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Now that we are officially
into the next (new) sunspot cycle, everyone is awaiting for the attendant
improvements in propagation on the higher frequency bands. AMers are especially
interested in 10 meters. This band has been nearly devoid of AM activity
(other than local pockets) for too long. Ten meters is a great band for
AM in that it offers plenty of room to spread out and generally, freedom
from QRM that is often prevalent on most of the other bands. Combine this
with the fact that using low power and simple, small antennas can easily
yield QSOs all over the U.S. and even world wide, and 10 meters is the place
to be!
I mentioned simple and small antennas. What could be more simple
and smaller (at least for HF use) than a 10 meter dipole or vertical. Either
will perform well, even a modest heights, like 20 or 30 feet. Remember,
30 feet is almost one-wavelength above ground on 10 meters. But, what if
you want more gain and directivity than can obtained with a dipole? Well,
on 10 meters, you can build antennas that can have quite a bit of gain and
directivity and they can still be small and simple. All antennas discussed
are made of wire, have a design frequency of 29 Mhz since this is where
most of the AM activity occurs on 10 meters, and are shown with antenna
patterns modeledusing EZNEC, with the antenna 30 feet above ground. Also,
in the name of simplicity, all antennas are fed with ladder line or open-wire
line to avoid more complex matching schemes (stubs, gamma matches, etc).
The easiest way to obtain more gain is to take your basic dipole
and extend its length to about 1.28 of a wavelength overall (0.64 for each
leg). This is known as a Extended Double Zepp (EDZ). This type of antenna
provides 3-4 dB of gain as can be seen in Figure 1.
Only a few lucky hams have room for this type of antenna
on 160 or 75 meters but on 10 meters the overall length is just 43 feet
long! This is smaller than a 40 meter dipole. With this arrangement, the
antenna will work well on all bands 40 through 10 but it will only works
as a EDZ on ten. On the other bands its will have about the the same gain
as a dipole. If coax feed is a must, a simple matching arrangement is shown
in Figure 2. With this setup, the
antenna will only work on ten meters.
If you only have one support but want still want gain as with the
Extended Double Zepp, consider the loop shown in Figure
3. It's actually rectangular and is very small. It can
be feed directly with 50 Ohm coax (preferably through a balun) and provides
more gain than the EDZ, and at a lower take-off angle. See Figure
4. How you support the wires in the loop (most especially
the horizontal ones) is up to you. However, a loop with the appropriate
dimensions could easily be made from bamboo, fiberglass rods or small diameter
PVC pipe and the wires attached to it. Then the antenna could be hoisted
aloft with just one rope. You'd probably need one other rope tied off to
the ground to keep the loop from spinning around. Using this approach would
allow you to rotate the loop, too. Of course you'd never have to rotate
it more than 90 degrees since it is bidirectional. |
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Another design that is often too big to be considered on the lower
frequency bands is the Lazy H. The name describes it, an H turned on
its side, as shown in Figure 5. The
two elements are one-half wavelength long, spaced one-half wavelength apart,
and are fed as shown. On 10 meters, these dimensions are workable
for most hams that own any real estate larger than a postage stamp. The are numerous other
antenna references (ARRL Antenna Handbook, etc.) that provide the details
of constructing open-wire line. Only about 16 feet is required, so the extra
construction requirement should not be too burdensome. As you can see from
Figure 6 , the Lazy H is bidirectional
but provides a nice 6 dB gain. This might be a good design for the AMer
who lives in the central part of the USA and wants to put a better signal
into both coasts with a minimum of effort. An extra bonus is that this antenna
will work on 20, 17, 15, and 12 meters, although without the gain obtained
on 10 meters (some people miss this part).
You can squeeze a little more gain out of the Lazy-H. The elements
are already a full wavelength (about 34 feet). By making each element just
a little longer (1.28 wavelength, just like the EDZ) another 1-2 dB can
be had with the right spacing. The element spacing should be increased from
one-half wavelength to anywhere from 0.64 to 0.75. The gain improvement
for 0.64 wavelength spacing is shown in Figure 7. One other change from the standard Lazy-H is required but
it is simple. Instead of reversing the connections of the feedline between
the upper and lower elements, as shown in Figure 8, DO NOT reverse the connections and feed at the center point.
That's it, you now have an EDZ Lazy H. It will also work well on all bands
between 40 and 10 meters and will have gain on 20 through 10.
A neat design that only requires one support is the Bisquare. If
you glanced quickly at Figure 8 you
might think it was a loop. But notice that it is open at the top. In this
respect it's more like a dipole with the legs bent upwards into a diamond
shape. This simple antenna gives you about 4 dB gain, as shown in Figure 9. It is a nice alternative to some
of the bidirectional designs presented previously, especially for the AMer
that has only one support. If you can devise a way to close the connection
at the top (with a relay or by hand), this antenna will work well on 20
meters too.
Of course there are many other antenna designs that can be used
on 10 meters. This article focused on some of the simple wire types that
provide gain and directivity. If you've tried any others that fit into this
category, share your design with us. Have fun and I'll hear you (hopefully
with an even better signal) on 10 meters. ;-)
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