Owen Duffy wrote:
Michael Coslo wrote in
:

...
Are you sure, Owen? I read it as the bottom part of the j-pole. I made
a j-pole once, the top was copper, and the support was thick walled
PVC. Obviously that wasn't an active part. The "u" shaped portion was
all above the support leg.

The antenna matched up well with moving the clips back and forth.
Still works okay. I don't look at them as any better or worse thatn a
1/4 wave gp, or vertical dipole, both of which I had before. Did not
measure for feedline radiation, but I have people at home who let me
know of such problems.

Mike,

Even if you put a J Pole on an insulating support, or build it with
flexible wire inside a rigid PVC tube, you can't pretend the the common
mode current path on the outside of the coax feedline does not exist.


No argument there.


If you take extreme care with symmetry of the J Pole, then common mode
current is lower, and with some further decoupling might be very
acceptable. I doubt that common constructions achieve that goal,
enquirers here and elsewhere often complain of a sensitivity of VSWR of
proximity of their body and other things to the feedline... a sign of
feedline common mode current.

Yes, I'll note that my installation was not particularly sensitive to
proximity.


If by a vertical dipole, you mean a coaxial dipole (though I note people
are now using 'coaxial dipole' to refer to the Double Bazooka), they are
also notoriously bad for feedline decoupling and need further sleeves
(Bazooka Balun) or radials to effectively decouple the feedline.

No, this is just a dipole hung vertically, mainly because it will be
used for repeater work.


Why is the attic so bad? You have to ask is the roof conductive or
variabile permittivity (under any weather) (steel, shingles, concrete
tiles etc), does it have sarking or other conductive insulation products
(such as foil aircell blankets) under it, or on the ceiling, does the
ceiling have foil backed insulation on it, what other conductors are in
the roof space (water, gas, flues, HVAC ducts, structural steel, framing
braces, wiring...etc). What works for one installation might not work
for another because the installer has no idea of the installation.

Agreed, for me at least, putting an antenna in an attic is a last
resort. I came into this thread sideways, hence some of the confusion,
but if I needed stealth, I'd probably make a ground plane out of pipe
that fit nicely over one of the vent stacks above the bathrooms.
Inside, I would go for a dipole of some sort, not a J-pole.

we're pretty much on the same page here, any issues are there because of
the point at which I jumped into the conversation.

I may ask some questions about the stack antenna in a new thread.

- 73 de Mike N3LI -