Hi Ron,
I had one question. When one splits the coax tfor the tuning section, is it
important to split it evenly or can it be a little to the left or right of
center?
God bless,
Mark
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-----Original Message-----
From: Jim Gammon
Sent: Saturday, March 22, 2014 10:10 AM
To: [log in to unmask]
Subject: How to construct a two meter j Pole Antenna
Hi Linda, Thanks to Ron for this I found it on my Apex, 73, Jim
WA6EKS How to Construct a 2-Meter J Pole
By Ronald E. Milliman, PhD. (K8HSY)
First, imagine the print letter J. This represents the shape
of the antenna.
It has one long side and one short side all connected at the
bottom or said
another way, one continuous length of conductor shaped into a J.
Now, turn or flip the J backwards as if you were writing it
with the short
side on the right rather than on the left. I say this simply
because this
is the way it is usually shown in the ham publications and on
most web
pages.
The J is one single "conductor" in the shape of a J.
From the top of the long side of the J to the bottom, for 2
meters, is about
58 inches long before it turns horizontally a couple of inches
and then goes
vertical for about 19 1/d inches. So you have a total length of
about 79 1/d
inches forming a single J shape. The center conductor of the
coaxial
feedline is attached to the long section of the J Pole and the
braid or
shield of the feedline is attached to the short section. These
feed points
for the coax are spaced up from the bottom of the J or antenna
about two
inches, and they need to be, initially, adjustable , until you
find the
optimum tap or feed point that gives minimum SWR. So, do not
make a
permanent feedline connection yet. Once you have these feed
points adjusted
to produce minimum SWR, you can solder them for a permanent
connection.
Again, the shield of the 50 ohm coax is attached to the short
section about
2 inches up from the bottom. The center conductor of the 50 ohm
coax is
attached to the long section about 2 inches up from the bottom.
Both the
center conductor and the shield should now be side by side on the
vertical
elements equal distance from the bottom; that is, one on one
vertical side
and the other on the other vertical side. BOTH are connected
equal distance
from the bottom of the antenna and are adjusted up or down
equally for
lowest swr. This is the tricky part. Some method of making
temporary
connections for tuning will be needed or they can just be tack
soldered for
testing and tuning.
For tuning, you can mount the J Pole on top of a short pole, a
length of
PVC, a push pole, etc., or you can mount it on a temporary
support like a
wooden ladder.
Then, once you have it tuned, you will need some method of
mounting the
antenna to a mast, pole, etc in your final installation. Most
builders just
add about 15 to 20 inches to the long side at the bottom for
mounting. This
extra length becomes invisible to the operation of the antenna;
thus, not
affecting its resonance. So, it can be metal tubing for a J
pole. Adding
this length would make the long side about 58 plus 15 inches
equals 73
inches...or 58 plus 20 inches equals 78 inches.
Of course, you want to mount the antenna in a location about
1/b wave length
away from any metal surface near the antenna in any direction
except the
bottom of the antenna.
The lengths and spacing of the J element is not very critical
due to the
variable tuning section. The main thing is to get the final swr
down as low
as possible.
Also, to keep stray RF off of the coax outer conductor, it is
highly
recommended to add an air choke at the bottom of the antenna.
For 2 meters,
the coil is 4 turns of coax with coils side by side in a neat
coil 5 inches
in diameter. You should get good results with the choke within 3
or 4 inches
of the feed point. You can simply use the last part of your
feedline as the
choke before attaching to the feedpoint!
Hopefully, I have described this antenna well enough so you can
construct
one if you want to give it a try. Alternatively, they are
available to
purchase either fully assembled or the parts that you can put
together
yourself.
Good luck!
Ron, K8HSY
----- Original Message -----
From: "Linda C. Knight" <[log in to unmask]
To: [log in to unmask]
Date sent: Sat, 22 Mar 2014 08:44:35 -0400
Subject: Re: Mag mount antenna and cookie sheet
Lol! This is why I love this group. Thank you.
Somebody mention a jPole. It slips my mind what is that.
Hugs and 73
Linda C. Knight & shirley
CallSign: kk4hrg
Please note email:
[log in to unmask]
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