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VE3WBZ > TECH 22.12.05 16:15l 106 Lines 4298 Bytes #999 (0) @ WW
BID : 21240_VE3FJB
Read: DL1LCA GUEST OE7FMI
Subj: J-Pole diagram found in cookbk
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0RES<ON0BEL<F1HPZ<ON4HU<VE3FJB
Sent: 051222/1410Z @:VE3FJB.#CON.ON.CAN.NOAM #:21240 [Barrie, ON] FBB
From: VE3WBZ@VE3FJB.#CON.ON.CAN.NOAM
To : TECH@WW
TO:TECH@WW
FR:VE3WBZ@VE3FJB.#CON.ON.CAN.NOAM
DT:Thursday,December 22nd.,2005 @ 0904hrs EST <JPST>
Can you believe this? This diagram and construction hints were found
in a YL NET cookbook put together in 1995. How it came to be placed
in a cookbook is beyond me? Perhaps <G> it was after adding some sprite
to a RumBall recipe that influence the J Pole be added <G> ????
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o\ Start of Original message:
Subject: TV Twin-lead J-pole design
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 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.
To get the best possible match, in step three above simply MARK
the "solder points" and measure from the mark for step 4 and 5.
Now solder straight pins to your conductor and your shield.
Insert the pins at the marked point and test for VSWR at the
design frequency (146MHz). If necessary, probe up or down till
you reach 1:1 (close as possible).
Solder at the best points. To try this, you may want to start
with the twin lead a little long and trim down to resonant
length -
note: you'll need to trim in a 3:1 ratio to maintain the 3/4
to 1/4 wave.
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.
This antenna is relatively broad-banded and will be more than
adequate if simply built as noted in steps above.
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).
--
Ed Humphries Texas Instruments, Inc. 512-250-6894
N5RCK Internet ed.humphries@hub.dsg.ti.com
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o\ End of original message............:
Being that this cookbook came out in "1995" for the YLs on packet at the
time, Ed N5RCK might not be under that call...and perhaps his email has
changed. Anyhow..if this is something that will help you, then great.
73 Peter VE3WBZ
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