pattern of a typical
three-element Yagi beam antenna in free
space. The Yagi’s boom is along the 0° to
180° axis.
RADIATION PATTERN
MEASUREMENTS
Given the basic radiation pattern and scales,
it becomes easy to define several useful mea-
surements or metrics by which antennas are
compared, using their azimuthal patterns.
Next to gain, the most commonly-used metric
for directional antennas is the
front-to-back
ratio
(
F/B
) or just “front-to-back.” This is the
difference in dB between the antenna’s gain
in the specified “forward” direction and in
the opposite or “back” direction. The front-
to-back ratio of the antenna in Fig 21.4 is
about 11 dB.
Front-to-side ratio
is also used
and is the difference between the antenna’s
“forward” gain and gain at right angles to the
Fig 21.5 — Elevation pattern of a
3-element Yagi beam antenna placed ½-
λ
above perfect ground.
21.4
Chapter 21
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