Build A 9 dB, 70cm, Collinear Antenna From Coax
By N1HFX
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 1/2 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 1/2 wavelength of
RG58/U coaxial cable with a frequency of 444 Megahertz:
1/2 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 1/2 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. Now its time to add the
additional elements to the top and bottom of the collinear array. First add a
1/4 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
1/4 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:
1/4 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 1/2 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.
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 1/4" 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 3/4" 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 1/2 wave coaxial elements result in an antenna too long for your
liking (over seven feet), then it is okay to use four 1/2 wave coaxial elements
but the SWR may be slightly higher (Attach four 1/4 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 1/4 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 N1HFX
As a foot note:
I used a matching stub instead of a bazzuka, ( free space 1/4 wavelength
piece of wire) and attained good SWR. I also used hot glue to
encapsulate the joints which gives a good measure of weather protection
and rigidity to stop shorts.
(I also believe 9 dB of gain optimistic, and gain isn't a linear increase
with
elements as there is a decreasing current distribution from the feed to the
top .) This is dead easy to build, good luck
VK4TCS
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