J POLE Question

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GDog
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J POLE Question

Post by GDog »

Hi All,
So having made myself a 2M 70cm plantpot antenna (link below)

I was quite pleased with its performance. Actually, I was more than pleased, I was impressed!

Always wanting to try new stuff, I went ahead and built a Copper Pipe J Pole Antenna. (link below)

I'm disappointed with its performance.

My question is this.... I have copper pipe below the J, should this be removed? I used this to mount to the mast and made sure it was insulated by sitting it inside some PVC pipe. Would the length of copper pipe below the J need to be adjusted? Some pictures I see online show some copper pipe below the J. Some dont.

I have since read from a post somewhere that J Pole antennas are not that great. I must admit, I am starting to think that myself.
I was under the false impression that copper pipe would be better than 'wire'. Have I built it incorrectly?

So to be clear, where the pipes 'join' below that should have no effect ? As the antenna is shorted?

Gregg
wa10
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Re: J POLE Question

Post by wa10 »

Anything connected to a j-pole below the stub effects its performance,

J-poles are notorious for common mode current on mast & coax which spoils the radiation pattern,

they are sensitive to mast & coax & ground wire length or the resulting common mode impedance of those conductors due to their electrical length

some people get lucky & have reasonable performance while others don't & have poor performance,

The J-pole in the video has the coax center connected to the short leg & coax braid connected to the radiator which makes cmc even worse,
it also has the feed-point tapped up the 1/4wave matching stub which also makes cmc worse, in short its a poor way to build a J-pole, about the worst configuration you could choose,

its easier to build than a bottom fed J-pole & allows some leeway in getting the stub spacing correct,

J-poles really need radials & to be isolated from a conductive mast and a good coax choke to help minimise cmc on the coax braid in order to get decent performance using random length masts coax or ground wire if you have one,

a starduster style groundplane antenna mounted at the same tip height will usually out perform a j-pole,

have a look at Tom Rauch's informative article on J-poles https://www.w8ji.com/end-fed_vertical.htm
W8JI
"An antenna with a poor ground using few radials cannot have a support mast grounded to the radial common point (at least it shouldn't if designed properly) There is no exception to this!
wa10
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Re: J POLE Question

Post by wa10 »

I just looked at the video,
He claims a slim jim has more gain & lower takeoff angle, that's an often parroted claim, don't fall for that old chestnut,

slim Jim's have about the same gain & angle as a regular j-pole & use more tube that increases wind load, they are a little shorter but its a waste of copper tube,

I don't think Fred Judd the inventor made the crazy claims you here from some folk,
minimising common mode makes much more difference than any small difference between the two designs,

That's also true of most antennas like end-fed verticals.
W8JI
"An antenna with a poor ground using few radials cannot have a support mast grounded to the radial common point (at least it shouldn't if designed properly) There is no exception to this!
GDog
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Re: J POLE Question

Post by GDog »

Hello again!
Looking at that article, it looks like I wasted my money on copper pipe :D Well maybe not...
Ok so the J-Pole is coming down this next weekend . Looks like far too much messing to really get this right.

Im going to make another, more accurate Plant-Pot antenna and put that up instead.
On a previous thread you may remember I mentioned to you that I was going for the foundation course, which I have now passed.
I am just waiting for my call sign, and really looking forward to getting out on 2m 70cm.
I was hoping the copper J pipe would be 'amazing' :D 2m 70cm. oh well never mind.

Just a thought here but...
Do you think its possible to use copper pipe instead of wire/coax on the 'plant pot' antenna to increase performance (largely keeping the same design - tube, balun and foil) ?
So I would basically get a larger diameter (20mm ish) to cover the 15mm pipe I already have, in a way this would be like the ground / shield of the coax. I can probably use something for spacers to keep one pipe central inside the other, maybe even good old electrical tape wrapped around the inner pipe. Or possible another sleeve of copper pipe instead of foil...

Or I will keep the copper as a ground plane for another build in the future, it only cost a few pounds but I hate to waste it.

Thanks for your help !
wa10
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Re: J POLE Question

Post by wa10 »

Congratulations on passing GDog :clap:

The only antenna i have seen but never made that you could be talking about is the flowerpot dual band antenna,
its a t2lt in a plastic tube that they put a coaxial phasing section over the feedpoint, they claim it makes it 2 x halfwave end-fed on the upper band,

having never built one or looked in any detail at them i can't answer your questions with any certainty,

sure you can make a copper version of a t2lt using pipe for the upper & lower 1/4wave & pass the coax up the lower pipe to make a sleeve dipole but the dimensions will be different to the coax version,
you could try making one for 2mtrs & if you get it working with low vswr then experiment with adding a sleeve over the feed-point,

if you get dimensions & spacing right it should ( may) work as they claim, far from everything you will read about antennas on the www is true,

LB cebik, Walt Maxwell & W8JI are good sources of info,

I like how you are imagining ways of making your own antennas from stuff you have at your disposal, That's amateur radio :thumbup:
W8JI
"An antenna with a poor ground using few radials cannot have a support mast grounded to the radial common point (at least it shouldn't if designed properly) There is no exception to this!
GDog
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Re: J POLE Question

Post by GDog »

Hi again.
So I spent a couple of weekends now messing around with antenna homebrew.
I went out, bought more stuff, drilled antenna mast into outside wall etc.
I tried the copper pipe, and while I am sure others have had great success with it I feel that my location doesn't help.

I've given up, gone out and purchased a 2.5 meter dual band antenna off the shelf.

To be fair the antenna builds I made taught me a little bit more about what makes a good homebrew antenna and completely put to bed the JPole ideas.
GDog
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Re: J POLE Question

Post by GDog »

Just to add the homebrew was covering about fifteen miles but I was barely making it. Still on 5 watts that's not too shabby.

Just for the mounting cost of copper pipe, coax, plastic pipe etc it was easier to get an SQBM500P and cut my cost losses.

Hopefully that will get me over the rooftops.
rpcomms
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Re: J POLE Question

Post by rpcomms »

The reasons it can be variable results,based on my controlled tests,samples worked flawlessly once fixes applied,a lot of mis-information out there in radio world about J poles I found personally.

1.Its a balanced antenna feed point.coax is unbalanced 50 ohms.
2.Lack of isolation between metal mast Ariel fixing point.
3.Lack of height of mast-ground affect can alter Ariel performance of J pole for Line of sight use.
4.Lack of ground planes to help reduce common mode and angle pattern even at UHF.
5.Gain is about 2.5-3dB in "real world"conditions.

1.Use a rf ground isolator transformer (binocular ferrite HF\VHF\UHF grade) 2 turns primary- 2 turns secondary=
50 ohms:50ohms,use a short flylead close to matching point.
pick the right ferrite grade for intended frequency of operation
2.Use fibre glass pole or a section of fibre glass pole to insulate Ariel from a metal mast
3.Mount it 50-60ft plus from ground or more for best results,if using metal mast ensure it is earthed for electrical safety (not performance purposes)and best line of sight use.
4 Add 3 or 4 1/4 ground planes horizontal from the bottom end of J pole where it shorts out.
5.Accept this will give about 1/2 a S point over a 1/4 wave ground plane mounted at same height,BUT being a DC shorted Ariel,its much lower noise and less chance of noise pick up
6.Due to the matching stub compared to a end fed 1/2 wave with a lossy transformer,J appears to be more efficient .

Hints
I would also advise the use of double shielded coax with foil,seems to resolve a host of issues including loss at VHF\UHF.RFI pick up from local QRM sources.


that should fix a host of issues and get better consistent results,give it a go!

Rob P
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bigpimp347
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Re: J POLE Question

Post by bigpimp347 »

Avanti J-poles work good..
i mean Sigma iv's
I want to Die Asleep like my Grandad did,
Unlike his Passengers, Screaming and Shouting.!
Alan Pilot
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Re: J POLE Question

Post by Alan Pilot »

GDog
How do you find the SQBM500P performs.
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