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Author Topic: BALUN for 2 meter J-pole  (Read 4298 times)
XY3LON4
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Posts: 84




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« on: April 11, 2007, 03:18:48 PM »

All of the directions for building a 2 meter copper cactus say for the BALUN, you should use 4 turns of coax 5" in diameter. BUXXCOMM sells what they call a VBALUN which is in the form of a can. I would like to complete my homebrew of my J-pole by making my own BALUN for it and make it a "CAN" style just to make it look a little nicer and professional. The can will be made out of 1.5" ID PVC pipe. How many turns am I going to need of coax if it is wound into a 1.5" coil instead of 5"? Anyone have plans for a can style BALUN?
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G8UBJ
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Posts: 478


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« Reply #1 on: April 12, 2007, 02:45:12 AM »

I have a copper J pole in my loft but never used a balun and it worked. But looking about I saw this with a comment at the end...

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=9295

"The simplest way to make a balun is
to get a split-core cylindrical ferrite (such as an Amidon 2X-43-251) and attach it to the outside of the coax 1/4 wavelength
from the feedpoint. On VHF frequencies some ferrite materials are not effective, so be sure to get type 43 material for best
results."
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WB6BYU
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Posts: 13573




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« Reply #2 on: April 12, 2007, 05:38:19 PM »

Actually 1/4 wave from the end is the WORSE place to put it.  If it
really acts as a high impedance (as it should) then the quarter length of
radiator (the outside of the coax) will transform that to a low impedance
at the feedpoint, which will encourage current to flow on the outside
of the coax.

Put it RIGHT AT the feedpoint, or a half wavelength away if you must.

When I've used a balun with a J-pole it has been a 4 : 1 coax balun.
The 200 ohm feedpoint on the matching stub is roughly twice as far
up from the bottom as the 50 ohm point is.

J-poles ARE prone to stray RF currents on the mounting mast or the
outside of the coax (or both!)  I know of one case where they couldn't
hit the local repeater until the J-pole was insulated from the mast it
was mounted on due to the stray currents messing up the radiation
pattern.

If you are going to use a choke balun the exact dimensions and number
of turns aren't critical - it needs to have a high impedance at the
operating frequency, but whether it is 400 ohms or 2000 ohms won't
cause a significant difference in operation of the antenna.
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XY3LON4
Member

Posts: 84




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« Reply #3 on: April 13, 2007, 05:05:15 PM »

Well, I have made a coil out of coax 4 turn coil with a diameter of 5". If that doesn't do it, I will try something else.
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XY3LON4
Member

Posts: 84




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« Reply #4 on: April 13, 2007, 06:10:14 PM »

I was just talking to another ham tonight and he said that a method for j-pole design that he has used and that works very well is to attach the RF connector to the middle of the cross piece and run the center connector wire to either the radiator or the stub with a capacitor in series with the wire. He starts off by tuning the location of the wire for the lowest SWR and then using a capacitor to fine tune it.
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XY3LON4
Member

Posts: 84




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« Reply #5 on: April 13, 2007, 07:01:37 PM »

With this design he finds that there is no need for a balun or choke.
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VK2XDZ
Member

Posts: 48




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« Reply #6 on: April 27, 2007, 06:53:54 PM »

I made a j pole out of copper water pipe,and it works well.
The ARRL handbook 2004 states that the choke balun should be 3 turns into a coil,about 8 inches in diameter.
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WB6BYU
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Posts: 13573




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« Reply #7 on: April 29, 2007, 10:50:05 AM »

>...a method for j-pole design that he has used and that works very well is to attach the RF connector to the middle of the cross piece and run the center connector wire to either the radiator or the stub with a capacitor in series with the wire.

I've seen a number of mobile J-poles built this way, and I use a similar
method for feeding quads.  An easy way to do it is to run the wire from
the coax connector up the center of the stub the n over to one side and
wrap it around the pipe - using insulated wire.  The capacitance between
the wire and the pipe forms the tuning capacitor, and it is easy to slide
the wrap up or down the pipe a bit and/or unwind a bit of wire for your
final tuning.  The exact dimensions you end up with will depend on the
size of the wire and the type of insulation.  Using the wrapped wire for
the capacitor is much easier than trying to weatherproof a standard
variable capacitor.

However, given that a J-pole is not a perfect design, you may still want
some sort of choke to keep RF off of the feedline.  The Zepp feed approach
as used in the J-pole was once described as, "not a good way to get an
antenna to radiate, but not a good way to keep it from radiating, either."
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