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!

On Wed, 10 May 2006 04:39:07 -0000, (Dave Platt)
wrote:

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.

My experience at my kids' place is that it has the same effect on the
vertical dipole as it does with the jpole. I have a dipole I created
by using wire inside a cpvc pipe. The coax was rolled up to form the
choke balun. The dipole antenna has the same problem with hitting the
local repeaters as the jpole did. In the same places, the two
antennas had such similar effects that I could literally not tell a
difference either in audio or signal strength as displayed by the
radio. SUper J-Poles, on the other hand, perform much better than
either the j-pole or the vertical dipole.


--
73 for now
Buck
N4PGW

"Dave Platt" wrote in message
...


snip

I haven't read any reports of actual experiments which attempted to
quantify the tuning stability of the classic J-pole ...

snip

That, sir is about to change:

I wondered about the effect of mast material, so I took a freshly built
j-pole and mounted it in the clear. I noted the XMIT reflected power (low)
and moved a ten-foot piece of galvanized steel mast around the antenna, I
found I had to get within about an inch to see the reflected power increase.

Next, I found a repeater with nearly continuous chatter and connected the
j-pole through an attenuator to my radio. I set the attenuator so the
received audio was very noisy -- another dB or two and the conversation was
unreadable. That same piece of mast material had essentially the same
effect: had to get really close to louse up the antenna's performance.

That said, if I take a j-pole and walk around with it, periodically
measuring the reflected power, I will see physical locations where it bumps
up. That's probably not detuning; it's probably reflected power off a wall
or fence, which could be what's happening in the OP's attic. (Navy ship
radars' reflected power monitors will "bump up" every time the antenna
rotates past the mast or some other bit of ship's structure.)

Is this great science? No. Is it worth noting? I hope so.

Sal M. Onella wrote:
"Dave Platt" wrote in message
...


snip

I haven't read any reports of actual experiments which attempted to
quantify the tuning stability of the classic J-pole ...


snip

That, sir is about to change:

I wondered about the effect of mast material, so I took a freshly built
j-pole and mounted it in the clear. I noted the XMIT reflected power (low)
and moved a ten-foot piece of galvanized steel mast around the antenna, I
found I had to get within about an inch to see the reflected power increase.

Next, I found a repeater with nearly continuous chatter and connected the
j-pole through an attenuator to my radio. I set the attenuator so the
received audio was very noisy -- another dB or two and the conversation was
unreadable. That same piece of mast material had essentially the same
effect: had to get really close to louse up the antenna's performance.

That said, if I take a j-pole and walk around with it, periodically
measuring the reflected power, I will see physical locations where it bumps
up. That's probably not detuning; it's probably reflected power off a wall
or fence, which could be what's happening in the OP's attic. (Navy ship
radars' reflected power monitors will "bump up" every time the antenna
rotates past the mast or some other bit of ship's structure.)

Is this great science? No. Is it worth noting? I hope so.



Maybe reflected power off of nearby objects is the cause of poor
performance of my J-Pole in the basement shop. And the attic has the
roof overhead and many, many cross braces, typical of pre-fab trusses.
Anyway, I am satisfied with the Arrow end fed antenna in the attic.
But, I am disappointed with my home made J-Poles. Maybe I will try a
little SO239 1/4 wave, just to see if there is any difference.

Thanks for the help, jimbo

On Thu, 11 May 2006 08:53:05 -0600, jimbo wrote:

Maybe reflected power off of nearby objects is the cause of poor
performance of my J-Pole in the basement shop. And the attic has the
roof overhead and many, many cross braces, typical of pre-fab trusses.
Anyway, I am satisfied with the Arrow end fed antenna in the attic.
But, I am disappointed with my home made J-Poles. Maybe I will try a
little SO239 1/4 wave, just to see if there is any difference.

I'm wondering if you are using a common mode choke? Also what type of
mast are you using to support the antenna?

Danny, K6MHE

In my many years I have come to a conclusion that one
useless man is a shame, two is a law firm, and three
or more is a congress. - John Adams

email: k6mheatarrldotnet
http://www.k6mhe.com/

Dan Richardson wrote:
On Thu, 11 May 2006 08:53:05 -0600, jimbo wrote:


Maybe reflected power off of nearby objects is the cause of poor
performance of my J-Pole in the basement shop. And the attic has the
roof overhead and many, many cross braces, typical of pre-fab trusses.
Anyway, I am satisfied with the Arrow end fed antenna in the attic.
But, I am disappointed with my home made J-Poles. Maybe I will try a
little SO239 1/4 wave, just to see if there is any difference.


I'm wondering if you are using a common mode choke? Also what type of
mast are you using to support the antenna?

Danny, K6MHE

In my many years I have come to a conclusion that one
useless man is a shame, two is a law firm, and three
or more is a congress. - John Adams

email: k6mheatarrldotnet
http://www.k6mhe.com/

I have tried with and without a coax loop, 5 turns about 6 inches in
diameter. I couldn't see any difference. I use a 1/8 inch diameter
rope to hang the J-Pole from a joist in the basement or from a roof
rafter in the attic.

jimbo

jimbo wrote:
Dan Richardson wrote:

I'm wondering if you are using a common mode choke? Also what type of
mast are you using to support the antenna?


I have tried with and without a coax loop, 5 turns about 6 inches in
diameter. I couldn't see any difference. I use a 1/8 inch diameter rope
to hang the J-Pole from a joist in the basement or from a roof rafter in
the attic.

A single choke blocks conducted common mode current. But a J pole (and
ground plane, for that matter) can also have considerable common mode
current due to coupling between the antenna and feedline, which can
distort the pattern and reduce the gain at the horizon. This can
actually be made worse by a choke in some cases. Two chokes, about a
quarter wave apart, are necessary to reliably reduce the coupled
current. The amount of conducted and coupled current you have without
chokes depend on the feedline length and orientation, so it can be small
in some installations and large in others. I suspect that's one reason
why people report such widely varying degrees of success with this antenna.

Roy Lewallen, W7EL

Roy Lewallen wrote:

jimbo wrote:

I have tried with and without a coax loop, 5 turns about 6 inches in
diameter. I couldn't see any difference. I use a 1/8 inch diameter
rope to hang the J-Pole from a joist in the basement or from a roof
rafter in the attic.


A single choke blocks conducted common mode current. But a J pole (and
ground plane, for that matter) can also have considerable common mode
current due to coupling between the antenna and feedline, which can
distort the pattern and reduce the gain at the horizon. This can
actually be made worse by a choke in some cases. Two chokes, about a
quarter wave apart, are necessary to reliably reduce the coupled
current. The amount of conducted and coupled current you have without
chokes depend on the feedline length and orientation, so it can be small
in some installations and large in others. I suspect that's one reason
why people report such widely varying degrees of success with this antenna.

Roy Lewallen, W7EL

Isn't 6 inch diameter a little large for 2 meters?

tom
K0TAR