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Re: J-pole antenna question



The coax should be attached to the neutral point halfway between the
longer and shorter stub.  Some ARRL publications and yes, even the AMSAT
Journal have been making this mistake for years!  Any doubters need only
measure the VSWR bandwidth to illustrate how good a match is being made.
    Most of these designs "choke" out the shield currents by winding a few
turns at the base of the J.  Why not do it right the first time?
    In the case of a copper water pipe J, mount a coaxial panel connector
midway between the vertical elements at the horizontal crossover.  Solder
a length of number 14 solid copper house wire to the center pin leaving
the insulation on the wire.  Extend the wire upwards for five inches
between the elements, then bend the wire at right angles to the longer
element.  Wrap four turns around the element as tightly as possible
securing the wire with tape and cable ties.  For any given frequency and
pipe diameter experimentally change the length and number of turns for a
VSWR of one to one.  Do experiment for a solid one to one match! 
     The theory is simple:  The five inch length of wire is an inductance.
The four turns of wire wrapped around the radiator is capacitor.  The
entire assemble is an L-C matching network.  Simple indeed!  No need to
use a "choke" of a few turns of coax at the base, but it will not hurt.
Cliff K7RR




On Thu, 7 Dec 2000, Douglas Braun & Nadia Papakonstantinou wrote:

> Hello,
> 
> I am building a dual-band J-pole antenna (for satellite use, naturally).
> I have seen some designs where the coax braid is attached to the main element
> and the center conductor is attached to the stub, and I have seen other designs
> where it is the other way around.  Does anyone have any idea which hookup
> works better?
> 
> Thanks,
> 
> Doug Braun
> NA1DB
> 
> P.S.:  A tip for J-poles:  Copper water pipe comes in two thicknesses (in the USA,
> at least).  If you are making a J-pole, look for the thinner-walled variety,
> which is identified with red markings.
> 
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