A newbie with a couple of questions.
 

I recently built a two meter j-pole.In the plan,I saw no mention of pvc
use.Yet,I had to place a piece of pvc between the two uprights to get
the swr to fall.Is there something I missed?
Also,I have been thinking about long-wire antennas.Every long-wire I've
seen has been in an East-to-West direction.How would it work if I divide
the wire lengths and transform a long-wire into multiple directions?I
realize most foreign lands are to the East and West,but what about the
North and South coverage?

Donnie B wrote:
Also,I have been thinking about long-wire antennas.Every long-wire I've
seen has been in an East-to-West direction.How would it work if I divide
the wire lengths and transform a long-wire into multiple directions? I
realize most foreign lands are to the East and West,but what about the
North and South coverage?

An unbalanced long-wire antenna that is longer than about 3/4WL doesn't
have much broadside radiation. Unless one understands what one is doing,
radiation patterns for long-wires are pretty much hit or miss.

BTW, radio waves follow great circles, not lines of longitude. From
Texas, a radio wave travels over Lake Erie and Newfoundland on its
way to England.
--
73, Cecil http://www.qsl.net/w5dxp



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hi donnie,

check out http://www.cebik.com for lots of neat info on antenna

jana

Donnie

I recently built a two meter j-pole.In the plan,I saw no mention of pvc
use.Yet,I had to place a piece of pvc between the two uprights to get
the swr to fall.Is there something I missed?
Also,I have been thinking about long-wire antennas.Every long-wire I've
seen has been in an East-to-West direction.How would it work if I divide
the wire lengths and transform a long-wire into multiple directions?I
realize most foreign lands are to the East and West,but what about the
North and South coverage?

Donnie,



The "open end" of the matching section of the J antenna (commonly called a
"J-Pole"), as you have found, does have a large effect on the
characteristics. The spacing of the two parallel lines or pipes changes the
match. I think that to the first approximation, this is an acceptable way
to adjust the match. The only problem I can foresee is that the top of the
shorter line/pipe is at a high impedance point and any changes there would
cause changes---such as dirt build up over time or deterioration of the PVC.
I would tend to put it a little ways down from the top to head off such
concerns. I recent built a J antenna for 1296 MHz and used a small piece of
plastic in this location for tuning it.


--
Steve N, K,9;d, c. i My email has no u's.

"Donnie B" wrote in message
...
I recently built a two meter j-pole.In the plan, I saw no mention of pvc
use.Yet, I had to place a piece of pvc between the two uprights to get
the swr to fall.Is there something I missed?
....

In the plan I have for one it calls for a piece of fiberglass rod, others
bakelite. to keep the two sections of the J at the right sepration. Somtimes
mechanical rigidity is enough, for one like the latter search for "copper
cactus".


"Donnie B" wrote in message
...
I recently built a two meter j-pole.In the plan,I saw no mention of pvc
use.Yet,I had to place a piece of pvc between the two uprights to get
the swr to fall.Is there something I missed?
Also,I have been thinking about long-wire antennas.Every long-wire I've
seen has been in an East-to-West direction.How would it work if I divide
the wire lengths and transform a long-wire into multiple directions?I
realize most foreign lands are to the East and West,but what about the
North and South coverage?

"Jimmy" wrote in message
. com...
In the plan I have for one it calls for a piece of fiberglass rod, others
bakelite. to keep the two sections of the J at the right sepration.

Are we talking regular J, or Super J here? The
antenna you're describing is a Super, a colinear
array with the middle phasing section. In a normal
J-pole, there is no insulator needed.

Here's a confusing point from the original post...


I recently built a two meter j-pole.In the plan,I saw no mention of pvc
use.Yet,I had to place a piece of pvc between the two uprights to get
the swr to fall.Is there something I missed?
/

Is the pipe used in an insulator fashion, or
is it used to spread the Q-section? I believe
it's the latter... if so, the newbie can just bend
or re-solder the pole so the spacing's correct.

And if the newbie is describing a 300ohm
twinlead J-pole, then I give up.

Somtimes
mechanical rigidity is enough, for one like the latter search for "copper
cactus".

A thing of beauty... I've got a nice copper
J for six that I'm considering converting to
the saguaro configuration.
__
Steve
KI5YG
..

" Stephen Cowell" wrote in message
. com...

"Jimmy" wrote in message
. com...
In the plan I have for one it calls for a piece of fiberglass rod,
others
bakelite. to keep the two sections of the J at the right sepration.

Are we talking regular J, or Super J here? The
antenna you're describing is a Super, a colinear
array with the middle phasing section. In a normal
J-pole, there is no insulator needed.

If a 2 meter J-pole construction is imperfect enough that
the short and long vertical portions are not exactly parallel
it might require an insulated piece of something to force them
to be parallel. If one builds a 6 meter J-pole the long and short
vertical parts are sufficiently lengthy and flexible that some
kind of insulating brace will probably be required to keep
winds from misaligning them. I recently built just such a 6 meter
J-pole that used a 102" whip to reduce weight and wind loading
on the top of the 1/2 wave radiator side. Even so the original plan
called for a pvc brace to keep the vertical tubes aligned. ( the
whip portion of the 1/2 wave radiator started about 7 inches
above the top of the 1/4 wave matching stub) Both the lower
portion of the radiator and all of the matching stub were of
3/4 inch copper pipe.

Harold
KD5SAK

If a 2 meter J-pole construction is imperfect enough that
the short and long vertical portions are not exactly parallel
it might require an insulated piece of something to force them
to be parallel.

For what it's worth - the "American Legion" aluminum-rod J-poles which
are quite popular in my area have just this sort of separator near the
top of the matching section. It seems likely to me that the junctions
between the rods, and the aluminum crosspiece which forms the bottom
of the J-pole are not rigid enough to keep the rods parallel over the
lifetime of the antenna, and that bracing is necessary.

These J-poles use a somewhat uncommon feed technique - the SO-259 is
attached near the center of the crosspiece and grounded there, and the
short side of the matching section is fed through a bent-wire
transmission line and gamma match.

http://www.pcarc.net/jpole.pdf shows the construction, including the
crossbrace.

I'm not sure just what they use for the cross-brace - the document
indicates acrylic. This doesn't seem to stand up to sunlight all that
well in the long term - if I see an American Legion J-pole which has
failed, it's usually due to the cross-piece breaking. I'd suggest
ABS, black Delrin, or some other plastic which is more UV-stable (or,
just coat clear or white acrylic with paint to shield it from UV).

--
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!

This is interesting, Dave.
They only give dimensions for the 2M.

I assume that the #12 wire wound around the long pole is the gamma
capacitor. I also wonder what the sun & time does to the insulation on this
wire.

In this version, the SO-239 is exposed to the WX as is the PL-259 screwed on
to it. In the more usual
"run-the-coax-up-to-the-feed-point-and-solder-or-clamp-it-on" method exposes
the coax end to the Wx - allowing seepage into the coax.

I am trying to figure out a good way to get the feed line completely
INTERNAL when using the "tap-the-coax-up-a-ways-on-the-matching-section"
feed method. This is so the feed line and the connector (if needed) is
not exposed to the Wx.

Sumptin' like this:
Drill a hole in the long pole, facing the short pole, where the feed line
will attach. However, if you try to solder the shield here and just extend
the center insulation and center conductor to the short pole, water can
enter the coax. Trying to put a SO239 or something smaller (and probably an
elbow version), inside the long pole at this point (to take advantage of its
"sealed" construction) is difficult. You could put a "thru joint" right
below that point to improve access for soldering such a connector -- that
may do it. Or you could use a pipe "T" with the lone opening facing the
short pole and do something in there to use an already sealed "something" to
go from the coax (inside the pipe) to the other side of the matching
section.
Any Ideas?



"Dave Platt" wrote in message
...
[...snipped...]
These J-poles use a somewhat uncommon feed technique - the SO-259 is
attached near the center of the crosspiece and grounded there, and the
short side of the matching section is fed through a bent-wire
transmission line and gamma match.

http://www.pcarc.net/jpole.pdf shows the construction, including the
crossbrace.
--
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!

In article ,
Steve Nosko wrote:

This is interesting, Dave.
They only give dimensions for the 2M.

I assume that the #12 wire wound around the long pole is the gamma
capacitor. I also wonder what the sun & time does to the insulation on this
wire.

Correct. The length and position of the wire form the usual
gamma-match transmission line out to the correct tap position. The
insulated wire wrapped around the pole forms the gamma match capacitor
and cancels out the inductance of the transmission-line stub.

The ones I've seen made by the Legion post all use commonly-available
insulated household wire (I think it's THNN insulation). This stuff
seems to be reasonably stable in sunlight, although not infinitely so.
Using the black variety might tend to prolong its life.

Rotting of the insulation on the exposed portions of the wire ought
not to matter. Only the insulation in the gamma-match capacitor area
is performing a functional task, and this has been shielded from
sunlight by the heatshrink tubing around it.

I've never seen one of these antennas fail due to deterioration of the
insulation. Their weak points seem to be the acrylic insulator, and
the set-screw arrangement used to fasten the rods to the base. The
commonest failure mode for these antennas seems to be a combination of
a deterioration of the insulator (sunlight) and a loosening of the
set-screw fastening (possibly due to metal creep, probably due to
mechanical stresses from the rods being wobbled back and forth by wind
pressure). Eventually, the insulator cracks and the rods flop around,
and your match and SWR to all to heck.

It ought to be possible to build a more robust antenna of this sort.
Sticking with aluminum, one might use one of the aluminum-soldering
kits on the market instead of (or in addition to) the set-screws. Or,
one could use brass rods and a brass crossplate, and ordinary plumbing
solder. Even a copper-pipe variety might be possible, although the
gamma match would probably have to be significantly redesigned to cope
with the larger diameter.

In this version, the SO-239 is exposed to the WX as is the PL-259 screwed on
to it. In the more usual
"run-the-coax-up-to-the-feed-point-and-solder-or-clamp-it-on" method exposes
the coax end to the Wx - allowing seepage into the coax.

Yup. On my Copper Cactus J-pole I found it necessary to glop the end
of the coax, and all of the wiring leading to the feedpoint
connections _very_ carefully with outdoor-rated black silicone
sealant. The first time I put it up, I missed a spot on the 440 coax,
and ended up with water in my coax and moss growing in the N connector
at the bottom (as Dave Barry says, "I Am Not Making This Up!").

I am trying to figure out a good way to get the feed line completely
INTERNAL when using the "tap-the-coax-up-a-ways-on-the-matching-section"
feed method. This is so the feed line and the connector (if needed) is
not exposed to the Wx.

http://www.antennex.com/hws/ws1002/sperrtof.pdf is one possibility.
In its essence, it's a J-pole with the matching section being
implemented coaxially rather than as a stub on one side, and the
attachments are internal. The insulating disk at the top ought to be
amenable to a complete waterproofing... a bit of epoxy would do the
trick.

I'm tempted to try making one, if I can figure out how to mill/lathe
the necessary discs.

Sumptin' like this:
Drill a hole in the long pole, facing the short pole, where the feed line
will attach. However, if you try to solder the shield here and just extend
the center insulation and center conductor to the short pole, water can
enter the coax. Trying to put a SO239 or something smaller (and probably an
elbow version), inside the long pole at this point (to take advantage of its
"sealed" construction) is difficult. You could put a "thru joint" right
below that point to improve access for soldering such a connector -- that
may do it. Or you could use a pipe "T" with the lone opening facing the
short pole and do something in there to use an already sealed "something" to
go from the coax (inside the pipe) to the other side of the matching
section.

Any Ideas?

Well, I've seen plans for J-poles which mount the socket on one pole
(combination of a screw, and solder), and then run a solid wire across
the other pole to make the feedpoint attachment. With UHF connectors
this is still potentially leaky. However, if you were to use an N
connector at this point, you'd end up with a connection which is far
easier to keep dry, thanks to the internal gaskets.

I use N connectors for all of my outdoor feedlines and antennas...
they're a bit more expensive than UHF, but I like 'em a lot better.

--
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!