John Passaneau wrote in news:hnqnnl$kk5a$1
@tr22n12.aset.psu.edu:

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

The support leg is just that a leg. You can make it any length that
works for you.

The 'support leg' as it is being discussed in not just a physical support
for the half wave.

If the upper half wave section carries current, there must be a non zero
current at the lower end of it. That current MUST exist as a common mode
current on the upper part of the 'support leg' as the OP terms it and you
concur.

That common mode current is a standing wave, and the distribution of it
on the 'support leg', feedline, and any conducting supporting structure
makes all of these elements part of the radiating system to some extent,
depending on the actual geometry and their coupling to other conductors
in close proximity.

So to represent that U shaped section as merely supporting structure is
to deny how the thing really works... but then that is the stuff of J
Poles.

Owen

Owen Duffy wrote:

So to represent that U shaped section as merely supporting structure is
to deny how the thing really works... but then that is the stuff of J
Poles.

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.


- 73 de Mike N3LI -

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.

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.

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.

The J Pole (and many other end fed antennas such as the Ringo) have a
matching arrangement that makes VSWR very sensitive to the antenna's
local environment. One of the most common questions is "I tuned this up
real good in the workshop, and when I put it in the attic, the VSWR is
3:1".

Sure, the attic is a challenging place to put an antenna, but a
narrowband matching network exacerbates the problem.

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.

An experienced eye sees things that an inexperience eye doesn't.

A well executed ground plane antenna is an antenna that a person with
little knowledge and experience can implement with a high level of
confidence that it is reasonably efficient and effective.

I didn't really want to can the J Pole, but to say to the OP if he
REALLY MUST put an antenna in the roof space, he ought consider a folded
dipole with half wave coax balun and dresss the feedline away
horizontally for a wavelength or two for minimal effects on pattern
(though that is not well controlled in the roof space).

BTW, a question that reveals the understanding of a J Pole by its
devotees is "should the braid go to one side or the other". All three
answers (yes, three) are offered in similar proportion, but I have never
seen anyone support them with measurement.

It seems to me that the preferred method from purely an electrical point
of view is none of those three. If you establish the tapping point,
drill a hole in either tube at that point, and another at the bottom of
the U, then pass the coax up through the bottom hole inside the tube
(bonding the shield to that point), exit through the other hole bonding
the coax shield to the tube and connecting the inner conductor to the
opposite side, you have built an integral balun which helps to reduce
common mode current. A further ferrite balun below the tube somewhat
using say #61 would give improved suppression at 144MHz.

Owen

Owen Duffy wrote:
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.

I assumed when he said a vertical dipole, he meant exactly that. Two 19 inch
wires center fed. It's cheap, versatile, and does a lot better than your
average rubber ducky.

You mount it verticaly for FM, horizontaly for ssb/cw and at an angle for
satellite work.

Geoff.


--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
New word I coined 12/13/09, "Sub-Wikipedia" adj, describing knowledge or
understanding, as in he has a sub-wikipedia understanding of the situation.
i.e possessing less facts or information than can be found in the Wikipedia.

"Geoffrey S. Mendelson" wrote in
:

....
I assumed when he said a vertical dipole, he meant exactly that. Two
19 inch wires center fed. It's cheap, versatile, and does a lot better
than your average rubber ducky.

I suspect you probably haven't read the thread. That was my original
suggestion with the addition of a balun and some detail on feedline
routing.

Owen

In article ,
Owen Duffy wrote:

A well executed ground plane antenna is an antenna that a person with
little knowledge and experience can implement with a high level of
confidence that it is reasonably efficient and effective.

That would certainly be another workable alternative for this
situation. An SO-239, a few feet of 10-gauge solid wire, and a bit of
soldering, and you end up with a ground-plane antenna with a hanging
loop at the top, two or four ground radials drooping at around a
45-degree angle, and a near-ideal match to 50-ohm coax. Cheap and
quick to make. Stick a ferrite or two on the feedline just below it,
and feedline radiation shouldn't be a problem. If I recall correctly,
this very design appeared in the "quick tips" freebie blurb I got from
the ARRL after I first got my ticket.

It seems to me that the preferred method from purely an electrical point
of view is none of those three. If you establish the tapping point,
drill a hole in either tube at that point, and another at the bottom of
the U, then pass the coax up through the bottom hole inside the tube
(bonding the shield to that point), exit through the other hole bonding
the coax shield to the tube and connecting the inner conductor to the
opposite side, you have built an integral balun which helps to reduce
common mode current. A further ferrite balun below the tube somewhat
using say #61 would give improved suppression at 144MHz.

I used that very design to build a two-arm "Copper Cactus", with
separate stubs and feedlines for 2 meters and 440. It seemed to work
well (although I never actuually measured the feedline currents).

One gotcha to this approach - you have to be *very* careful to
waterproof the point where the coax exits from the upper hole and has
its shield bonded to the pipe! If you don't get a thoroughly
waterproof coating of silicone sealant (or something similar) here,
the exposed braid will wick up water during every rainstorm which
comes along, and your coax will turn into a hose.

The SWR on mine went sky-high after a winter up in the California
rains. When I disconnected the N connectors down at the base of the
antenna, I found them full of water and algae(!) and had a devil of a
time getting things dried out again.

--
Dave Platt AE6EO
Friends of 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!

Owen Duffy wrote:

I suspect you probably haven't read the thread. That was my original
suggestion with the addition of a balun and some detail on feedline
routing.

Some people have more agressive killfiles than others. :-)

Geoff.
--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
New word I coined 12/13/09, "Sub-Wikipedia" adj, describing knowledge or
understanding, as in he has a sub-wikipedia understanding of the situation.
i.e possessing less facts or information than can be found in the Wikipedia.

On Mar 17, 5:36*pm, (Dave Platt) wrote:
In article ,
Owen Duffy wrote:

A well executed ground plane antenna is an antenna that a person with
little knowledge and experience can implement with a high level of
confidence that it is reasonably efficient and effective.

That would certainly be another workable alternative for this
situation. *An SO-239, a few feet of 10-gauge solid wire, and a bit of
soldering, and you end up with a ground-plane antenna with a hanging
loop at the top, two or four ground radials drooping at around a
45-degree angle, and a near-ideal match to 50-ohm coax. *Cheap and
quick to make. *Stick a ferrite or two on the feedline just below it,
and feedline radiation shouldn't be a problem. *If I recall correctly,
this very design appeared in the "quick tips" freebie blurb I got from
the ARRL after I first got my ticket.

That is my choice for a vertical antenna hung from rafters.
I've got one in my attic. Quarter wave ground plane with
8 radials. Hung from the rafters with fishing line. I'm lucky
that my roof is fairly non conductive. All the ducts etc are
well below the antenna. I've never really like J poles for all
the reasons already mentioned. The radiator is 19 inches,
and ditto for the radials, except being as they slope at about
a 45 degree angle, they require less height than if the lower
half of the antenna went straight down from the feed point.

BTW, if you model them, the usual difference between a
1/4 wave GP and a 1/2 wave vertical usually amounts to
about .3 db... Not enough to worry about, and not enough
to make using the taller antenna really worth the extra trouble.
Not to mention the GP is much easier to decouple from the
line. And decoupling from the line is much more critical than
any extra gain from a longer radiator when used for VHF/UHF
and the very low angles most local contacts will use.
In many cases, the GP will outperform the J pole due to it's
better decoupled feed line.

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 -