On 25 Apr 2006 16:43:11 -0700, wrote:


jimbo wrote:
SNIP
jimbo - AJ7IM

Hi Jimbo, it has been my experience, and everybody else's that an
indoor or attic J-pole is unpredictable. A high impedance end fed
antenna in an attic environment will be hard to predict. Moving it one
foot can effect things.
You might consider the SO-239 ground plane. It can be built and
tuned in 30 minutes, is 50 ohms, so it will not be so concerned about
its environment. Likely you could not tell a difference in performance
from a J-pole especially if they are both in the attic.
Oh yes, the standard-if the SWR is below 3.0, don't worry about it.
Gary N4AST


I started building dipole antennas. they can be mounted horizontal or
vertical and easily moved around. I use cpvc with a dipole wire
inside and a coax-choke balun. They work quite well.

Just another option.
--
73 for now
Buck
N4PGW

The J-pole crowd seems to tout fantastic claims about this antenna and I
have never been impressed. Many people have tested the J-pole against
other easy to make antennas and the end fed half wave J-pole usually
does not stand up to even it’s brother, the center fed half wave dipole.
I assume the decrease in performance is due to mistakes in assembling
and tuning the matching section. Here is a link to a group that tested
some J-poles against other antennas in the CA desert and the J-pole lost
to even simple 1/4 wave ground plane types.
http://www.geocities.com/SiliconVall...5/anttest.html
My point is, why not make a simple ground plane for the attic that may
not get bothered by surroundings as much as the J-pole.
Bob


Buck wrote:
On 25 Apr 2006 16:43:11 -0700, wrote:

jimbo wrote:
SNIP
jimbo - AJ7IM
Hi Jimbo, it has been my experience, and everybody else's that an
indoor or attic J-pole is unpredictable. A high impedance end fed
antenna in an attic environment will be hard to predict. Moving it one
foot can effect things.
You might consider the SO-239 ground plane. It can be built and
tuned in 30 minutes, is 50 ohms, so it will not be so concerned about
its environment. Likely you could not tell a difference in performance
from a J-pole especially if they are both in the attic.
Oh yes, the standard-if the SWR is below 3.0, don't worry about it.
Gary N4AST


I started building dipole antennas. they can be mounted horizontal or
vertical and easily moved around. I use cpvc with a dipole wire
inside and a coax-choke balun. They work quite well.

Just another option.

Bob wrote:
snip

My point is, why not make a simple ground plane for the attic that may
not get bothered by surroundings as much as the J-pole.
Bob


I made one of those antennas right after I got my Tech license. Hit
the local repeater with no problems. I guess I forgot or thought it
was too simple for the current application.

jimbo

"Bob" wrote in message
. com...
The J-pole crowd seems to tout fantastic claims about this antenna and I
have never been impressed. Many people have tested the J-pole against
other easy to make antennas and the end fed half wave J-pole usually
does not stand up to even it’s brother, the center fed half wave dipole.
I assume the decrease in performance is due to mistakes in assembling
and tuning the matching section. Here is a link to a group that tested
some J-poles against other antennas in the CA desert and the J-pole lost
to even simple 1/4 wave ground plane types.
http://www.geocities.com/SiliconVall...5/anttest.html

snip

A quick look at the numbers doesn't make the j-pole look so all-fired bad.
"Losing" by 1 dB or so isn't a serious loss. Ham radio isn't exactly the
Kentucky Derby.

I don't know what "fantastic claims" you've heard, but my affinity for the
j-pole comes from its simplicity and durability. It's a halfwave vertical
that requires no ground plane and can often be matched to 1:1 at the sweet
spot. Mounting is a dream -- it even works if you drop the low end into a
plumbing vent pipe. (How would I know that? ;-)

I have never built a center-fed halfwave, but we use a bunch of them in the
Navy -- call them "stovepipes" because of the way the fiberglass housings
look -- and they work very, very well. The elements are large metal
cylinders, tending to make them quite broadband. I do not know how they're
fed ... balun or what.

John , KD6VKW

The J-pole crowd seems to tout fantastic claims about this antenna and I
....[snip]....

... my affinity for the j-pole comes from its simplicity and durability.
... Mounting is a dream -- it even works if you drop the low end into a
plumbing vent pipe. (How would I know that? ;-)
....[snip]....

I once made a J-pole from 300-ohm twinlead and tweaked it until it worked
OK while hanging from the ceiling, but when I moved it over next to a
wood-framed picture window (hung it from the window casing), the SWR went
to pot. Did I do something wrong?

(I concluded the J-pole must be VERY sensitive to its surroundings.)

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

wrote:
I once made a J-pole from 300-ohm twinlead and tweaked it until it worked
OK while hanging from the ceiling, but when I moved it over next to a
wood-framed picture window (hung it from the window casing), the SWR went
to pot. Did I do something wrong?

What kind of metal was behind the window frame?
--
73, Cecil http://www.qsl.net/w5dxp

I once made a J-pole from 300-ohm twinlead and tweaked it until it worked
OK while hanging from the ceiling, but when I moved it over next to a
wood-framed picture window (hung it from the window casing), the SWR went
to pot. Did I do something wrong?

What kind of metal was behind the window frame?

The house was old enuf that I'm sure it was made of wood, and
since it was a PICTURE window, there were no sash weights, either.

So the question remains: are J-poles sensitive to nearby materials?

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

On 9 May 2006 20:43:50 -0500, wrote:

So the question remains: are J-poles sensitive to nearby materials?

yes, and also, RF doesn't always saturate an area.

The wire in the j-pole is affected by the insulating material the twin
lead is made from. Different types have different dimensions for the
j-pole lengths.

Though it doesn't make sense sometimes, moving an antenna near a wall
may cause a negative effect. I have that problem at my kids house.
They are hams and I setup a hook in the ceiling to hang a pole. It
worked, but their mom didn't want it hanging so far from the wall and
moved it against the wall. It doesn't work. So we found a compromise
spot near another wall, a corner actually. It works but it is leaning
at about 20 degrees coming from the corner.

There is also a problem of RF holes. Mobile operators and HT or
portable operators experience them the most. It is the cause of
picket-fencing. The mobile moves from a hot spot to a not-so-hot
spot. A portable rig can usually find the hot spot or the dead spot
easily. I can hit the local repeater from inside my room with low
power on my ht. outside about three feet away, I can't hit it on high
power. It is further from the wall there.

Good luck
Buck

--
73 for now
Buck
N4PGW

In article , wrote:

What kind of metal was behind the window frame?

The house was old enuf that I'm sure it was made of wood, and
since it was a PICTURE window, there were no sash weights, either.

So the question remains: are J-poles sensitive to nearby materials?

That depends on the materials. Conductive, or lossy-dielectric
materials seem to have a significant effect especially when located
near the high-impedance points.

When I built a ladder-line J-pole and hung it next to my house's
outside wall, it de-tuned quite badly. Its performance stank so badly
that I could barely reach a nearby repeater.

When I hung it by an inside wall, it was not de-tuned enough to affect
the radio's performance appreciably (I didn't have an MFJ meter at the
time and so don't know the actual degree of de-tuning). I was able to
hit the repeater quite well even though the antenna was further inside
the house.

The first (outside) wall was stucco... with embedded chicken wire.
The second (inner) wall was drywall and lumber.

I suspect that exterior brick, or stone, or wet wood, would fall
somewhere in between the two in terms of de-tuning potential.

My guess is that the classic J-pole may be somewhat more vulnerable to
being de-tuned by nearby materials than a center-fed half-wave
radiator. The quarter-wave matching section is often fairly critical
of adjustment - I've read reports that even small changes in the
configuration of the upper potion of the matching section (e.g.
material near the open end of the stub, or changes in the spacing) can
cause some fairly large changes in the J-pole's feedpoint impedance.
Standard J-pole antennas seem to work most stably when built of rigid
materials and mounted "in the clear".

I haven't read any reports of actual experiments which attempted to
quantify the tuning stability of the classic J-pole, compared with
variants like the Arrow (a.k.a. Cebik's nontypical design), the
open-sleeve, and the sperrtopf coaxial design, or compared with a
center-fed dipole or other varieties of halfwave radiator. It'd be an
interesting study to try to put together.

--
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!

wrote:
I once made a J-pole from 300-ohm twinlead and tweaked it until it worked
OK while hanging from the ceiling, but when I moved it over next to a
wood-framed picture window (hung it from the window casing), the SWR went
to pot. Did I do something wrong?

What kind of metal was behind the window frame?

The house was old enuf that I'm sure it was made of wood, and
since it was a PICTURE window, there were no sash weights, either.

So the question remains: are J-poles sensitive to nearby materials?


My limited experience with three J-Poles indicates that they are very
sensitive to nearby materials. I tried tuning in my shop which is
fairly cluttered and in the basement of a stucco finished townhouse. I
used thin rope to hang the J-Poles from an overhead floor joist.
Changing the location by only several inches changed the SWR. I found
a spot about four feet from the nearest object which gave the lowest
SWR. A ladder line J-pole was still very difficult to tune and it
changed significantly when it was moved to the attic. A copper J-Pole
was easier to tune but it also changed when it was moved to the attic.
The Arrow end fed was "store bought" so I didn't make any adjustments
in the basement. It had a reasonable SWR in the basement and it didn't
change when it was moved to the attic.

I did some more tuning of the copper J-Pole on my deck. SWR changed
with location, even though the deck is fairly open. The best SWR was
with the J-Pole in a 5 gallon pickle crock sitting near the center of
the deck.

jimbo