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Handmade Slim Jim Antenna's by N9TAX
Let me start by saying that since discovering the slim jim antenna
design I have really come to respect these antennas. The design uses a
matching system like a j-pole antenna. This is called the j integrated
match (jim) the antenna itself is very slim hence the name Slim
Designs for these antennas date back to the early 1930's the best I can
tell. I have found them in QST magazine as far back as the 1940's.
I started building these antennas and selling them because I saw no one
else with a commercial product like this and I figured that I would
help fill a gap. The antennas have become a great success thanks to a
lot of hard work and some very dedicated customers. Before I get into
the specifics of the antennas I first want to thank everyone who has
purchased or helped promote this wonderful product. Without you none of
this would be possible.
The basic Slim Jim is an end fed folded dipole. It has a radiation
pattern peaking between 5 and 8 degrees above the horizon. It is this
and this alone that makes this one of the best terrestrial
communications antennas I have ever seen for the price. The layout of
the antenna is as follows.
The top section is 1/2 wave the bottom is 1/4 wave matching section.
The antenna is fed near the bottom and the feedpoint impedance is 50
ohms. Ladder line was an excellent choice for these antennas as it
presented two well spaced parallel conductors and is virtually
You can hang the antenna in many places. The only exception is near
metal objects. They will interfere with the tuning and performance of
the antenna. Hanging it up against drywall or plaster as many people do
will also affect the antenna but if this is your only option it will
still perform adequately. Open free space is the best of all options
and as with all antennas will yield the best performance.
Enter the Dual Band antenna.
After many requests for a dual band version of this
antenna. I searched the internet and books high and low to no avail. I
then began research on what it would take to make this antenna perform
well on 2m and 70cm. What I came up with was a trapped end fed folded
dipole. The antenna acts like a slim jim on 2m and a jpole on 70cm.
The secret formula I will leave for the history books. But this design
yields excellent results compared to many other dual band antennas. The
best I can tell there is no compromise to the performance on 2 meters
in fact some claim it works better on 2 than the single band vhf (I
have yet to scientifically prove that). Some ask well dont j-poles
resonate on the third harmonic of 145mhz anyway? First this antenna is
in the j-pole family but is not a true j-pole. Second J-poles
will resonate on 440 without traps but realistically for lack of a
better term they suck.
If anyone tries to sell you an un-modified j-pole and says it is a dual
band antenna they are wrong they dont work, I have seen swr of 3:1 on
unmodified j-poles on the 70cm band!
So what are my SWR's
Well I have a really nice meter that spits out some
great charts to show the band sweeps on these antennas.
Here is the 2m side.
And the 440 plot
Just a little disclaimer here this plot was taken with N connectors
throughout the system. Your mileage will vary a bit with other
connectors. Dont get me wrong this antenna will perform fine with
pl-259's but they do introduce impedance bumps at higher frequencies.
There have been a few customers lately that have broke their antennas
by putting them in bow mounts under too much pressure. There should be
very little pressure in the antenna. This can be achieved a number of
ways. 1st I always suggest the use of the thin (not sch 40) 3/4" pvc.
It is easier to bow and does not keep as much pressure on the antenna.
The 2nd method is to pre-bend the bow. This can be achieved by
overbending the bow with string wire or rope and then putting it in the
hot sun for a few days or using a heat gun to permanently shape the
Here is a photo that one of my customers sent me about pre-bending the
Original BOW info
One thing I get a lot of requests for is a way to
rigidly mount this antenna for mast or tower mounting. Most people want
to mount the antennas inside pvc. This makes them extremely hard to
tune because the velocity factor of pvc is not predictable. I have
developed a solution that may not be the prettiest but is
definitely usable. I call it the bow and anyone can build one. It
consists of a piece of 3/4" pvc about a 6" longer than the antenna two
90 degree elbows another foot of pvc and a few zip ties. Basically you
drill holes in the ends of the long piece of pvc and bow it to the
length of the antenna and zip tie the ends to the holes you just made.
Affix the elbows as on the bottom to bring the short piece parallel
with the antenna and you have a rigid mount that will not interfere
with the antenna's tuning. Here is a photo of mine on my tower.
I'll tell you what that thing really talks up there I have worked
repeaters over 100 miles away.
Introducing the flag
It is kind
of a bad photo but this is an atv flag pole with the antenna attached.
This is still not the perfect answer to rigid mounting but it is pretty
good. As you can see there is an SAE 1/2" stud with a nut and
lockwasher at the bottom. This makes it very easy to come up with a
mount for the antenna. You can also use a fishing pole the same way but
if you want a permanent roof installation I would stay with the flag
mast. I purchased this mast at the local Honda ATV shop for $10.70
punching hole pairs in the antenna like is done at the bottom to hold
the coax in place we can get the fiberglass rod into the middle of the
antenna where it can do the least amount of de-tuning to the antenna.
And now the bad news.....
Before (Antenna in open space)
After (Antenna attached to flag mast)
As you can see we have lost our perfect tune to 146.52. This is not the
end of the world as we still have a very good SWR on the entire band.
As with everything in life there are tradeoffs. Lets look at 440.
Before (Antenna in open space)
After (Antenna attached to flag mast)
As you can see we have the same effect on 440. The antenna feels
electrically longer as the mast has changed the velocity factor of the
ladder line. Still it is not that bad.
In conclusion, this is a
workable mount for the antenna. Many of my customers want to purchase
these antennas and put them into PVC. I can understand as it is an
inexpensive alternative to purchasing a $150 fiberglass dual band
vertical antenna. Please stay away from the insertion into PVC because
if you think the rod has affected the antenna badly pvc has about 20X
the affect. The bow remains the only non frequency altering method of
roof mounting the antenna.
The antennas are available with an assorted connectors
and with or without long coax cable.
The VHF only antennas are custom tunable from 136 - 999 mhz at no extra
charge. (Tuning from 88-136 are available with slight extra charge for
added material and shipping weight.) Antennas from 400-999 are made
from 300 ohm ladder wire as they perform better with the thinner wire.
Thank you for your interest in my antennas.
The link below will take you to my webstore where you can buy one.