Catalog 2005 - Radio Works
One band just isn’t enough
Most hams want to be operational on as many
bands as possible. This requires a separate
dipole for each band. Since we now have eight
HF bands, that means that eight different
coax-fed dipoles are required to cover all the
bands from 80 through 10 meters. Eight separate antennas are unacceptable to the average Ham. Most of us want 80 through 10
meters coverage with a single antenna.
Figure 1
Figure 1 is a treasure map. Contained within
this diagram is one of Mother Nature’s important
secrets. The mystery of wonderfully loud signals
emanating from insignificant looking antennas
strung between two cooperative trees is explained
in this simple chart. This map to the “Treasure
Island” of incredible antenna performance was
borrowed with permission from the 14th edition
of the ARRL Antenna Book, page 7-1.
Popularity does not guarantee high
antenna performance The all time favorite wire antenna is the halfwavelength dipole, center-fed with coax. Building
it is easy, and it performs adequately. The dipole
is the reference used to measure the performance
of the other antennas. It is the base line. The
dipole is a suitable reference for amateur radio
purposes because you can actually build one. In
contrast, all the antenna patterns used in this
publication are referenced against an ‘isotropic
antenna.’ The results are expressed in dBi.
Isotropic Antenna
An Isotropic Antenna is a hypothetical antenna
radiating or receiving equally in all directions.
Such antennas do not exist physically but
represent convenient reference antennas for
expressing directive properties of actual antennas.
Figure 2
20 meter dipole @ 50 feet
Takeoff angle show = 200
There are several ways to achieve this goal of
a single antenna for 80 - 10 meters. The most
popular technique for achieving multiband operation with a single antenna is to divide the
antenna into appropriate lengths with ‘traps.’
Traps act like electronic switches, which isolate the correct portions of the antenna wire to
achieve half-wavelength resonance within each
desired band. A five-band trap antenna can
use as few as two traps or as many as eight. It
all depends the trap design and the desired
SWR performance of the antenna. In general,
the more traps, the better behaved the antenna
is in terms of operating bandwidth and acceptable SWR.
More High Performance Wire Antennas
There is a high price to pay for the convenience of traps. First, trap antennas are usually expensive.
Second, there is a loss of operating bandwidth if a low SWR is a requirement. Besides the bandwidth loss,
there may be a slight loss in efficiency caused by the traps themselves. This loss may not be significant if the
traps are well designed.
Disadvantages of trap antennas
1. Dipole performance at best
2. Trap antennas often require trimming
3. Traps fail
4. High cost of traps
5. Hard to troubleshoot
6. Detuning effects of nearby objects
7. They are heavy and often difficult to support
1. No tuner required, if you are willing to operate
within narrow portions of the bands.
2. Slightly shorter than full size dipoles
3. One antenna can cover many bands
It is my argument that trap antennas are not an
efficient use of available antenna space. If you
put up a 120' long 80 - 10 meter trap antenna, why
use only 33' of it on 20 meters? There are significant
advantages to using the full antenna length on all
bands. Let me show you some interesting data.
Figure 3 is the radiation pattern of and ‘Extended Double Zepp’ or ‘EDZ.’ An EDZ built for 20
meters is about 85' long. Notice the difference in
the pattern of this antenna and the curve for the
dipole in figure 2. The narrowing of the lobes concentrates the energy radiated by the antenna. Concentrating energy is another way of saying the antenna develops gain. The Extended Double-Zepp
produces between two and three dB gain. A 3
dB gain is the equivalent of doubling your output
power. Note the four ‘minor lobes.’ Operation is not
limited to the direction of the major lobes.
The point of comparing these two curves is
to show that it is possible to build better antennas
than dipoles.
This is not the end to the story. In fact, let’s
go back to the beginning. Figure 1 gives us the most
important reason for using the full length of 80 or
40 meter antennas on higher bands where antenna
length becomes long.
Figure 3
Extended Double Zepp @ 50 feet
Pattern represents a 150 takeoff angle
In figure 1, look at the curve marked “GAIN.”
It rises as the length of the antenna increases beyond
½ wavelength. These gain figures are not large. For
example, on 10 meters, the gain of an 80 meter dipole
will be only 3 dB. But, that’s three dB you wouldn’t
have if you were using a dipole. It is the same as
doubling your power. Remember that every dB
counts. 3 dB here, another 3 dB there, they all
add up to a big signal. Figure 1 proves a very
important point. Put up a lot of wire, and get a lot of
gain. It’s that simple.
Combine the advantage of a long antenna
with techniques that lower the radiation angle, and
the result is unbeatable. This is the secret of high
performance wire antennas.
It’s not magic. It is
just taking advantage of physical laws.
The RADIO WORKS specializes in high
performance wire antennas that take advantage
of these principles. Take a close look at the
CAROLINA WINDOM series of antennas,
CAROLINA BEAMs, SuperLoops, and G5RV Plus.
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