Questions about a 2m J-pole
 

Frank

Wouldn't you be concerned over presenting a slight Z bump of a 75 ohm
piece of material? I'll admit I cant see it making a huge difference,
but as one is building test gear it might pay to be pedantic.

I'd also be concerned that pushing wire into the air gap would lower the
Z in an unpredictable way. Pushing it just under the shield keeps it at
a reasonably fixed dielectric spacing from the inner conductor.

If it works however, then I won't argue!

Bob VK2YQA


Highland Ham wrote:
Instead of using RG213 coax I would use a short piece of 75 Ohms coax
(here in the UK used for sat dish feeder) with air gap dielectric .
The 10 mm (3/8 inch) OD variety is particularly suitable .
Without removing the outer sheathing a bare or teflon insulated wire can
be readily pushed through.

Questions about a 2m J-pole
 

Bob Bob wrote:
Frank

Wouldn't you be concerned over presenting a slight Z bump of a 75 ohm
piece of material? I'll admit I cant see it making a huge difference,
but as one is building test gear it might pay to be pedantic.

I'd also be concerned that pushing wire into the air gap would lower the
Z in an unpredictable way. Pushing it just under the shield keeps it at
a reasonably fixed dielectric spacing from the inner conductor.

If it works however, then I won't argue!
==========================
I'll check the anticipated 'bump' with the MFJ259B analyser.
First connected to a high quality professional dummy load good for freqs
to 2 GHz (ex cellnet industry),thereafter with the homebrew SWR sensor
inserted,testing in 2 stages : First with just the short piece of 75
ohms coax (obviously having the braid connected to chassis at one end
only) ;Secondly with the wire pushed into the air gap.

This is a useful and interesting NG thread.

Frank GM0CSZ / KN6WH

Questions about a 2m J-pole
 

Highland Ham wrote:
In the US, copper water pipe comes in three grades, K, L and M, in
increasing order of quality/cost. We also have "DWV" copper pipe for
drains
and vents (non-pressure applications, I guess). Same over there?
================
Question : Would any of the above grades of copper pipe affect the RF
radiation characteristics of the antenna ?


The answer depends upon whether you mean an effect that is noticible or
not. ;^)

73 de Mike KB3EIA -

Questions about a 2m J-pole
 

Michael Coslo wrote:
Highland Ham wrote:
In the US, copper water pipe comes in three grades, K, L and M, in
increasing order of quality/cost. We also have "DWV" copper pipe for
drains
and vents (non-pressure applications, I guess). Same over there?
================
Question : Would any of the above grades of copper pipe affect the RF
radiation characteristics of the antenna ?


The answer depends upon whether you mean an effect that is noticible
or not. ;^)
============================================
My question is directed to the different grades K,L and M ,whether any
particular grade would give better RF radiation performance .
It could be that the grade with the highest copper content gives the
best performance ,although I doubt that it will be measurable.
J-pole antennas I am familiar with are all made of aluminium,but copper
is of course better long term in an outdoor environment and electrical
contacts are reliable following soldering.

Frank GM0CSZ / KN6WH

Questions about a 2m J-pole
 

"Highland Ham" wrote in message
...
Michael Coslo wrote:
Highland Ham wrote:
In the US, copper water pipe comes in three grades, K, L and M, in
increasing order of quality/cost. We also have "DWV" copper pipe for
drains
and vents (non-pressure applications, I guess). Same over there?
================
Question : Would any of the above grades of copper pipe affect the RF
radiation characteristics of the antenna ?


The answer depends upon whether you mean an effect that is noticible
or not. ;^)
============================================
My question is directed to the different grades K,L and M ,whether any
particular grade would give better RF radiation performance .
It could be that the grade with the highest copper content gives the best
performance ,although I doubt that it will be measurable.
J-pole antennas I am familiar with are all made of aluminium,but copper is
of course better long term in an outdoor environment and electrical
contacts are reliable following soldering.

Frank GM0CSZ / KN6WH


-------------


I have made emergency VHF antennas from a hunk of 2x4 and two pieces of 3/8"
aluminum grounding wire. Then I get a piece of mini8x (any 50 ohm coax will
do), skin back the insulation until I have enough material to solder an
alligator clip onto the center lead and one onto the equi-distant piece of
shielding. I then drill two 3/8's diameter holes approximately 4" apart in
the hunk of 2x4. Calculate the 1/2 wavelenth of the band you wish to operate
on. Cut a hunk of 3/8" grounding wire to that length, straighten and then
insert it into one of the previously drilled 3/8" holes in the hunk of 2x4.
Cut another hunk of 3/8" aluminum grounding wire to exactly one half the
length of the first piece and then insert it into the remaining hole in the
2x4.

Connect the coax to your VHF rig via the PL-259 that you soldered onto the
end opposite the end with the alligator clips and plug it into your rig.
Clip the alligator clip fastened to the center coax conductor to the longest
piece of 3/8" wire (half wavelength) and the shield connected alligator clip
to the shorter wire protruding from the 2x4. Adjust the height of the two
clips (should be even with one another) for the lowest SWR. Voila! You're on
the air with an instant J-Pole antenna.

All of this can be enclosed in PVC plastic piping for use outdoors.


Ed, NM2K

Questions about a 2m J-pole
 

In article ,
Highland Ham wrote:

My question is directed to the different grades K,L and M ,whether any
particular grade would give better RF radiation performance .
It could be that the grade with the highest copper content gives the
best performance ,although I doubt that it will be measurable.

My recollection is that the basic difference between the various
grades is the thickness of the copper walls. The thicker-walled pipe
is what's preferred for outdoor/buried use and/or higher pressures
(e.g. use on the supply side of a house's water-pressure regulator).

RF current flows only through the skin of a conductor... it doesn't
penetrate very far into the body. One web calculator I found says
that the skin depth of a 144 MHz signal in copper is around 200
micro-inches, or 1/5000 of an inch. If the tubing wall is
much thicker than this, the additional thickness will have no
significant benefit in reducing resistive losses.

Since even the thinnest-walled (Schedule M) copper pipe has a wall
thickness which is about a hundred times more than the skin depth,
there's no electrical benefit to using thicker-walled tubing (Schedule
K or L).

Unless there's some mechanical reason to want to use the heavier-
schedule tubing, I'd just stick with Schedule M - it's lighter, less
expensive, and should be plenty stiff and strong for most
applications.

--
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!

Questions about a 2m J-pole
 

Highland Ham wrote:
Question : Would any of the above grades of copper pipe affect the RF
radiation characteristics of the antenna ?

No.

Roy Lewallen, W7EL

Questions about a 2m J-pole
 

On Sep 19, 9:29 am, "Ed Cregger" wrote:
"Highland Ham" wrote in message

...



Michael Coslo wrote:
Highland Ham wrote:
In the US, copper water pipe comes in three grades, K, L and M, in
increasing order of quality/cost. We also have "DWV" copper pipe for
drains
and vents (non-pressure applications, I guess). Same over there?
================
Question : Would any of the above grades of copper pipe affect the RF
radiation characteristics of the antenna ?

The answer depends upon whether you mean an effect that is noticible
or not. ;^)
============================================
My question is directed to the different grades K,L and M ,whether any
particular grade would give better RF radiation performance .
It could be that the grade with the highest copper content gives the best
performance ,although I doubt that it will be measurable.
J-pole antennas I am familiar with are all made of aluminium,but copper is
of course better long term in an outdoor environment and electrical
contacts are reliable following soldering.

Frank GM0CSZ / KN6WH

-------------

I have made emergency VHF antennas from a hunk of 2x4 and two pieces of 3/8"
aluminum grounding wire. Then I get a piece of mini8x (any 50 ohm coax will
do), skin back the insulation until I have enough material to solder an
alligator clip onto the center lead and one onto the equi-distant piece of
shielding. I then drill two 3/8's diameter holes approximately 4" apart in
the hunk of 2x4. Calculate the 1/2 wavelenth of the band you wish to operate
on. Cut a hunk of 3/8" grounding wire to that length, straighten and then
insert it into one of the previously drilled 3/8" holes in the hunk of 2x4.
Cut another hunk of 3/8" aluminum grounding wire to exactly one half the
length of the first piece and then insert it into the remaining hole in the
2x4.

Connect the coax to your VHF rig via the PL-259 that you soldered onto the
end opposite the end with the alligator clips and plug it into your rig.
Clip the alligator clip fastened to the center coax conductor to the longest
piece of 3/8" wire (half wavelength) and the shield connected alligator clip
to the shorter wire protruding from the 2x4. Adjust the height of the two
clips (should be even with one another) for the lowest SWR. Voila! You're on
the air with an instant J-Pole antenna.

All of this can be enclosed in PVC plastic piping for use outdoors.

Ed, NM2K

Normally, a J-pole has parallel conductors at the bottom 1/4
wavelength, with one of the conductors extending above that 1/4 wave
section for an additional 1/2 wavelength above that. That would be a
3/4 wave piece of wire and a 1/4 wave piece of wire. If I'm reading
your description correctly, you're describing an antenna in which
there's the 1/4 wave of parallel conductors, but then only 1/4 wave
above that of the "unaccompanied" part of the half-wave piece. Also,
do you not connect the two pieces of wire together at the end?

I've seen J-pole designs fed at the bottom end, and I've seen them
with the bottom end shorted and the feed line tapped up on the
parallel section, but haven't heard of one fed tapped up on the
parallel section without the bottom shorted.

Cheers,
Tom

Questions about a 2m J-pole
 

"K7ITM" wrote in message
ups.com...
On Sep 19, 9:29 am, "Ed Cregger" wrote:
"Highland Ham" wrote in message

...



Michael Coslo wrote:
Highland Ham wrote:
In the US, copper water pipe comes in three grades, K, L and M, in
increasing order of quality/cost. We also have "DWV" copper pipe
for
drains
and vents (non-pressure applications, I guess). Same over there?
================
Question : Would any of the above grades of copper pipe affect the RF
radiation characteristics of the antenna ?

The answer depends upon whether you mean an effect that is
noticible
or not. ;^)
============================================
My question is directed to the different grades K,L and M ,whether any
particular grade would give better RF radiation performance .
It could be that the grade with the highest copper content gives the
best
performance ,although I doubt that it will be measurable.
J-pole antennas I am familiar with are all made of aluminium,but copper
is
of course better long term in an outdoor environment and electrical
contacts are reliable following soldering.

Frank GM0CSZ / KN6WH

-------------

I have made emergency VHF antennas from a hunk of 2x4 and two pieces of
3/8"
aluminum grounding wire. Then I get a piece of mini8x (any 50 ohm coax
will
do), skin back the insulation until I have enough material to solder an
alligator clip onto the center lead and one onto the equi-distant piece
of
shielding. I then drill two 3/8's diameter holes approximately 4" apart
in
the hunk of 2x4. Calculate the 1/2 wavelenth of the band you wish to
operate
on. Cut a hunk of 3/8" grounding wire to that length, straighten and then
insert it into one of the previously drilled 3/8" holes in the hunk of
2x4.
Cut another hunk of 3/8" aluminum grounding wire to exactly one half the
length of the first piece and then insert it into the remaining hole in
the
2x4.

Connect the coax to your VHF rig via the PL-259 that you soldered onto
the
end opposite the end with the alligator clips and plug it into your rig.
Clip the alligator clip fastened to the center coax conductor to the
longest
piece of 3/8" wire (half wavelength) and the shield connected alligator
clip
to the shorter wire protruding from the 2x4. Adjust the height of the two
clips (should be even with one another) for the lowest SWR. Voila! You're
on
the air with an instant J-Pole antenna.

All of this can be enclosed in PVC plastic piping for use outdoors.

Ed, NM2K

Normally, a J-pole has parallel conductors at the bottom 1/4
wavelength, with one of the conductors extending above that 1/4 wave
section for an additional 1/2 wavelength above that. That would be a
3/4 wave piece of wire and a 1/4 wave piece of wire. If I'm reading
your description correctly, you're describing an antenna in which
there's the 1/4 wave of parallel conductors, but then only 1/4 wave
above that of the "unaccompanied" part of the half-wave piece. Also,
do you not connect the two pieces of wire together at the end?

I've seen J-pole designs fed at the bottom end, and I've seen them
with the bottom end shorted and the feed line tapped up on the
parallel section, but haven't heard of one fed tapped up on the
parallel section without the bottom shorted.

Cheers,
Tom


--------------


Try it, just as I described it. It works and works well.

No, the bottoms are not connected, but there is a version that works with
that connection.


Ed Cregger, NM2K

Questions about a 2m J-pole
 

"Highland Ham" wrote in message
...
In the US, copper water pipe comes in three grades, K, L and M, in
increasing order of quality/cost. We also have "DWV" copper pipe for
drains
and vents (non-pressure applications, I guess). Same over there?
================
Question : Would any of the above grades of copper pipe affect the RF
radiation characteristics of the antenna ?



I don't think so -- just the cost. Aluminum, which is less conductive than
any common copper alloy, makes fine antennas.

By the way, please refer to the copper pipe section at
http://www.homedepot.com/webapp/wcs/...?pn=Pipes&lang
Id=-1&storeId=10051&catalogId=10053 (I think I had the price/quality
expressed backwards in my original post.)

"Sal"