In message , Owen Duffy
writes
ai8o wrote in news:1bcefed7-a087-4fda-976d-
:

I am thinking of making a 2 meter J-Pole from 3/4 or 5/8 copper pipe.

Most plans that I can find on the web call for a 58 inch main leg and
a 20 inch support below, making the antenna
78 inches or so in height.

I want to install it in my attic where I have 66 inches of clear
space.

Can I cut the lower support leg down to 8 inches without a major
impact on the performance of the antenna?

End fed antennas are an implementation risk.

What's an 'implementation risk'?

Have you considered a vertical dipole hung from the roof structure, and
taking the feedline away horizontally for a couple of wavelengths, then
down. You could make it a folded dipole (TV ribbon?) and use a half wave
coax balun.

Are you suggesting that a centre-fed dipole is better than a J-Pole?
There is absolutely no problem with the J-Pole / Slim Jim type of
antenna, or with feeding it.
--
Ian

On Wed, 17 Mar 2010 08:22:52 +0000, Ian Jackson
wrote:

End fed antennas are an implementation risk.

What's an 'implementation risk'?

Having it come out mismatched, a very much higher probability.

Have you considered a vertical dipole hung from the roof structure, and
taking the feedline away horizontally for a couple of wavelengths, then
down. You could make it a folded dipole (TV ribbon?) and use a half wave
coax balun.

Are you suggesting that a centre-fed dipole is better than a J-Pole?

A center fed dipole is simpler to tune and keep tuned.

There is absolutely no problem with the J-Pole / Slim Jim type of
antenna, or with feeding it.

The majority of users who come here for solutions to theirs would
suggest otherwise.

73's
Richard Clark, KB7QHC

Ian Jackson wrote in
:

In message , Owen Duffy
writes
....
End fed antennas are an implementation risk.

What's an 'implementation risk'?


We will see if the OP, like so many who implement J poles in the roof
space, becomes aware of problems and comes back for advice on how to
solve them.

The commonest problem is that VSWR doesn't meet expectations, but that it
is the most mentioned problem speaks more of the visibility of VSWR that
that it is the greatest problem with the J Pole.

Like many if not most end fed antennas, the J Pole does little to deal
with the common mode current that must flow on the feed system / support
structure. Explanations of how they work usually deny the issue.

But for all their faults, they spray RF around, and people would foolish
to deny that they 'work' to some extent.

When you look at the strengths and weaknesses of a J Pole, they are not a
good choice IMHO for in-roof-space deployment.

Owen