The Desert Slim Jim Antenna
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It's “OK”. It's no longer the 20th Century. Really.
This is a unique way to make a slim jim antenna for the 2 or 3 three meter band out of some fiberglass reinforced packaging tape, a ten foot length of thin wall PVC pipe, some wire, spray paint and that gift from God, Duct tape. This version is strong and simple to build. Installation or use could be dangerous, -use a qualified contractor.
Start by looking at the drawing that says "Aerial Construction" on the top. Download, Print and Study it.
Slim
Jim Schematic
Study everything until you Grok it so well you
could build it in your sleep.
You start with a 3/4"
(19mm?) by ten foot length of 200 PSI PVC pipe. It is very thin, and
very light weight. Do not use schedule 40 or other thick, heavy
plastic pipe.
Cover the PVC pipe lengthways with fiberglass
reinforced packaging tape.
This tape is almost clear & you
can see the reinforcing strands of fiberglass running lengthwise. 3M
makes the good stuff. Apply the tape so the whole pipe is covered two
layers thick from top to bottom lengthwise. Next, run another
complete layer half way up from the bottom. 3/4" or 1" wide
tape is good to use. When done, the pipe will have three layers
covering the bottom half and two layers covering the top half. -all
the way around.
You end up with a length of pipe that is very
stiff, flexible to high winds, lightweight and contains no metal.
Calculate,
Measure, & Mark
Do the math to convert the
given lengths in the drawings for a 98mhz antenna in meters to the
lengths needed for the frequency you want. Convert other lengths,
Feedpoint, Breakpoint, by the same method.
For example: (The drawing is spec'd for) 98mhz. x 2.205M = 216.09 divided by the new frequency (for example) 95.3mhz = a new overall-length of 2.267M
Apply the same math formula to acquire correct feedpoint length. 111mm = 114mm
Apply the same math formula to acquire correct breakpoint length. 710mm = 730mm
The "open gap" length at 37mm, (1.5 “) is fine for all 2 or 3 meter frequencies.
( Converter for working with
feet : http://www.simetric.co.uk/metres_to_feet.php
)
List of measurements
for the lazy or the math challenged.
Measure from the top for
placement of the overall-length and make your mark. Next, apply the
other two measurements starting at the bottom of the overall-length
mark, measuring "up" from the bottom of the overall-length
mark, not the bottom of the pipe.
I suggest 18 gauge multistrand
bare copper wire, it weighs very little and will easily handle 120
watts. Use 14 gauge multi strand for higher wattage.
Start by running the wire up
one side of the pipe starting at the top of the 37mm Gap, make a
small notch across the top of the pipe to set the wire into and run
it down the other side to the bottom mark. A small 1/8th inch hole
will have to be drilled clear through the pipe at the overall-length
mark.
Pass the wire through the holes and run it back up
stopping at the bottom of the 37mm Gap. Trim off excess wire. With
more of that reinforced fiberglass tape secure the wire every foot or
so, wrapped around 4 times, taking care not to put tape over the
feedpoint area plus or minus two inches.
Print out and Study the
Feedpoint-detail.gif
Take the coax and tin each
end of the wire after making about a one inch lead out of each
conductor. "tack" solder the coax at the measured
feedpoint. For simplicity, the outer coax lead is attached on the
"gap side" at the feedpoint, while the inner coax lead is
attached to the opposite side where the wire runs up and over the
top. (the long side) Set the slim jim up vertically on a temporary
pole or taped to a wooden ladder in an open area. Check the SWR using
a very low power setting, less than one quarter watt should work if
your SWR meter is sensitive enough. Move both wires up 1/4 inch or
down 1/4 inch till you find a 1 to 1 “flat” SWR.
Sometimes no movement is necessary. Permanently solder the leads
after bending the leads around the wire a bit, don't use too much
heat. Note: This antenna will show a dead short at DC, but is
resonant at the designed frequency.
BTW: It is not necessary
to have the wire stretched too tight, a wee bit loose is OK !
After
the solder cools, take some silicone RTV Blue automotive gasket maker
sealant, cover the end of the coax all over so both leads are
immersed in the silicone and a bit of the outer coax covering is
overlapped with silicone sealant. From the ends back to the outer
covering on the coax. No more. Savvy? Less than a teaspoon of RTV
Blue is used, total.
Take a one inch dia. length of
common TV mast pipe from Radio Shack or wherever. Saw off the small
"male", formed end of that pipe that normally fits into the
larger (female, straight) end. Slip the bottom of the Slim Jim inside
the mast pipe, till the bottom of the over-all length is 8 inches
away from the steel mast pipe, (the bottom “overall-length
mark” where the wire runs through the pipe) you will need a bit
of that left over fiberglass tape to wrap around the antenna base to
make it a very snug fit. Plan to Drill one 5/16" hole near the
bottom end of the PVC pipe. Drill at 9'- 10" down, measured from
the TOP of the PVC pipe. Secure the antenna with a 1/4”
by 1.5” bolt in place with lockwasher and nut. Locktite is nice
if you have it.
****Do not drill the hole near the top of the
of the mast pipe!****
****It would weaken the plastic
pipe near the highest stress point! ****
The drill hole
measurement point will fall a ways down from the top of the mast.
This is good.
This measurement will vary according to the
Electrical frequency/overall length of the Desert Slim Jim.
Drill
with the plastic pipe inserted so that 8 inches is in between the
mast and the bottom of the overall mark.
See Antenna
prior to covering solder joints with RTV blue.
See picture of
completed Antenna
with paint!
Duct tape the coax on the side of the Desert Slim Jim, leaving a bit of slack (partial loop) just below the feedpoint at the point where antenna and steel mast pipe join. That way the soldered leads will not rip off in strong wind. This antenna is designed to bend in high winds a bit, which is good! Use duct tape tape every three feet attaching the coax to the side of the mast pipe. Four layers is a good idea at each tape point. Paint the whole plastic part of the antenna with some cheap white titanium based paint, the $1.98 stuff from ACE hardware works very well. Use three thin coats of paint, let it dry between coats. The paint is for UV protection of the plastic pipe and fiberglass tape and does not effect antenna performance measurably.
Safety Warning
! Be safer and ground the base of the mast pipe for your
protection from Electrical Power lines and Electrical storms. Make
sure the antenna site is 1.5 or more times farther from any overhead
electrical hazard than your antenna is high.
If you are going
up twenty, thirty, or forty feet, Belden 9258 RG-8/X gives good
performance for the money & about one Db of transmission loss
which is nothing. Times LMR-240 is even better. I would consider it
over the Belden cable if the total coax length is over 50 feet.
http://rfparts.com/
is a good place to buy coax. I attach the entire length to the
antenna and that way I only need one connector at the Exciter. At 40
feet high, this antenna when paired with a 1 watt exciter will reach
cars for a solid 2+ miles over flat terrain. Note: Shielded audio
cable is cheaper than coax. If you can, place the Exciter almost
“under” the base of the antenna.
Savvy?
