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Pobierz
Long-Wire and
Traveling-Wave
Antennas
Chapter
13
The power gain and directive characteristics of
electrically long wires (that is, wires that are long in terms
of wavelength), as described in
Chapter 2
, make them useful
for long-distance transmission and reception on the higher
frequencies. Long wires can be combined to form antennas
of various shapes that increase the gain and directivity over
a single wire. The term
long wire
, as used in this chapter,
means any such configuration, not just a straight-wire
antenna.
edged sword too. We’ll discuss this aspect in some detail in
this chapter.
GENERAL CHARACTERISTICS OF
LONG-WIRE ANTENNAS
Whether the long-wire antenna is a single wire running
in one direction or is formed into a V-beam, rhombic, or
some other configuration, there are certain general principles
that apply and some performance features that are common
to all types. The first of these is that the power gain of a
long-wire antenna as compared with a half-wave dipole is
not considerable until the antenna is really long (its length
measured in wavelengths rather than in a specific number
of feet). The reason for this is that the fields radiated by
elementary lengths of wire along the antenna do not combine,
at a distance, in as simple a fashion as the fields from half-
wave dipoles used in other types of directive arrays.
There is no point in space, for example, where the
distant fields from all points along the wire are exactly in
phase (as they are, in the optimum direction, in the case of
two or more collinear or broadside dipoles when fed with
in-phase currents). Consequently, the field strength at a
distance is always less than would be obtained if the same
length of wire were cut up into properly phased and
separately driven dipoles. As the wire is made longer, the
fields combine to form increasingly intense main lobes, but
these lobes do not develop appreciably until the wire is
several wavelengths long. See
Fig 1
.
The longer the antenna, the sharper the lobes become,
and since it is really a hollow cone of radiation about the
wire in free space, it becomes sharper in both planes. Also,
the greater the length, the smaller the angle with the wire at
which the maximum radiation lobes occur. There are four
main lobes to the directive patterns of long-wire antennas;
each makes the same angle with respect to the wire.
Fig 2A
shows the azimuthal radiation pattern of a 1-
LONG WIRES VERSUS
MULTIELEMENT ARRAYS
In general, the gain obtained with long-wire antennas
is not as great, when the space available for the antenna is
limited, as you can obtain from the multielement phased
arrays in
Chapter 8
or from a parasitic array such as a Yagi
or quad (Chapters
11
or
12
). However, the long-wire antenna
has advantages of its own that tend to compensate for this
deficiency. The construction of long-wire antennas is simple
both electrically and mechanically, and there are no
especially critical dimensions or adjustments. The long-wire
antenna will work well and give satisfactory gain and
directivity over a 2-to-1 frequency range. In addition, it will
accept power and radiate well on any frequency for which
its overall length is not less than about a half wavelength.
Since a wire is not electrically long, even at 28 MHz, unless
its physical length is equal to at least a half wavelength on
3.5 MHz, any long-wire can be used on all amateur bands
that are useful for long-distance communication.
Between two directive antennas having the same
theoretical gain, one a multielement array and the other a
long-wire antenna, many amateurs have found that the long-
wire antenna seems more effective in reception. One possible
explanation is that there is a
diversity effect
with a long-
wire antenna because it is spread out over a large distance,
rather than being concentrated in a small space, as would be
the case with a Yagi, for example. This may raise the average
level of received energy for ionospheric-propagated signals.
Another factor is that long-wire antennas have directive
patterns that can be extremely sharp in the horizontal
(azimuthal) plane. This is an advantage that other types of
multielement arrays do not have, but it can be a double-
λ
long-wire antenna, compared with a
1
/
2
-
dipole. Both
antennas are mounted at the same height of 1
λ
above flat
ground (70 feet high at 14 MHz, with a wire length of
70 feet) and both patterns are for an elevation angle of 10
λ
,
an angle suitable for long-distance communication on
20 meters. The long-wire in Fig 2A is oriented in the 270
°
°
to
Long-Wire and Traveling-Wave Antennas
13-1
Fig 1—Theoretical gain of a long-
wire antenna, in dBi, as a function of
wire length. The angle, with respect
to the wire, at which the radiation
intensity is maximum also is shown.
90
direction, while the dipole is aligned at right angles so
that its characteristic figure-8 pattern goes left-to-right. The
1-
°
long-wire has about 0.6 dB more gain than the dipole,
with four main lobes as compared to the two lobes from the
dipole.
You can see that the two lobes on the left side of Fig 2A
are about 1 dB down compared to the two lobes on the right
side. This is because the long-wire here is fed at the left-
hand end in the computer model. Energy is radiated as a wave
travels down the wire and some energy is also lost to ohmic
resistance in the wire and the ground. The forward-going
wave then reflects from the open-circuit at the right-hand
end of the wire and reverses direction, traveling toward the
left end, still radiating as it travels. An antenna operating in
this way has much the same characteristics as a transmission
line that is terminated in an open circuit—that is, it has
standing waves on it. Unterminated long-wire antennas are
often referred to as
standing wave antennas
. As the length of
a long-wire antenna is increased, a moderate front-to-back
ratio results, about 3 dB for very long antennas.
Fig 2B shows the elevation-plane pattern for the long-
wire and for the dipole. In each case the elevation pattern is
at the azimuth of maximum gain—at an angle of 38
λ
°
with
respect to the wire-axis for the long-wire and at 90
for the
dipole. The peak elevation for the long-wire is very slightly
lower than that for the dipole at the same height above
ground, but not by much. In other words, the height above
ground is the main determining factor for the shape of the
main lobe of a long-wire’s elevation pattern, as it is for most
horizontally polarized antennas.
°
Fig 2—At top, comparison of azimuthal patterns for a
1-
λ
long-wire antenna (solid line) and a
1
/
2
-
λ
dipole
(dashed line) at an elevation angle of 10
. Each
°
antenna is located 1
(70 feet) over flat ground at
14 MHz. At bottom, the elevation-plane patterns at
peak azimuth angles for each antenna. The long-wire
has about 0.6 dB more gain than the dipole.
λ
13-2
Chapter 13
Fig 3—A 3-D representation of the radiation pattern
for the 1-
long-wire shown in
Fig 2
. The pattern is
obviously rather complex. It gets even more
complicated for wires longer than 1
λ
.
λ
Fig 4—An azimuthal-plane comparison between a
3-
(209 feet long) long-wire (solid line) and the
comparison
1
/
2
-
λ
The shape of the azimuth and elevation patterns in
Fig 2
might lead you to believe that the radiation pattern is simple.
Fig 3
is a 3-D representation of the pattern from a 1-
λ
dipole (dashed line) at 70 feet high
(1
) at 14 MHz.
λ
λ
long-
wire that is 1
high over flat ground. Besides the main low-
angle lobes, there are strong lobes at higher angles. Things
get even more complicated when the length of the long-wire
increases.
λ
direction. This is the case even if a multielement array and a
long-wire antenna have the same peak gain in the favored
direction.
Fig 5
compares the same 3-
DIRECTIVITY
Because many points along a long wire are carrying
currents in different phases (with different current amplitudes
as well), the field pattern at a distance becomes more
complex as the wire is made longer. This complexity is
manifested in a series of minor lobes, the number of which
increases with the wire length. The intensity of radiation
from the minor lobes is frequently as great as, and sometimes
greater than, the radiation from a half-wave dipole. The
energy radiated in the minor lobes is not available to improve
the gain in the major lobes, which is another reason why a
long-wire antenna must be long to give appreciable gain in
the desired directions.
Fig 4
shows an azimuthal-plane comparison between
λ
long-wire with a
4-element Yagi and a
1
/
2
-
dipole, again both at the same
height as the long-wire. Note that the Yagi has only a single
backlobe, down about 21 dB from its broad main lobe, which
has a 3-dB beamwidth of 63
λ
. The 3-dB beamwidth of the
long-wire’s main lobes (at a 28
°
°
angle from the wire axis) is
far more narrow, at only 23
.
For amateur work, particularly with directive antennas
that cannot be rotated, the minor lobes of a long-wire antenna
have some advantages. Although the nulls in the computer
model in
Fig 5
are deeper than 30 dB, they are not so
dramatic in actual practice. This is due to irregularities in
the terrain that inevitably occur under the span of a long
wire. In most directions the long-wire antenna will be as
good as a half-wave dipole, and in addition will give high
gain in the most favored directions, even though that is over
narrow azimuths.
Fig 6A
compares the azimuth responses for a 5-
°
a 3-
λ
dipole. The long-wire now has 8 minor lobes besides the
four main lobes. Note that the angle the main lobes make
with respect to the axis of the long-wire (also left-to-right in
Fig 4) becomes smaller as the length of the long-wire
increases. For the 3-
λ
(209 feet long) long-wire and the comparison
1
/
2
-
long-
wire (350 feet long at 14 MHz) to the same 4-element Yagi
and dipole. The long-wire now exhibits 16 minor lobes in
addition to its four main lobes. The peaks of these sidelobes
are down about 8 dB from the main lobes and they are
stronger than the dipole, making this long-wire antenna
effectively omnidirectional. Fig 6B shows the elevation
pattern of the 5-
λ
λ
long-wire, the main lobes occur 28
°
off the axis of the wire itself.
Other types of simple driven and parasitic arrays do
not have minor lobes of any great consequence. For that
reason they frequently seem to have much better directivity
than long-wire antennas, because their responses in undesired
directions are well down from their response in the desired
long-wire at its most effective azimuth
compared to a dipole. Again, the shape of the main lobe is
λ
Long-Wire and Traveling-Wave Antennas
13-3
mainly determined by the long-wire’s height above ground,
since the peak angle is only just a bit lower than the peak
angle for the dipole. The long-wire’s elevation response
breaks up into numerous lobes above the main lobes, just as
it does in the azimuth plane.
For the really ambitious,
Fig 7
compares the
performance for an 8-
(571 feet) long-wire antenna with a
4-element Yagi and the
1
/
2
-
λ
dipole. Again, in actual practice,
the nulls would tend to be filled in by terrain irregularities,
so a very long antenna like this would be a pretty potent
performer.
λ
CALCULATING LENGTH
In this chapter, lengths are discussed in terms of
wavelengths. Throughout the preceding discussion the
frequency in the models was held at 14 MHz. Remember
that a long-wire that is 4
λ
long at 14 MHz is 8
λ
long at
28 MHz.
There is nothing very critical about wire lengths in an
antenna system that will work over a frequency range
including several amateur bands. The antenna characteristics
change very slowly with length, except when the wires are
short (around one wavelength, for instance). There is no need
to try to establish exact resonance at a particular frequency
for proper antenna operation.
The formula for determining the lengths for harmonic
wires is:
Fig 5—A comparison between the 3-
λ
long-wire (solid
line) in Fig 4, a 4-element 20-meter Yagi on a 26-foot
boom (dotted line), and a
1
/
2
-
dipole (dashed line),
again at a height of 70 feet. The main lobes of the long-
wire are very narrow compared to the wide frontal lobe
of the Yagi. The long-wire exhibits an azimuthal pattern
that is more omnidirectional in nature than a Yagi,
particularly when the narrow, deep nulls in the long-
wire’s pattern are filled-in due to irregularities in the
terrain under its long span of wire.
λ
984
N
f MHz
(
−
0 025
.
)
(
)
Length feet
=
(Eq 1)
(
)
Fig 6—At left, the azimuth responses for a 5-
long-wire
(350 feet long at 14 MHz—solid line)) to the same
4-element Yagi (dotted line) and dipole (dashed line) as
in Fig 5. Above, the elevation-plane responses for the
long-wire (solid line) and the dipole (dashed line) by
themselves. Note that the elevation angle giving peak
gain for each antenna is just about the same. The long-
wire achieves gain by compressing mainly the azimuthal
response, squeezing the gain into narrow lobes; not so
much by squeezing the elevation pattern for gain.
λ
13-4
Chapter 13
Fig 8—Methods for feeding long single-wire antennas.
Two arrangements for using nonresonant lines are given
in
Fig 8
. The one at A is useful for one band only since
the matching section must be a quarter wave long,
approximately, unless a different matching section is used
for each band. In B, the
Fig 7—The azimuthal-plane performance for an 8-
λ
(571 feet) long-wire antenna (solid line), compared
with a 4-element Yagi (dotted line) and a
1
/
2
-
λ
dipole
(dashed line).
/4 transformer (Q-section)
impedance can be designed to match the antenna to the line,
as described in
Chapter 26
. You can determine the value of
radiation resistance using a modern modeling program or
you can actually measure the feed-point impedance.
Although it will work as designed on only one band, the
antenna can be used on other bands by treating the line and
matching transformer as a resonant line. In this case, as
mentioned earlier, the antenna will not radiate as a true long
wire on even multiples of the frequency for which the
matching system is designed.
The end-fed arrangement, although the most convenient
when tuned feeders are used, suffers the disadvantage that
there is likely to be a considerable antenna current on the
line. In addition, the antenna reactance changes rapidly with
frequency. Consequently, when the wire is several
wavelengths long, a relatively small change in frequency—
a fraction of the width of a band—may require major changes
in the adjustment of the antenna tuner. Also, the line becomes
unbalanced at all frequencies between those at which the
antenna is resonant. This leads to a considerable amount of
radiation from the line. The unbalance can be overcome by
using multiple long wires in a V or rhombic shape, as
described below.
λ
where N is the antenna length in wavelengths. In cases where
precise resonance is desired for some reason (for obtaining
a resistive load for a transmission line at a particular
frequency, for example) it is best established by trimming
the wire length until the standing-wave ratio on the line is
minimum.
Tilted Wires
In theory, it is possible to maximize gain from a long-
wire antenna by tilting it to favor a desired elevation take-
off angle. Unfortunately, the effect of real ground under the
antenna negates the possible advantages of tilting, just as it
does when a Yagi or other type of parasitic array is tilted
from horizontal. You would do better keeping a long-wire
antenna horizontal, but raising it higher above ground, to
achieve more gain at low takeoff angles.
Feeding Long Wires
A long-wire antenna is normally fed at the end or at a
current loop. Since a current loop changes to a node when
the antenna is operated at any even multiple of the frequency
for which it is designed, a long-wire antenna will operate as
a true long wire on all bands only when it is fed at the end.
A common method of feeding a long-wire is to use a
resonant open-wire line. This system will work on all bands
down to the one, if any, at which the antenna is only a half
wave long. Any convenient line length can be used if you
match the transmitter to the line’s input impedance using an
antenna tuner, as described in
Chapter 25
.
COMBINATIONS OF LONG WIRES
The directivity and gain of long wires may be increased
by using two wires placed in relation to each other such that
the fields from both combine to produce the greatest possible
field strength at a distant point. The principle is similar to
that used in designing the multielement arrays described in
Chapter 8
.
Long-Wire and Traveling-Wave Antennas
13-5
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