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Author Topic: How to tune a 2mtr "Slim Jim" antenna??  (Read 2434 times)
WB0AXN
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Posts: 90




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« on: April 29, 2008, 06:36:15 PM »

I've built several 2mtr ground plane antennas and was able to tune the resonant frequency up or down, by adjusting the length of the vertical.
A few days ago, I read several articles on the web, about "Slim Jim" 2mtr antennas and it's advantage over a ground plane.  So, I decided to build one out of 12 guage wire to see if it works.  I got it built in an hour or less and checked out the SWR...it was way off.  So, I moved the feed line up about an inch and got a flat SWR a little above 144 mhz.  I would like to get a 1:1 match at 146 mhz.  Since I'm working with three elements, instead of one....what section of the antenna do I trim shorter??  The 1/4 wave, 1/2 wave or 3/4 wave?? ....or all three sections?
Rather than start cutting here or there, I decided to post this query and maybe someone here had experience with this antenna and could solve my problem.
In the short amount of time I was testing, it seemed this antenna was much broader than a J-pole I am using now.  
If I get the Slim Jim to work like I want, I will build one with 1/2" copper pipe and replace the J-Pole.

Roland
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ZL1KMN
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Posts: 99




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« Reply #1 on: April 29, 2008, 09:10:45 PM »

3/4 wave (longest section = 8415 / fMHz = inches
1/2 wave section   = 5610 / fMHz = inches
1/4 wave section   = 2805 / fMHz = inches
1/4 wave freespace = 2953 / fMHz = inches  

To make a good match, move the coax connection point  up or down in the slim jim, until you get good match.
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WB6BYU
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Posts: 13151




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« Reply #2 on: April 29, 2008, 11:13:06 PM »

First of all, a "Slim Jim" has no advantage over a J-pole unless you are
trying to use up wire/pipe.  Personally I'd cut off the funny half wave
part on the side of the radiator, but you can leave it there if you prefer.
(If you do leave it, make sure that the insulation between the top of
the matching stub and the bottom of the added half wave piece is VERY
good:  this is a high impedance point (as is the top of the stub itself)
and using poor materials will increase losses.

Note on terminology:  the two important components of a J-pole are
the radiator (the portion of the long piece sticking up above the top of
the matching stub) and the matching stub (the portion at the bottom
where the two conductors are parallel.)  While it may be built mechanically
from a 3/4 wave piece and a 1/4 wave piece, electrically you have to
see it as a half wave radiator and a quarter wave matching stub.

There are three adjustments on a J-pole:  the length of the radiator,
the length of the matching stub, and the tap point for the feedline.
(With the "Slim Jim" the radiator is the two pieces in series:  the
effective length is roughly the total length up from the top of the stub,
across and down, divided by two, but this will vary depending on a
number of factors.)

Here is what works for me when I tune one:

First calculate the length for the radiator based on the diameter of the
material you are using.  EZNEC tells me that 37" is about right for 1"
pipe, or 39" for #12 wire.  Generally I'd probably just pick 38" and call
it good.

Next I set the length of the matching stub to about 1/4 wavelength.
It probably will end up a bit shorter than that, though.  Tap the coax
up about 2 inches above the bottom of the matching stub using some
sort of a balun.  (If you use a 4 : 1 balun, perhaps the standard type
with 1/2 wave of coax, start about 4" up from the stub.)

Adjusting the length of the stub shifts the resonant frequency.  Moving
the feedline tap point changes the impedance at resonance.  Generally
the adjustable part of the stub is at the bottom, in which case try to
keep the feedpoint the same distance above the shorting bar when you
move it.  From that point it is fairly easy to find a good match.

One problem, though, is that many J-pole designs (especially those
made with copper pipe) don't provide for adjusting the length of the
matching stub.  In that case you'll have to experiment with varying the
length of either the radiator or the top of the stub to see what affect
it has.  If you understand how the J-pole works you can see that
trimming the short side makes the stub shorter and the radiator longer
(since the top of the stub is the beginning of the radiator).  Trimming
the radiator leaves the stub length the same.  (For the "Slim Jim", trimming
the bottom of the added half wave piece is equivalent to trimming the
radiator length.)  If you following an existing set of plans then you may
get a good match.  Otherwise I'd recommend making the stub a few
inches longer than 1/4 wavelength and making a movable shorting bar
to change the effective length.

Of course, with 3 variables to set two values (resistance and reactance)
there will be a number of combinations that will give a match.  So
don't worry too much about getting exactly the RIGHT lengths.  Just
build it and adjust it for a good match.

Many of the designs I've seen don't really tell you all the pitfalls.  Any of
the following can change the way that the antenna matches:

- the diameter of the elements in the radiator and the matching stub
- the spacing between the conductors of the matching stub
- the way the coax is attached (especially the lack of a balun)
- where/if the antenna is grounded (ideally the center of the bottom of the stub.)
- coax length (if you aren't using an effective balun)
- type and location of insulators
- environment:  walls, antennas, inside a pipe, other nearby metal objects, etc.
- the accuracy with which the author can actually measure antenna performance

An adjustable matching stub can accomodate a lot of variation and
still get a good match.
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