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Author Topic: J-pole feeding question  (Read 4222 times)
W8JI
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« Reply #15 on: August 23, 2011, 11:22:46 AM »

Understand there are different levels to antenna design.

The basic level is making the user happy......... and part of that is SWR.

Because something doesn't work as claimed, it doesn't mean it can't be satisfying. Very few things won't work at all, but many things don't work as claimed.

Let me give an example. I can work Europe on 160 meters SSB from my mobile. My 160 antenna is less than 1% efficient, and is the equivalent of a 20 meter vertical just one foot tall!!!  Not only is it far more difficult to work a long distance on 160, I am doing it with a terrible antenna.

This shows how bad an antenna can be an make someone happy.

:-)

So it is useful to learn how things work, but not to ever think because they don't work well or as planned they won't make us happy.

That is really very important to remember.

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WB0KSL
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« Reply #16 on: August 23, 2011, 11:37:56 AM »

That is possibly the wisest comment I have seen on eHam yet.  Life itself is a series of compromises.  we just have to recognize it when it happens.  As my wife has been known to say, "Grow where you're planted."  I know I can't have what I want in the way of antennas where I live now, but I'm trying to do the best I can with what I have to work with.

Again, great comment, Tom.

73 de John - WB0KSL
 
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AD4U
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« Reply #17 on: August 23, 2011, 12:03:00 PM »

Two Meter J Pole Daffy-nission:

J Pole ....... A 50 inch long (+/-), very complicated, expensive to build, hard to construct, difficult to feed, almost impossible to match, antenna that works almost as well as a simple 19 inch ground plane.

Dick  AD4U
« Last Edit: August 24, 2011, 05:10:49 AM by AD4U » Logged
K0BG
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« Reply #18 on: August 23, 2011, 01:14:09 PM »

Dick I agree. You can build a simple vertical out of an SO239, and 5 pieces of building wire, that will out perform a J-pole. The problem is, it isn't exotic! It is off-center fed, so I guess that is why it is as popular as the so-called OFC windom!
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W8JI
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« Reply #19 on: August 23, 2011, 06:55:36 PM »

But you guys forget...the goals with a J-pole almost always are:

1.) work better than an rubber duck

2.) have a low SWR

3.) very easy to build and handle


It isn't how it compares to other the same or better antennas....it is how it compares to worse antennas and how easy it is that sells them.



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WB6BYU
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« Reply #20 on: August 23, 2011, 08:59:52 PM »

But it can't be TOO simple to build, or it won't seem like you are putting enough effort into the antenna.

The ground plane is easier to build than a "copper cactus" J-pole, but it doesn't take a whole afternoon
and give you an excuse to fire up the propane torch.  After all, you have to drink your beer pretty fast
to have an excuse to get a second one before you finish a ground plane.  It just isn't same antenna
construction experience...
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K8AXW
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« Reply #21 on: August 23, 2011, 09:18:56 PM »

These last few comments made me laugh out loud!  Reason is because after building my first J-Pole, doing a bang-up job on it and then looking at it for a couple weeks, I concluded this piece of pipe crap can't work as an antenna.  I threw it in the garbage without ever hooking it up.  Was that a mistake!  The second on is still up and does a fine job.
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HAPLO
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« Reply #22 on: August 23, 2011, 10:25:27 PM »

But it can't be TOO simple to build, or it won't seem like you are putting enough effort into the antenna.

The ground plane is easier to build than a "copper cactus" J-pole, but it doesn't take a whole afternoon
and give you an excuse to fire up the propane torch.  After all, you have to drink your beer pretty fast
to have an excuse to get a second one before you finish a ground plane.  It just isn't same antenna
construction experience...

The Super-J was a fun thing to build. I need to buy a mount for it yet, and I'm thinking of getting http://www.ronard.com/ychim.html to mount it to my chimney... I chose to build it because I had 40+ feet of 5/8 copper lying around my house, so other than some odds and ends, I had the material to do the job. I'm open to other suggestions for mounting it on my roof though.

H.
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KC9TNH
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« Reply #23 on: August 24, 2011, 04:31:14 AM »

But you guys forget...the goals with a J-pole almost always are:

1.) work better than an rubber duck

2.) have a low SWR

3.) very easy to build and handle


It isn't how it compares to other the same or better antennas....it is how it compares to worse antennas and how easy it is that sells them.
Bingo.  Grin I built the roll-up because it provides (perhaps) an option in a ziploc that goes in the field bag with other stuff that may not meet others' red-flanneled criteria, although it meets mine. Full tests still to be done. Didn't have a 1<-->2 beer stopwatch on me; time spent took most of a mug of coffee, applying the old "measure twice (or 3x) cut once" advice. I got a 30' stock of twin-lead for pennies from a long-established mom & pop TV/appliance store on the main drag of my little burg.
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
K8AXW
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« Reply #24 on: August 24, 2011, 09:07:54 AM »

Bingo.  Grin I built the roll-up because it provides (perhaps) an option in a ziploc that goes in the field bag with other stuff that may not meet others' red-flanneled criteria, although it meets mine. Full tests still to be done. Didn't have a 1<-->2 beer stopwatch on me; time spent took most of a mug of coffee, applying the old "measure twice (or 3x) cut once" advice. I got a 30' stock of twin-lead for pennies from a long-established mom & pop TV/appliance store on the main drag of my little burg.
[/quote]

THIS is ham radio at it's finest!
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WB6BYU
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« Reply #25 on: August 24, 2011, 09:31:10 AM »

Quote from: KC9TNH
Bingo.  Grin I built the roll-up because it provides (perhaps) an option in a ziploc that goes in the field bag with other stuff...

Yes, and they are great for that.  I have a couple that I use the same way - a quick rope over a tree
branch and you can get by on low power where the rubber ducky was marginal at 5 watts.


To answer your earlier question on repeatability:  there are a number of designs floating around, and
most are pretty close.  One of the biggest variables is the exact characteristics of the twinlead, which
will vary from one manufacturer to the next, and in some cases from one end of a roll to the other.
As a result, you have to expect to do a bit of tweaking, though I've had some that came out close
enough without it.

One of the biggest problems, however, is that most designs really don't give good advice on how to
adjust the antenna when needed.  The radiator (the length above the notch) is not subject to the
standard transmission line velocity factor, so that length probably doesn't need to change.  If you
trim the top of the stub at the notch then you are simultaneously shortening the stub AND lengthening
the radiator.  My preference is to adjust the length of the matching stub for resonance, and the
location of the coax feedpoint along the stub for SWR at resonance.  With most designs the length
of the stub is not conveniently adjustable, since the twinlead is cut and shorted at that point.

So I'd recommend building it to spec and trying it out first.  If the SWR at the minimum isn't as good
as you'd like, try moving the coax tap point up and down the stub a bit.  If the frequency of minimum
SWR is not where you'd like it, try shortening the stub a bit (which may also require readjusting the
tap point.)

When I'm building my own rather than following an existing set of plans (like my first J-pole that
I built for 15m) the twinlead can get rather battered with all this experimentation.  I've resorted to
using some straight pins to punch through the insulation for the coax tap point and the short across
the end of the stub.  Then I cut the twinlead a bit long and cut the notch for the radiator at about the
right distance down from the top to form the radiator.  (The radiator wires can optionally be shorted or
open at either end, or the unused wire can be removed entirely - it really doesn't matter.)  Then I make
a guess for the stub length and coax tap point, stick the pins through to contact the wires inside the
insulation, and proceed with experimenting.  If you have plenty of twinlead and want something more
tidy, build a second one using the dimensions you found from the first one.
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KC9TNH
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« Reply #26 on: August 24, 2011, 09:52:56 AM »

WB6BYU (RE whole post above) Thanks very much for all that; also didn't occur to me that there might be the variance you describe with the twin-lead within a given roll.

I'll play with this one and if satisfied it can be a jig for a less-mangled one if needed.
Plenty of winter up here to do stuff like that.
Thanks again!
Smiley
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
HAPLO
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Posts: 54




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« Reply #27 on: October 03, 2011, 07:43:45 AM »

It can make a big difference in pattern because the impedances of the short section and long section are not exactly equal, and neither are currents on the shield and center.

http://www.w8ji.com/end-fed_vertical.htm

The pattern sensitivity to shield and center connections occurs because the J-pole is an unbalanced half-wave element fed with a balanced quarter wave stub section fed by unbalanced coax. It won't stop the antenna (and coax) from radiating, but it does make the antenna sensitive the feedline and any mast length, feedline and any mast grounding, and the shield and center connections to the stub.

73 Tom

I read your webpage mentioned above regarding shield vs center for feeding the J-Pole. Does the same hold true for a J-Pole with an extra 1/2wave radiator, such as this: http://www.n7qvc.com/amateur_radio/copper.html which is the one I built. The author of the article seems fairly confident in his feeding, which is backwards from your website's article. I don't have a way to test the pattern, nor do I know enough yet to model this particular antenna to know if the feed point needs to be reversed.

Thanks for your time,
H.
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N4CR
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« Reply #28 on: October 03, 2011, 09:06:28 AM »

I don't have a way to test the pattern, nor do I know enough yet to model this particular antenna to know if the feed point needs to be reversed.

Making the antenna collinear doesn't change the need to feed it correctly. W8JI has it right. Your author has it wrong.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
WB6BYU
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Posts: 13038




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« Reply #29 on: October 03, 2011, 09:44:22 AM »

Quote from: HAPLO
...Does the same hold true for a J-Pole with an extra 1/2wave radiator, such as this: http://www.n7qvc.com/amateur_radio/copper.html which is the one I built...


Tom's analysis is for a directly-fed radiator, not one with a shorted matching stub.  (The extension to the
radiator isn't a factor in this discussion.)  I wouldn't be surprised, however, if there were a similar effect,
as it is due to the relative phase of the current on the mast / coax shield relative to that in the radiator.

But looking at your copper pipe model, notice that the coax shield is connected an inch or two up from the
bottom of the stub, at the same height as the center conductor tap point.  The mast is connected to one
side of the bottom of the stub.  Because there is voltage between the center and shield to drive the
antenna, about half of that voltage drop will appear between the coax shield and the bottom of the stub.
As a result, the coax shield and the mast are not at the same RF potential.  (It looks like a dead short
from a DC perspective, but it isn't at VHF.)  That means that either the mast or the coax shield (or more
likely BOTH) is NOT at ground potential, and RF currents will flow on them based on their lengths and
what they are connected to.  This is what makes J-poles more unpredictable in behavior.
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