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1-8 of 8 messages
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Page 1 of 1
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VHF J-Pole
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by KC8PTB on December 6, 2000
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I am looking for guidlines for building a 2m band j-pole type antenna which I can use both in my home and in other places I set-up to operate. I have seen many ideas for specialized 2m antennas, but have not found j-pole data. Can someone point me to it?
Jim
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RE: VHF J-Pole
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by KG4DED on December 6, 2000
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I have built 2 J-poles from plans I found on the internet. Go to one of the search engins like Alta Vista and do a search you should find more then you can handel. Here are a few i have found.
Steven
KG4DED
kg4ded@hotmail.com
http://www.umars.umsu.umanitoba.ca/umars/research/jpole.html
http://www.qsl.net/wb8erj/146jpole.htm
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RE: VHF J-Pole
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by WB6BYU on December 7, 2000
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There are three basic construction approaches for 2m J-poles
that appear to be the most common:
1) twinlead roll-up version. One design was published in the
ARRL Emergency Coordinator's manual, and several others are
floating around.
2) copper tubing (often called a "copper cactus".)
3) mobile version using threaded whips in an aluminum block, which
typically also includes a coax fitting (usually matched with a single
wire from the coax connector wrapped around one of the uprights.)
This can also be built by attaching a shorter element on the side of
a metal CB whip.
Although the construction methods differ, the basic steps for tuning
and making it work are the same.
1) The radiator is the portion of the long element above the top of
the short element. This should be half a wavelength (36" for tubing,
38" for wire is probably close enough.)
2) The matching stub is the shorter element and the corresponding
section of the longer element. This should be about a quarter
wavelength long with a short circuit at the bottom, with provision to
adjust the length (position of the shorting bar) somewhat.
18" to 19" is typical. (If you are using twinlead, the quarter
wavelength should include a correction for the velocity factor,
so it would be around 15".)
3) The feedline tap point is about 1 1/2" to 2" up from the shorting
bar at the bottom of the matching stub for starters. It doesn't
matter which side of the coax connects to which side of the
antenna: in fact, I have had my best results using balanced feed
with a J-pole for permanent mounting (a 4 : 1 coax balun, with the
feed taps twice as high up the matching stub.)
4) Connect the coax and check the SWR. Vary the shorting bar
position and the coax tap point for best match. DO NOT change
the length of the radiator section.
5) Putting the twinlead J-pole in a piece of plastic pipe will detune
it, due to the change in the velocity factor of the matching section.
(This will work if you adjust the match for best match IN THE PIPE.)
Hope this helps! - Dale WB6BYU
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RE: VHF J-Pole
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by N3JIY on December 13, 2000
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If copper tubing is okay, there's a nice plan on the internet at http://www.arrl.org/tis/info/JPole-V.html
I have not built this one, but would like to. Let us know which one you decide to build.
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RE: VHF J-Pole
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by KB1FPZ on December 14, 2000
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There are plans for a copper J-Pole in the 1998 ARRL handbook if you can get your hands on a copy of it. If not, you can probably find the plans somewhere on the ARRL website (www.arrl.org)
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RE: VHF J-Pole
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by KC8PTB on December 18, 2000
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A quick note of thanks to those who pointed me in the right direction. I now have lots of plans for J-pole antennae (old Latin student). I am going to make a "roll-up" type using 300-Ohm wire this week. I have a 1/4 wave ground plane under construction and will wait on the copper pipe J-pole until next spring when I can work outside. A might cold here right now.
Thanks again!
Jim
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RE: VHF J-Pole
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by N6JSX on June 17, 2001
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Hi, Well in about an hour I will be making another Copper pipe "J" antenna - my take it easy Fathers day project. I published an article in 73 Magazine a few years ago called the J" Compendium" covering about 5 different ways of making/using the J.
Recently, I've been trying to make a tri-band 2m/220/440 "J" with only one feed line. I've found this to be very difficult to get the VSWR below 2:1 on all bands. So I may have to go to two feed lines one for 2m/440 and the other for 220.
In this process of designing this tri antenna I've found some better dimensions for optional performance on 2m/440 - wider band width almost flat VSWR across both bands. The standard 57" needs to be stretched to 63". Which you will find is the optimal for 440 and yet 2m meters work very very good and the VSWR is nearly flat even up into the 150's.
I've been in the midst of writting another article for eHAM, looks like I need to get hot.... I will try and get something here in the next week or so about my newer design w/pictures, diagrams, & graphs. Otherwise write me I can send you my old WORD.doc file.
I find the "J" antenna to be the best flat land antenna a HAM could use for the $$$. Best range, easy to make - very surprized that some big antenna maker hasn't built a fiberglass "J" whip like thay have on the marine bands.
73, Kuby, N6JSX /8
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RE: VHF J-Pole
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by KC9HLR on October 29, 2009
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i have a jpole of copper i have no balun and using 20ft of rg58 coax but it wont let me put full power into the antenna at 5w i only get one bar and at 25w i only get 7 bars on signal strength the guy i got it from used it for years with no promblems with swr im not sure if i need a balun or need to switch coax maybe the 58 is too lossy
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