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Author Topic: A Ladder Line Slim Jim for 6m.  (Read 2492 times)
GILGSN
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Posts: 201




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« on: January 06, 2013, 10:49:55 PM »

Another 6m antenna!

http://radiopreppers.com/index.php/topic,328.0.html

Gil.
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KB4QAA
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Posts: 2339




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« Reply #1 on: January 07, 2013, 12:07:26 AM »

How is it "More of a Slim Jim than a Jpole"?    I have always heard the names used interchangeably.
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AC4RD
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Posts: 1236




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« Reply #2 on: January 07, 2013, 04:05:10 AM »

I don't know what a "slim jim" IS, but that 6m J-pole (or slim jim or whatever) at  http://www.astromag.co.uk/j-pole/ works great.  I've used it for portable operating with an FT817, and a bonus is that it works pretty well on 17M as well.  Smiley 
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AJ3O
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Posts: 124




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« Reply #3 on: January 07, 2013, 05:40:22 AM »

A J-pole is just what the name implies, a J shape

A slim jim antenna is the same antenna but the long vertical section is extended and folded back down to meet the J tail with a tuning space in between the two ends. Imagine an exagerated C that is almost closed up.

http://www.n7qvc.com/amateur_radio/copper.html

http://www.hamuniverse.com/slimjim.html

Hope that helps.
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GILGSN
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Posts: 201




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« Reply #4 on: January 07, 2013, 09:13:26 AM »

Quote
and a bonus is that it works pretty well on 17M as well.

Thanks! I would not have thought to check that out!

Quote

That's where I got the idea for my 2m Slim Jim!

Great antennas, they work. I wonder though if the SWR van go below 1.5:1, or if the minimum impedance of the design precludes that..?

Gil.
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N4CR
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Posts: 1663




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« Reply #5 on: January 07, 2013, 10:34:29 AM »

I wonder though if the SWR van go below 1.5:1, or if the minimum impedance of the design precludes that..?

The matching section of a J-Pole or a Slim Jim is a variable impedance transformer. At the bottom of the U, it's 0 ohms. At the top of the U it's around 3000 ohms. The coax attachment point is somewhere between the two ends of the possible range of impedances. Down near the bottom of the U but up the range somewhere there is a 50 ohm spot. If you attach your 50 ohm coax there, it will be a 1:1 match.

It usually requires experimentation to find that spot the first time.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
GILGSN
Member

Posts: 201




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« Reply #6 on: January 07, 2013, 10:53:25 AM »

Quote
up the range somewhere there is a 50 ohm spot.

Great, thanks.

Gil.
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WB6BYU
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Posts: 13168




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« Reply #7 on: January 07, 2013, 11:11:01 AM »

A "Slim-JIM" is just a J-pole with a double wire radiator connected at the top.
It has no advantage compared to a standard design:  shorting the conductors
at the top, at the bottom, both or neither makes little difference in performance
because the wires are closely coupled.

Using a single wire for the radiator (rather than both wires of a length of
twinlead) makes it lighter to carry, and works just as well, except for needing
to make a mechanically secure connection to the matching section.

When the radiator is cut to the proper length, all matching can be done by
adjusting the stub length and the tap point.  Because trimming the top of
the stub effectively lengthens the radiator, the easiest way to match is to
use some sort of sliding short circuit at the bottom of the stub, along with
an adjustable tap point.  This allows for variations in the impedance and
velocity factor of the stub.
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