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1994-05-01
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4KB
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94 lines
SUBJECT: Wire antennas - part one
Dollar for dollar, the wire antenna is an Amateur Radio
Operators (AROs) best bet when it comes to inexpensive antennas.
In the lower bands, 160, 80, and 40m, the wire antenna is
nearly the only means for most AROs to work those bands.
Wire antennas perform from great to poor depending on many
factors even when the antenna is cut for the frequency desired.
I'll discuss some of my findings with wire antennas in this ten
part series. Since most of the wire antennas are published in
numerous antenna books, the specifics of most wire antennas
will be left to the reader to investigate for the band they
wish to operate in. Instead, I will discuss some helpful
hints, findings, and misgivings about wire antennas that
the ARO can use in overcoming the difficulties encountered
with their first wire antennas and some that the old pros may
find usefull.
I will be discussing the advantages and disadvantages of
wire antennas on different bands, wire diameter, ground height,
matching, multi-band wire antennas, and a few other odds and
ends.
---------------------------------------------------------
First, let's look at the capture area of a wire antenna cut
as a dipole. We can examine this at 10m for ease of calculations
and understanding.
Many hams use a #12 or #10 wire when constructing their wire
antennas. It's cheap and easy to obtain. Some wire antennas
come as kits and use strained wire verses the solid conductor
many hams use in their home construction. Looking at the
diameter of the wire, whether strained or not, it can be easily
seen that a dipole made from aluminum tubing has a greater
capture area, without picking up a calculator to find out the
difference. Larger diameter elements produce greater
bandwidths, thus a wire dipole on 10m would have a limited
bandwidth compared to it's aluminum tubing counterpart.
How does one overcome this? Through trial and error,
and a tight budget when I first got into Ham radio, I found
that cutting the wire antenna slightly shorter that the
designed frequency and adding some tubing to the end of the
dipole, I was able to expand the bandwidth of the wire
antenna upto three times the orignal value.
It doesn't take much tubing or a large diameter to
accomplish this. I used two of these units (of course, one
on each end). There are two parts to the extention. Use
a 12-in x 1/2-in diameter tube with an 8-in x 3/8-in tube
telescoping inside the former. By placing a vertical cut
into the 12-in section, a clamp can be used to lock the
two tubes together. Now you have tunable end pieces.
Run the support rope through the tubing and attach to
the wire then secure the wire to the 1/2-in end of the
tubing. A good electrical contact is a must. The rope
will support the tunable end piece.
The end pieces are also much greater in diameter than
the wire, adding to the capture area. Also they provide a
tunable method for zeroing the dipole to the frequency
desired or changing the frequency of the dipole later.
At 10m, I would recommend an aluminum tubing dipole and
save this technique for the 30m and below antennas. For
the lower frequency wire dipoles and inverted vees, the
tubing can be made much longer for better results. There
is a fine line between weight and performance that needs
to looked into when using this method and it's a function
of the antenna support and support lines.
I stumbled across this technique while putting around
with a 40m inverted vee. I just could not get the bandwidth
and SWR right. It was either too high for the resonate
frequency or too low no matter what the calculator thought.
The SWR was 2:1 and I wanted an antenna that did not require
a tuner for my solid state radio. I grew tired of soldering
and cutting wire and decided to add the tubing extenders.
Not only did I get the Inverted Vee on target, 7.15 MHz, but
I covered the entire 40m band and the MARS frequency I was
required to attend on. The SWR was below 1.5:1 across the
entire band also! :-)
It worked so well I decided to bring it to field day,
1991. The antenna worked very well and without a tuner.
If you have any questions about the modification, let me
know on HAM_TECH.
-WS