Dave Platt wrote:
It is just I have some conduit and I can weld it. I thought is would
be
interesting.

Sure. No reason not to do this. You may find that there are, or are
not significant differences from a copper-pipe J-pole. The
finding-out is part of what's interesting, no?

I was thinking of building one of those "slim jims' but
think the mounting might prove to be a problem.

As others have pointed out, a classic J-pole and a Slim Jim are the
same basic sort of antenna. The Slim Jim simply has a double-section
radiator, shorted at the top. The behavior and performance of the
two
will be very similar.

See http://www.cebik.com/jp1.html for a lengthy discussion of such
J-pole variants.

I am just so confused over this j pole. So many people have
different
opnions. One says it should never be grounded or touch a metal mast
others say it does not matter. SOme say it should use a choke of
some
type others say it does not matter. It is confusing. What do you
think
on that?

Here's my take on it. At www.cebik.com you can find out a bunch of
other info and discussion.

The classic copper-pipe J-pole is neither a fully-balanced design nor
a fully-nonbalanced design. The impedances "seen" by the two sides
of
the feedline are similar, but not identical, and thus there are going
to be somewhat-different currents flowing on the two sides of the
matching arm.

Now, some people feel that a J-pole gives the best and most
consistent
performance overall if you force it to act as much like a balanced
antennas as possible... you force the currents on the two sides of
the
matching stub to be as equal as you can, and you eliminate the
pathways which would allow RF current to flow down to ground through
other paths (that is, through a grounded mast, and along the outside
of the feedline).

To implement this approach, you insulate the J-pole (or Slim Jim)
from
ground, and you use some sort of balun or choke on the feedline to
cut
off the outside-of-the-feedline radiation. Some people loop the coax
a few times to create a choke, some people run the coax down the
inside of the copper pipe, some people stick a few ferrite beads on
the outside of the coax, and some people use a half-wavelength 4:1
coaxial balun.

This approach, carefully implemented, *may* give you a J-pole or Slim
Jim whose radiation pattern closely approximates that of a classic
half-wave vertical dipole. You're keeping RF currents from flowing
in
any places which could cause radiation which disturbs the pattern. A
J-pole or Slim Jim of this sort would tend to be insensitive to small
variations in the length of the feedline, because the feedline is
choked off and is presenting a high impedance.

So, this is the right and best approach? Not necessarily.

The opposite approach can also work very well... you just build a
J-pole, stick it on a metal mast, run the feedline down to your
transmitter without choking, and get on the air. There are many,
many
such J-poles in operation today, with very satisfied owners.

Such J-poles *may* end up with a nontrivial amount of RF flowing down
the mast, or down the outside of the feedline. This may disturb the
antenna's radiation pattern somewhat (making it less than ideally
omnidirectional), or may cause the SWR seen by the transmitter to
change somewhat if the feedline length is varied or if the mast is
touched (both of these can change the outside-of-the-feedline RF
impedance and currents).

However, these can very easily be small and insignificant factors in
many installations. Radiation from the feedline and the mast might
actually help an antenna's coverage more than it hurts, in certain
circumstances... it might "bend" the antenna's pattern into a
direction where the slight additional gain is helpful, at the expense
of a gain loss in another direction where there doesn't happen to be
anyone that the owner doesn't want to talk to. A shift in SWR due
to mast or feedline RF-current effects can often be "tuned out" of
the
system just by moving the feedline attachment points a fraction of an
inch... in a base-station application such an adjustment, once made,
would probably not need to be re-done unless you start changing
cables
or moving the mast around.

So... an insulated-and-forcibly-balanced J-pole may well be a more
theoretically-perfect antenna in some sense, but a grounded J-pole
with no attention paid to balancing or choking could quite easily
work
just as well in any given installation.

My own copper-pipe J-pole uses a hybrid approach (well, that's the
polite term... "it's schizophrenic" is equally valid). It's grounded
to the mast, but the feedline is well choked (it runs inside the
copper pipe and through a couple of ferrite beads therein, and
there's
a four-turn choke loop just outside the end of the pipe). Why did I
do it this way? Why not?! It works fine, it's been working fine for
close to three years, and if an insulated-and-perfectly-balanced
version gave me a dB or two better gain uniformity I would very
probably never notice the improvement.

Just BUILD SOMETHING, guy. Don't wait around until you figure out
the
One True Way To Do It Right. There isn't one... or, rather, there
are
at least a thousand.

--
Dave Platt
AE6EO
Hosting the Jade Warrior home page:
http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!


If I build a j out of conduit and make it more like a "U" cutting the
long and short and crossbar at 45 degree angles so they can be welded
togther, how would I measire the length to cut each piece? Can I mount
the cross bar to the mast since I wont have a bottom "leg"?