J-POLE  HELP,  Phil Karras/KE3FL


I became interested in building foldable J-Pole antennas when I started
using 2-meters.  I joined ARES and needed a better antenna for emergency
use - something easy to carry along and set up.  The foldable J-Pole fit
this bill nicely.  The article I used to get started was printed in my
local (Carroll County, MD) club newsletter NewsWaves.  Shortly after that
one was published, another one came out in QST: September 1994, pg 61-62

My first experience building one of these antennas was wonderful.  It
tuned up right on 146.0 MHz and the SWR was 1.1 to 1.  In November, 1994,
I became the Emergency Coordinator for Carroll County, MD, and wanted
every ARES member to have one of these types of antennas.  They cost less
than $5 to build and give at least a 6dB gain over a rubber duck antenna. 
I built a number of these antennas and experimented along the way.  While
helping others build their antennas, I discovered a good method for
broadening the bandwidth and later I found two methods for lowering a high
SWR.  These methods are described below:

BANDWIDTH: The bandwidth of a wire antenna is not going to be as broad as
           a well-built copper-pipe J-Pole.  However, I have been able to
           get it to be 4 MHz between 1.5:1 SWR points.  Do not feel bad
           if you only get it to be 4MHz between 1.7:1 points.  To
           increase the bandwidth, the type of connection from the coax to
           the antenna is important.  I found the best connection to use
           is a V-type connection with the wire leads being about 1/2 inch
           long.  Longer doesn't seem to help, and shorter makes the
           bandwidth narrower.

           The second thing to try if the BW is extremely narrow is to cut
           wire off the top of the antenna.  I had an antenna that had a
           BW of 0.6 MHz.  I cut one inch off the top of the antenna and
           the BW went right up to 4 MHz.  I had to make SWR and frequency
           adjustments after this, but the BW remained at about 4 MHz
           between 1.7:1 SWR points for the ham band.  Cutting off one
           inch on a 2-meter antenna is rather drastic, but since I tried
           a longer design to begin with, I knew that this was OK.  If you
           follow the construction article found in QST, make small
           changes since - even with wire of a different velocity factor -
           you're probably off by a small amount.  You can make the
           antenna too short.  I had an antenna that was too short and I
           had to add wire to the top and the bottom before I was able to
           tune it up, so go slowly.  (This was another test to see if a
           shorter design would tune up.)

SWR: The lower the SWR at the center frequency, the better (as long as
     radiation resistance is not lowered or radiation angle increased in
     the process.) You should be able to get the SWR to be 1:1 at or near
     146.0 MHz simply by moving the coax connection points up or down.  If
     you find that this is not the case, first try pushing the connections
     apart.  <| |>.  This will work if the SWR is close to 1:1, but won't
     get it to be 1:1 if the SWR is more than 1.3:1.

The second method requires adding more wire to the bottom of the antenna. 
I have seen a number of antennas whose lowest SWR was 1.5 to 1.7:1, and
this was almost the worst SWR across the entire 4MHz 2 meter band.  To
improve this, I found that it helps if you add about one inch of extra
wire to the bottom of the antenna, remove the old shorting bar and install
a new one at the new bottom.  Try moving this bar up until you find a
point at which the antenna tunes up to 1:1 at some frequency.  (It should
end up being close to 146.0 MHz.) STOP! Now go back and move the coax
connection points to get it 1:1 at 146.0 MHz.  (Move up to increase the
frequency, down to lower it.) Remove any excess wire at the bottom of the
antenna and you are done.

One thing that does NOT work is adding or removing wire from the top of
the antenna.

I have never had to add a balun to improve either SWR or bandwidth.  You
may want to add it for other reasons, but not these.  


ABOUT THE AUTHOR:
Philip Karras/KE3FL has a BS and MA in Physics.  He received his novice
ticket (KB3AEG) in June 1992 and two weeks later ran the Novice station
for the BRASS club.  In December of 1992 he passed his extra exams.  He
holds the ARRL appointments of: VE, ORS, OES, and EC for Carroll County. 
He recommends CW slow net training for learning about traffic and
increasing your code speed, he is a graduate of the Maryland Slow Net
(MSN) training courses.


73 de KE3FL

Phil