On Tue, 7 Nov 2006 12:21:06 -0500, Dr M J DiGirolamo wrote:
>Any comments or suggestions for 160M and 80M antennas would be
appreciated.
I just moved to a similar situation here in CA, but with a lot more
space -- I own 8.5 acres of an old-growth redwood forest! I have two
suggestions. Do both if you can, under the theory that you can never
have too many antennas. There WILL be times when one will work better
than the other to whoever you are trying to work. I run each of them
through a tuner, then put them through a switch so that I can select
between them in a contest (or when calling someone).
1) Hang a dipole for 80 and 160 as high as you can get it. A couple of
locals came over (with a WB6ZQZ pneumatic tennis ball launcher) and
helped me do that a week ago. I watched as the tennis ball sailed over
the top of a redwood that is at least 130 ft tall, then again as the
launch for the other end topped a 130 ft or taller Douglas Fir. By the
time we got heavy rigging lines in, they were on solid branches at
about 110 ft, and the antenna is roughly 100 ft above ground. That
antenna kicks some serious butt. (So does the 20/15/10 fan that is in-
line with it).
Don't be afraid to use loaded dipoles if that's what it takes to fit it
between your available skyhooks. The dipole above is full size on 80,
with loading coils added to make it play on 160. I also have a 40 meter
element in a "fan" arrangement, using 15" pieces of 0.5" PVC conduit to
keep it about 12" from the 80 meter element. Sure, a full size dipole
might be a dB or so more efficient, but height is more important.
Also, ANY 160 dipole you are likely to be able to hang is probably
going to be < 0.25 wavelength high, so it isn't going to be terribly
dead off the ends, so don't be TOO worried about azimuth.
2) Hang a tee top-loaded vertical between a couple of trees, and put
as many radials under it as you can. There are several forms this
antenna can take, depending on the cards nature has dealt you. The
tee differs from the inverted L in that the top radiation tends to
cancel itself, so you end up with a better low angle signal. That
doesnt mean you wont work locals, it simply means you wont be as
strong close-in. Besides, your dipole will be low enough to allow you
to work locals.
My current top-loaded antenna is actually an 80/40 loaded dipole ab
about 65 ft (at 90 degrees to the higher one). The feedline is some old
72 ohm kW twinlead, and I tie both ends of it together to use it as a
vertical. This antenna works out to be a quarter-wave on 80 and 1/8
wave on 160, but the top loading makes the transmitter see it as 1/4
wave (low Z) on 160 (easy to load) and 1/2 wave on 80 (difficult to
load).
To load it on 80, I improvised an autoformer using a BIG inductor from
a hamfest purchase several years ago. The inductor is resonated with
the capacitance of about 10 ft of RG-8.
Another form the antenna might take is the vertical equivalent of a fan
-- that is, a 67 ft element (for 80) with spacers to a taller element
for 160. The top of the tee connects only to the longer 160 wire, and
the length of the top of tee will depend on how high the vertical
element is (that is, adjust it until you have something approaching 1/4
wave resonance at the base of the antenna). Do NOT tune it for minimum
VSWR -- the loss in the ground system will give you a wrong answer to
that one. Then use a suitable matching network or tuner to make up the
difference. Im planning to replace my current top-loaded vertical
(80/40 dipole) with this version. Actually, Ill move the 80/40 to
different trees and dedicate it to use as a dipole, and add this taller
vertical.
The loading coils Im using came from Hypower Antenna Co, which is
Barry, NU3X, in his basement, I think. They seem to be standing up to
my kW in contests. Google to find him, or see his ad in QST.
Follow N6LFs guidelines in the latest (20th) ARRL Antenna Book on how
to best spend your time and money on radials.
I own a Big Shot from Sherrill Tree Svc (in NC), which is essentially
a big slingshot. Its good for about 80 ft going straight up. Nice
people.
73,
Jim Brown K9YC
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