Build A 9dB, 70cm Collinear Antenna


Build A 9 dB, 70cm,
Collinear Antenna From Coax

By

Recently the RASON technical
committee was hard at work at the repeater site repairing our 2
meter repeater antenna. One of the members commented to me that I
should write an article about collinear arrays so that we could
all build our own. While it is not always feasible to home-brew a
commercial quality antenna designed to take hurricane force
winds, it is very feasible to built a collinear antenna for
average use. This article describes a collinear antenna made from
very inexpensive RG58/U coaxial cable and encased in PVC pipe.

Before we start building we need
to cover some ground about the characteristics of coaxial cable.
First remember that there is something called the velocity factor
for coaxial cable. For RG58/U coax it is typically .66. This
means that when we calculate the length of � wavelength in free
space we need to adjust its size by multiplying it by the
velocity factory. Simply put, RF slows down by the velocity
factor when traveling through coaxial cable. All that aside now,
lets calculate the � wavelength of RG58/U coaxial cable with a
frequency of 444 Megahertz:

� wavelength of
coax = 300 / F / 2 * V

Where F = Frequency
in Megahertz

V = Velocity
factory of Coax

300 / 444 / 2 * .66
= .2229 meters or 223 millimeters

To allow for cutting the ends of
our coax, we will need to add 8 millimeters to each � wave
length for a total of 231 millimeters.

To get started, we will need 8
half wave lengths (231 millimeters) of RG58/U coaxial cable to be
cut and connected in the manner shown in Figure 1. First cut back
4 millimeters of the outer jacket, braid and dielectric exposing
the center conductor as in Figure 2. Now cut back the outer
jacket another 4 millimeters to expose the braid and push the
braid back about a millimeter to prevent it from shorting with
the center conductor. It is best to lightly tin the braid with
solder at this point. Now solder each half wavelength as shown in
Figure 1. Attach a few feet of RG58/U to the bottom of the array
as in Figure 1 for feeding the antenna.

Build A 9dB, 70cm Collinear Antenna

Build A 9dB, 70cm Collinear Antenna #2

Now its time to add the additional
elements to the top and bottom of the collinear array. First add
a � wave element to the top of the antenna as shown in Figure 3.
Use #16 solid wire or similar and solder it to the center
conductor only. The length of the � wave element is calculated
as follows:

1/4 wavelength
radiator = 300 / F / 4

Where F = Frequency
in Megahertz

300 / 444 / 4 =
.1689 meters or 169 millimeters

At the bottom of the array we will
slide a 5/16 inch aluminum tube over the coax and crimp it to the
braid of the antenna feed point only. If copper is used, it is
okay to solder. The length of the tube is calculated as follows:

� wavelength of
tubing = 300 / F / 4 * V

Where F = Frequency
in Megahertz

V = Velocity
factory of Tubing. (Use .95 for 5/16" tubing)

300 / 444 / 4 * .95
= .1604 meters or 160 millimeters

Because a collinear antenna is hot
with RF along the shield of the coax, it is necessary to prevent
the RF from coming back through the coax. Slide three FT50-43 or
almost any similar sized toroids over the bottom end of the coax
as shown in Figure 3. The toroids should be placed about � wave
length from the bottom of the array. Use the same formula for
calculating a half wave length of coax. If you prefer, apply RF
to the antenna at this point and slide the toroids up and down
until minimum SWR is found. Tape the toroids to the proper point
on the coax using electrical tape or similar means.

Build A 9dB, 70cm Collinear Antenna #3

After completing the basic
assembly of the collinear antenna, apply a small amount of RF
with the antenna on the floor or ground. Relatively low SWR
should be observed at this point. The SWR will be much lower once
the antenna is mounted in the air. If the SWR is greater than 2
to 1 across the entire band, a connection may separated or a
short occurred. It will be necessary to correct the problem
before proceeding. After good SWR is obtained, place heat shrink
tubing along all connections or wrap tightly with electrical
tape.

For final mounting, attach the
antenna to a �" wooden dowel using tie wraps about every 3
inches. It may not be possible to obtain a wooden dowel for the
complete length so attach two dowels together by using a 1 inch
sleeve of 5/16" tubing and crimping the tubing at each end.
Check SWR again to insure that no connections have separated or
shorted. Carefully insert the coax and dowel assembly into
several feet of �" PVC pipe for final mounting. Because of
the tie wraps, it is not necessary to use spacers but may be
necessary if larger size piping is used. Drill a hole for the
coax at the bottom end cap and place an end cap on the top of the
PVC. Do not cement end caps until the SWR has been doubled
checked. Cement end caps and water proof coax opening on the
bottom. Use whatever type of coaxial connector is desired on the
bottom of the coax end but do not use RG58/U for your complete
feed line. Use a low loss coax such as RG8/U for the main feed
line to the transceiver. Don’t forget to water proof all coax
connectors.

If the eight � wave coaxial
elements result in an antenna too long for your liking (over
seven feet), then it is okay to use four � wave coaxial elements
but the SWR may be slightly higher (Attach four � wave vertical
ground radials at the antenna feed point to help lower SWR.). If
9 dB gain is still not enough for you then increase the number of
coax elements from eight to sixteen. You will probably need to
attach guy lines to the antenna. Although only a 70 CM antenna
was described in this article, the formulas can be easily
calculated for the 6 meter, 2 meter or 1� meter bands.
Millimeters were used for many of the measurements but can be
converted to inches by dividing millimeters by 25.4 for those who
are not familiar with the metric system. After installing one of
these antennas, be prepared to hear stations and repeaters that
you never heard before.

DE


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