"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