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1994-08-24
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7KB
From frbspd@crl.crl.com Sat Dec 4 11:04:42 1993
Date: Sun, 28 Nov 1993 01:40:45 -0800
From: Stephen Dunifer <frbspd@crl.crl.com>
To: frbspd@crl.com
Newsgroups: rec.radio.amateur.antenna
Subject: (fwd) Re: Copper j-pole antenna - where?
Path: nntp.crl.com!decwrl!ames!elroy.jpl.nasa.gov!sdd.hp.com!col.hp.com!srgenprp!news.dtc.hp.com!hpscit.sc.hp.com!hpuerca.atl.hp.com!hpuerca!edh
From: edh@hpuerca.atl.hp.com (Ed Humphries)
Newsgroups: rec.radio.amateur.antenna
Subject: Re: Copper j-pole antenna - where?
Date: 22 Nov 1993 08:46:07 -0500
Organization: Hewlett-Packard NARC Atlanta
Lines: 133
Distribution: world
Message-ID: <edh.753975822@hpuerca>
References: <2catqs$hbo@samba.oit.unc.edu> <PARTOS.93Nov16134213@larry.larc.nasa.gov>
NNTP-Posting-Host: hpuerca.atl.hp.com
Since I ended up emailing so many copies the last time
this question came up, this time round I'll just go ahead
and post my jpole articles. Enjoy!
Ed Humphries - N5RCK Hewlett Packard Atlanta GA
----
Thanks for asking!
The following two articles are 59 lines long each.
Just cut at the "--cut here--" lines and the articles
will print neatly on one page for easy reference.
Hope you enjoy building and using one or both!
Cheers & 73 Ed Humphries - N5RCK
--cut here--
A Copper Tube J-pole
by Ed Humphries - N5RCK
edh@hpuerca.atl.hp.com
The following is a description of a J-Pole antenna made from copper
pipe. You can use 1/2 inch to 1 inch pipe (wall thickness will
affect stiffness/stability AND price, but not performance). Larger
diameter pipe increases bandwidth, but 1/2 inch is fine for amateur
frequencies. Start with a 10 foot (standard) length of pipe, 1 90
degree (right angle) fitting, 1 "tee" fitting, 2 end-caps, 2 hose
clamps (worm-gear adjustable of the appropriate diameter), and your
coax (end stripped, braid separated, center conducter stripped, and
coax sealant to close opening in coax to keep water out). Use a tube
cutter (for best/easiest results) and cut the 10 foot pipe according
to the dimensions needed following the diagram below:
__
| <=endcap
|
|
5/8 | for 146 MHz
|
|
wave |
|
|
|
-- | | <=endcap
| | 1/4
| | wave
| |
coax ctr conductor=>* *<= coax shield
| |
@ 5"-->| |
tee => --- <= elbow
|
| < "leftover" pipe
|
For best general purpose use, the 5/8th wave version should be
used. The dimensions to cut are: 66 1/2 inches (5/8 + 1/4 matching
section), 19 inches (other half of 1/4 matching section), 3/4 inch
(joins the tee and the elbow), and the "leftover" 33 3/4 inchs that
forms the base. Use standard plumbing solder methods to join main
section to base using the tee. Use the 3/4 inch piece and the elbow
to attach the 19 inch piece. Be careful to keep pieces parallel.
This will give you a center frequency of 146 MHz. Attach the coax
as shown using the hose clamps. Adjust the swr at 146 MHz by sliding
the connections up or down as needed -- you should be able to reach
very close to 1:1 (best to do this in approximately where you intend
to use the antenna - the base can be attached directly to a mast by
two hose clamps). Try not to be standing right by the antenna!
It has been noted that this design can lead to rf coupling onto the
feedline. To avoid this, put a ferrite on the coax at the feedpoint,
or use 3 turns (@1") of the coax taped together at the feedpoint.
Other center freq dimensions: (adjust 5/8 section accordingly).
144 =19.25 inches, 145 =19.12, 146 =19, 147 =18.86, 148 =18.73.
--cut here--
TV Twin-lead J-pole
By Ed Humphries - N5RCK
edh@hpuerca.atl.hp.com
The following is a description of a J-Pole antenna made from 300 ohm
TV twin-lead. They have quite a few advantages which include improved
performance for 2-meter HTs, portability, and low cost.
| | do not short this end.
| | (when trimming for vswr, cut both sides)
| |
| |
| |
3/4 | | Technically-speaking, this is a 1/2
wave | | wave end-fed antenna with a 1/4 wave
| | matching section.
| |
| 1/4" gap
| | (trim for vswr _below_ gap)
| | 1/4
| | wave
| |
coax ctr conductor=>* *<= coax shield
1 1/4"-| |
-*- solder the twin leads together at bottom
For a center frequency of 146 MHz:
1. Start with @54" of TV twin lead (flat, NOT foam core)
2. Strip 1/2" of insulation at bottom and solder wires together.
3. Measure 1 1/4" from soldered wires and strip insulation on both
sides. This is the solder point for a coax feedline.
4. Measure 16 3/4" from coax shield solder point and cut out 1/4" notch.
5. Measure 50 1/3" from coax center conductor solder point and trim
off twin lead at that point.
6. Feed with a length of RG58U coax. Tape coax at feedpoint to the
twin lead for strength and seal coax for weather protection.
It has been noted that this design can lead to rf coupling onto the
feedline. To avoid, put ferrite beads on the coax at the feedpoint,
or use 3-5 turns of coax (1"-2") taped together at the feedpoint.
You may attach an alligator clip to the plastic on the top of
the antenna in order to easily hang it. Alternately, punch a
hole near the top and use a length of fishing line to hang.
This design appears on many BBSs, in club newsletters, and in books;
the earliest reference that I know of is a Jan. 1984 D.A.R.C. antenna
article by James Burks, KA5QYV.
FYI, the 1/4 wave sections for other center frequencies are:
144 MHz =17 inches, 145 =16.88, 146 =16.75, 147 =16.65, 148 =16.54
I usually just go ahead and solder the coax in place and trim
down to as close to 1:1 vswr as I can get. I use the MFJ vhf
antenna analyzer and a frequency counter then afterwards test
with a radio and in-line swr/power meter. When done, the antenna
should also present 1:1.2-3 vswr in the center of 444MHz band as
well (demonstrated on my dual-band meter and Alinco DJ-580). You
will need to trim in a 3:1 ratio to maintain the 3/4 to 1/4 wave.