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1994-06-23
|
3KB
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60 lines
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.