Military Affiliate Radio System
"Providing the reins of command in emergencies"
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Military Affiliate Radio System "Providing the reins of command in emergencies" |
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Over the 25 + yrs I have been a licensed amateur, this antenna style has been
a real workhorse for me. It has been used in lots of locations (I’m retired US Air Force now),
some less than optimum (like taped to the eves of a motel). When stationed in Louisiana,
I even scaled one for 160 meters. Don’t remember how well it worked, or what the bandwidth
was, but will remember the look on the kids face at “Radio Shack” when I asked for a single
section of RG-58 that needed to be 260 ft long. He called three stores to see who had that
much on a spool.
My 2-meter beam is an 8-element unit with a coaxial bazooka for the driven element.
It is taped to a splint (5/16 wooden dowel rod). I also have an 8-element beam for 440 MHz.
I added a couple of layers of electrical tape to stiffen the coax bazooka up enough to keep
it straight.
While stationed in South Carolina, I built a 15-meter unit for WA4WYT. It was slipped
inside of two 15 ft fiberglass-fishing poles, and mounted on a boom with a full sized
director and reflector. It worked good, really good, at 40 ft above ground.
The antenna is operated at DC ground. Your handy dandy ohmmeter will read a short even
when the antenna is operating properly. It can be a real bear to troubleshoot if you
get it to hot while soldering feedline on. The heat can cause the shield to melt through
the insulation and short out to the center conductor.
Protective Coating Company makes an epoxy called “PC-7”. It comes in small cans, and
I get at my local hardware store. It is thick and sticky, but can be molded into a center
support. Use waxed paper to control/contain drips. A small eyebolt and be inserted before
it hardens, or layers can be built-up and a hole drilled for a rope to raise and lower.
Play with the math and should be able to make one for about any frequency. Some emergency
management agency folks are looking at this style for dependable communications from
“deployed” locations. If you need an antenna for portable use, this one works nice.
Roll it up and toss it into the closet until you need it. Screw a feedline on it; toss
a couple of lines into some trees, hoist it up, and you’re on the air. I have used these
for our HF Field Day antennas.
The old ARRL Handbook (about ’76 vintage I think) had a “double bazooka” designed by
Mass. Institute of Tech. for radar use. The center section is longer than this one,
and the ends are open wire line. Formulas are:
Total length = 460/f
center coax section = 325/f.
Feed system is the same.
I haven’t built one of these, but know a MARS operator who likes it.
Good Luck
73,
Edward E. “Sludge” Switzer
WB8TCT
This dipole takes on some very interesting characteristics unlike its predecessor the
“simple dipole”. Unlike the simple dipole, this antenna is very broad banded. Normally,
the bandwidth of this coaxial dipole is around 500 KHz wide, with average installation
considerations, with an SWR of under 2:1. The higher SWR occurs, of course, at the band
edges. The SWR will be absolutely flat when the antenna is of the proper length at the
design frequency.
By proper choice of design frequency, on may have an SWR on one band edge equal to
the opposite band edge. If this is your preference, you want to establish a design
frequency somewhere in the top half of the band.[ii] As an example, for the 80-75 meter
band, the design frequency should be about 3.900 MHz. At the 4 MHz edge the SWR will
be about 1.5:1; and at the 3.5 MHz end about 1.5:1.[iii] This is without the aid of a
“matchbox”. Antenna parameters may vary slightly from one location to another.
There are at least several reasons why this antenna is so broad. One reason is the
antenna is matched to the feedline. Another is that it incorporates its own balun.
Also, this dipole has a large circular-mil area over its entire surface, thus a low “Q”.
Over a 5-year period of testing, K7UAE reports an arithmetic mean gain of 1.5 db
over a simple dipole cut to the same frequency and installed at the same height
and configuration. From the author’s findings, this gain figure could be a conservative
representation. Also he reports a –6 db noise figure do to static charge build-up common
to the open wire construction of the simple dipole. Since this antenna is completely
covered by a vinyl jacket, it greatly reduces static charge build-up, which discharges
causing a popping noise in the receiver. The vinyl jacket covering is usually found
on most well designed mobile antennas.
This antenna also greatly reduces harmonics of the operating frequency.
Any signal fed into the antenna that is harmonically related to the antenna’s
operating range is reduced by a considerable amount when compared to a simple dipole.
This feature, as well as other features, amount to a savings in cost for the station
accessories such as low-pass filter, balun, matchbox, etc.
In summary, the advantages of this antenna are:
1). Broad bandwidth
2). Almost unaffected by environment
3). Positive gain with respect
to a common dipole under the same relative conditions
4). Greatly reduces
harmonics
5). Substantial decrease in static charge build-up
6). Essentially non-directional.[iv]
For antenna lengths, see Table 1. For illustration, the 80-meter antenna will be used.
It is suggested at this time that one may use any 52 ohm coaxial cable for both the
construction of the antenna and for the feedline.[v] The common choice of coaxial cables
to meet this requirement is RG-58A/U or RG-8U. It may be noted that RG-58A/U is
a very good choice in that it is the least expensive.[vi] As for attenuation, within
the HF bands, losses are considered negligible. Also, with this antenna, one may
use RG-58A/U at maximum legal power without fear of feedline breakdown.
For 80-meters, measure from the center of the antenna out on each side 30 ft. 6 in.
Solder the inner conductor (center) to the outer conductor (shield).[vii] This forms the
52-ohm matching section and balun. When completed, these connections will need to
be water/weatherproofed.[viii] Next, at both outer ends of the antenna, the center
conductor and shield need to be shorted. Weatherproofing is not done on these ends,
as the antenna may require some pruning to get the lowest SWR at the desired frequency.
At the center of the antenna, remove a maximum of one inch of the vinyl jacket
(1/2 inch both sides of center). At the center, CAREFULLY cut the shield all the
way around. Do not cut the insulation or the center conductor. Fan the shield out
to form two leads. Connect the feedline to the antenna by soldering one lead to the
feedline center conductor and taping, the other lead to the feedline shield,[ix x].
When done connecting feedline to the antenna, the feed-point will need to be
weatherproofed and strengthened, as this is the weak point in the antenna.
As for feedline lengths, random lengths may be used. However lengths of 57 ft, 87 ft,
and 103 ft are optimum for 80/75-meter use. Separate feedlines must be used for
each antenna.
After erecting the antenna, measure the SWR and trim the antenna to length
at the desired resonant frequency. The SWR should be 1:1. Be sure to solder
the ends when tuning. Weatherproof the ends when done.[xiii]
Follow these procedures for antennas for other bands. This antenna works well
as a flattop or as an inverted-vee.
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80/75-meters 40-meters 20-meters 15-meters 10-meters 2-meters 70-cm |
30 ft, 6 in. 16 ft., 9 in. 8 ft., 9 in. 5 ft., 7 in. 4 ft., 2 in. 9 in. 3.5 in. |
29 ft., 6 in. 14 ft., 9 in. 8 ft. 5 ft.,10 in. 4 ft., 5 in. 9 in. 3.25 in. |
120 ft 63 33 ft., 6 in. 22 ft, 10 in. 17 ft., 2 in. 3 ft. 1 ft., 1.5 in. |
i. Original information and text by Lynn, WA9PIV & was given to me by WB4FFX. WB8TCT
ii. Although the SWR is low and the antenna has a wide bandwidth, antenna
efficiency has been reported to decrease as you move away from design frequency.
With an SWR of less than 2:1 at the band edges, radiation efficiency may be less than
50 percent of what it is at the design frequency. WB8TCT
iii. The current 40-meter Inverted-Vee at this QTH has an SWR of 1.6:1 at 6.9 MHz and
at 7.4MHz, and 1:1 in the phone band. This antenna was built 15 yrs ago.
A 10-meter version at KC8FCZ’s QTH has an SWR of less than 2:1 from 28 to 29.7 MHz.
Both of these antennas are fed by SWR sensitive solid-state rigs. WB8TCT
iv. Have not verified omni-direction characteristics at 14 MHz and above.
I suspect it has some directional characteristics when installed as flat-top at
heights greater than ½ wavelength above ground. WB8TCT
v. I have made a couple of antennas from RG-59 (73-ohm) coax. WB8TCT
vi. It weighs less too. WB8TCT
vii. I prefer to prepare the feed-point (next step) first.
That way you can check the feedline and antenna for any shorts caused by
excessive heat from soldering. Once the balun shorts are done, an ohmmeter
is about useless unless you are looking for an open. WB8TCT
viii. Quick setting epoxy and some heat-shrink tube work great.
Mix the epoxy, dab it on the joint, and slip the shrink tubing over top.
Heat it enough to cause the epoxy to squeeze out, and let it cure. WB8TCT
ix. I have soldered an SO-239 connector at the feed-point so coax can just be
screwed on. WB8TCT
x. Use the minimum amount of heat. Be careful not to melt the shield into the
center conductor. WB8TCT
xi. Epoxy makes a strong, weatherproof seal. Not all epoxy is an insulator at RF.
Mix a small amount and let cure, then place in a microwave (with a cup of water for a load),
and turn on for a minute. If the epoxy is warm it may not be a good insulator for RF,
even if the ohmmeter reads an open. WB8TCT
xii. Most epoxies do not stick to waxed paper. A mold can be formed, and when cured,
the waxed paper peels away from the epoxy. Additional layers can be added to build up
the area, and a hole drilled for a rope. WB8TCT
xiii. Once again, quickset epoxy and heat-shrink tube make a strong, weatherproof
connection. WB8TCT
xiv. Measurements not in original data but do work on antenna I built for this band.
WB8TCT
xv. Measurements not in original data but do work on antenna I built for this band. WB8TCT