J-POLE Question

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barrylewis
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J-POLE Question

Post by barrylewis » Sat May 19, 2018 3:12 pm

Hi all,

Im building a J pole out of copper tubing. I uderstand how the j pole works, matching section at bottom and the halfwave radiator above it etc etc

Does anybody know if this antenna needs to be isolated from the mast or can the mounting section (the bit below the 1/4 wave matching section) be grounded/earthed on the mast? im wondering how I can isolate this if need be.

And also im using 22mm thick copper tube and have a 48mm spacing between them. I need a 48mm spacing between poles as im using a Jpole aerial connector I found on ebay, therefore cannot change that measurment. The J pole calculators give different measurments for the spacing, is it important?

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yellowbeard
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Re: J-POLE Question

Post by yellowbeard » Sat May 19, 2018 8:08 pm

They used to say you could connect it to the mast back in the day, but with the advent of computer modelling the current thinking is to isolate it and let it float. Its a debate - beware internet debates!

Code: Select all

http://www.hamradio.me/antennas/j-pole-antenna-should-i-ground-it.html
I have always connected them to the mast and it always worked for me. The most important thing is to use a choke balun of about 8 turns of coax, 8 inches diameter just below the antenna stub, this stops the outer of the coax from radiating.

Albert H
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Re: J-POLE Question

Post by Albert H » Sun May 20, 2018 1:34 am

The choke balun really helps. I had a pile of ferrite tubes, and used to use those for my balun (actually an Un-Un, but you know what I mean). The radiator can be earthed. Ignore the ranting nutters on the ham sites - most of them couldn't match into a 50Ω dummy load! I've improvised J-poles by using just a ¼-wave stub clamped to a metal pole (scaffolding could be temporarily used). My connections to the metalwork was either with alligator clips (for a very temporary lash-up) or with Jubilee clips for something a bit more permanent. I've loaded into scaffolding, part of a tower crane, electricity pylons, and almost anything that's reasonably tall and conductive. I've even used the lightning conductor with an earthed strip running down the side of a building!
"Why is my rig humming?"
"Because it doesn't know the words!"
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nrgkits.nz
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Re: J-POLE Question

Post by nrgkits.nz » Sun May 20, 2018 1:38 am

Connecting it to the mast is the preferred way, then you have short to ground at DC which is very useful when struck by lightning. Yellowbeard is also correct with the 8 turns of coax, although I've gotten away with 3 turns and still had a good SWR. I use a VF of 0.97 when calculating the length of my pipes, I used Aluminium of approx the same diameter. Once you've set the distance between the pipes, temporarily attach the feed point with 8 turns of coax bundled up and then move it up and down to find the point with best SWR, this will be the point at 50ohm. You can also make a 4:1 balun out of coax and then you won't need the 8 turns anymore but the impedance is transformed to 300ohm and so the feedpoint on the antenna is going to be slightly higher above the point where it's 50ohm. Moving all the way up the top of the short stub the impedance is very high, and some very serious RF voltages can develop there.

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radionortheast
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Re: J-POLE Question

Post by radionortheast » Thu May 24, 2018 9:34 am

barrylewis wrote:
Sat May 19, 2018 3:12 pm
And also im using 22mm thick copper tube and have a 48mm spacing between them. I need a 48mm spacing between poles as im using a Jpole aerial connector I found on ebay, therefore cannot change that measurment. The J pole calculators give different measurments for the spacing, is it important?
they’ll have to be apart if their not they’ll act like a coaxial cable, i’ve made smaller versions of a jim, but difference been its connected directly at the bottom, (2 separate parts, just like a jim but without a loop at the bottom) use a wire for the shorter peace, the distance between the 2 is really critical. i can tape afew cms of the wire to the long peace at the top and bottom part, the rest of the wire has to be away by 5 or 6 cms, to stop like i say it becoming coxial. Where its taped the distance is away depends on the lenght of the main element..I have one now made from 2 rods, the spacing on that is just 2 cms so 4.8 should be more than enough

i’ve messed with folded dipoles i’m pretty sure the boom is connected to the aerial going to the mast, problaly the reason they can say their lighting protected,
a silm jim much the same as both the ground and signal are connected to the same wire or mental tubing or whatever it is, so in thoery it might work you might beable to connect the bottom of the aerial to the mast.

barrylewis
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Re: J-POLE Question

Post by barrylewis » Sat May 26, 2018 12:37 am

Cheers people. The build is complete and worked really well. Ill grab some photos in next few days. Soldering the copper was a bit of an art, i rivited the copper elbows in place before i fired up the blow torch.
I just did 5 turns in the coax as I couldnt add a 1:4 balun due to the moulded so239 plug in the jpole connector, I used this here if anyone interested:

https://www.ebay.co.uk/itm/DIY-J-POLE-A ... 2659591355

I tapped the holes on this fitting with a 4mm tap so i could add bolts to easily adjust the positioning, once im happy with the position i will remove the m4 bolts and rivit it onto the copper. I just need to mount onto a high pole now and check the swr again. Im getting around 1.2 swr at my frequency.

barrylewis
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Re: J-POLE Question

Post by barrylewis » Sat May 26, 2018 10:35 am

radionortheast wrote:
Thu May 24, 2018 9:34 am
i’ve messed with folded dipoles i’m pretty sure the boom is connected to the aerial going to the mast, problaly the reason they can say their lighting protected,
A folded dipole (or any dipole) is 0 ohms at the centre hence why you can connect the folded side of a dipole to the boom/mounting arm and earth/ground it which in theory will help with lightning and static.

mgb
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Re: J-POLE Question

Post by mgb » Sat May 26, 2018 8:13 pm

I have a 1.1 SWR without a balun and my coax don't radiate.

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