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Building a Coaxial Collinear Antenna
Discussion
Vertical antennas "with gain" are usually
pretty tough to build utilizing tools found around the
average home. Hard to construct matching coils are normally
needed as well as hardware and tubing which can be difficult
to find.
The Coaxial Collinear is rarely mentioned in radio
circles, odd....since many commercial antennas are based
on it's design. Appreciable gain can be had from them
if one wishes to spend a little extra time consructing
additional elements. It's design is super simple and
since it needs no radials, it takes up little horizontal
space; its weight is negligible. Best of all, it is
made from coaxial cable.
The only tools needed are a soldering iron with
solder, a sharp cutting tool and an optional plumber's
tubing cutter. A little tape or shrink tube to seal
it up and an old antenna tube or piece of PVC pipe to
mount it in.
First, determine what the working frequency will
be. Though this antenna design will work on any frequency,
versions for frequencies below about 90 MHz will tend
to be quite long.
Divide 2757 by the working frequency to obtain 1/4
wavelength through air (1/4 WLA). Many people use 492
and get 1/2 wavelength in feet, then convert usually
giving them a dimension much too long. I use 2757 with
excellent results. Next, select the type of coax to
be used. If Teflon insulated type is used, multiply
1/4 WLA by 0.88; the resulting figure will be 1/4 Wavelength
of Teflon coax. Preferably you will decide on polyethylene
coax (the cheap stuff); the 0.66 Velocity Factor will
result in much shorter and easier to work with antennas.
For polyethylene coax, multiply
1/4 WLA by 0.66 to obtain the length of a 1/4 wave section
of polyethylene coax.
Teflon coax, also referred to as "low loss"
or "foam" type, is usually marked as such.
If not, it can be identified by its very soft white
insulation between the center conductor and braiding.
**Note: Either RG-58/U or RG-8/U will work. RG-58/U
is ok for receive only or for transmitting under 20
watts.**
Construction
Step#1
The top section is made from #12 or #10 copper wire.
The bottom "balun stub" is made from a peice
of #14 wire or equivalent.
Step#2A
Cut pieces utilizing the above formulas, plus 2-1/4".
Step#2B
Cut 1-1/8" of black outer insulation from each
end.
Step#2C
Tin copper braiding with solder.
Step#2D
Using a sharp cutting tool or tubing cutter, cut 5/8"
of tinned braiding
off each section end.
Step#2E
Cut 1/2" of insulation from center conductor, leaving
1/8" between
center conductor and braiding.
Image
1
Step#3
Section splicing
Image
2
After soldering all sections and balun stub together,
test SWR with an antenna analyzer or transmitter and
SWR meter. If formulas were followed, all should be
OK. If not, trim or add to the 1/2 wave section(s).
The more 1/2 wave sections you use,
the more gain you get. About 6dB gain with 8 halfwave
sections and 9dB with 16 halfwave sections.
Radiation patterns are Omni-directional.
Image
3
When you are happy with the SWR, cover each joint with
tape or shrink tubing for insulation and strength. Insert
entire assembly into a piece of PVC tubing or a hollow
fiberglass antenna tube, such as used with marine antennas.
PVC pipe is sometimes made
with materials containing metal. This type of pipe will
now work with this antenna. I test PVC by putting it
in the microwave oven. If it gets warm, I don't use
it for antennas.
For down tilt, but antenna sections 2% shorter than
the calculated length and adjust VSWR with the balun.
Mount antenna as high as possible away from any
power carrying wires.
De W1KLM
*Special thanks to Leonard/WB3AYW for use of images.*
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