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

On Sun, 07 May 2006 00:39:24 GMT,
wrote:

Measurements are required. A suitable SWR meter of some form
is needed or your just guessing.

Judging an antenna's performance based upon SWR is a very poor
indicator of how it will radiate.

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/

jimbo wrote:

I have been trying to get an acceptable 2 meter antenna installed in my
attic. A J-Pole seemed to me the best solution for my attic space. You
snip
jimbo - AJ7IM

Here is a link to one I have had good luck with. Works quite well out
of the box. I use two chaokes, one at the base, and one 19 inches below
that. You may need to trim the length of the half wave section, so cut
it a smidge long.

http://www.hamuniverse.com/2mladjpole.html

tom
K0TAR

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!

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

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

"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