Date: Tue, 22 Mar 1994 07:55:39 -0500
From: Ed Humphries <
[email protected]>
To: Stephen Dunifer <
[email protected]>
Newsgroups: rec.radio.amateur.antenna
Subject: Re: 5/8 over 1/4 design info needed
Hi Stephen!
In rec.radio.amateur.antenna you write:
>
> Having read several references to 5/8 over a 1/4 vertical
>antenna, I have not been able to find any fully detailed construction
>plans in the usual sources. Can anyone provide the details or point the way
>to an appropriate source ? I assume that the antenna would consist of a
>5/8 element, 1/4 element and some sort of type of coil between the two
>elements along with the usual 4 ground radials.
If you are thinking about a collinear antenna, they usually have
1/2 wave elements stacked with 1/4 wave phase sections in between:
sometimes a carefully matched 5/8 wave section is used instead of 1/2.
However, from your description I suspect you are thinking about the
typical 5/8 wave antenna where the radiating portion sits on top of
a 1/4 wave _matching_ section (no radials required inthis configuration,
but may be added at the base of the 5/8 wave section (NOT base of the
matching section)). The most typical example of this is the Jpole.
So, if you'd like to have one of these, simple: build it from my
enclosed plans! I wrote this article several years ago (note: I
did not _design_ this, but written plans were pretty scurrvy before
I researched and wrote this. If you circulated the article further,
do please keep my name as author intact.
Many people have built this Jpole, literally from around the world!
The following article is 59 lines long.
Just cut at the "--cut here--" line and the article
will print neatly on one page for easy reference.
Hope you enjoy building and using!
Cheers & 73 Ed Humphries - N5RCK
Hewlett Packard NARC Atlanta GA
--cut here--
A Copper Tube J-pole
by Ed Humphries - N5RCK
[email protected]
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.
