J Pole Help !!
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- proppa neck!
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J Pole Help !!
Unfortunately i cant get hold of the guy who made these brilliant antennas, so im attempting to make one. Im sure this part circled in red is a balun but could someone correct me ? and what is it doing. How can i work out the correct length and/or velocity for my frequency ?
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- Bton-FM
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Re: J Pole Help !!
I'm not 100% sure but I think that is a 4:1 balun, transforming the impedance from 50 to 200 Ohms, so that the J-pole feedpoint can be higher up the pole.
- rigmo
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Re: J Pole Help !!
This is nor open dipole ho need balun. I never use balun for open dipole and all working perfect...
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- proppa neck!
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Re: J Pole Help !!
What would be the benefit of doing this ? Would it be so it effectively becomes slightly smaller because you could mount it from further up aswell?
- Electronically
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Re: RE: J Pole Help !!
Yea it's a 4:1 balun lamda half coaxXXL wrote:Unfortunately i cant get hold of the guy who made these brilliant antennas, so im attempting to make one. Im sure this part circled in red is a balun but could someone correct me ? and what is it doing. How can i work out the correct length and/or velocity for my frequency ?
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- yellowbeard
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Re: J Pole Help !!
Dat guy is on ebay and faecebook - or at least that is where the image is from:
https://www.ebay.co.uk/itm/4-Metre-J-Po ... 3430568937
https://www.facebook.com/Ceecom-Amateur ... 186974066/
The j-pole is a balanced antenna, to feed it with coax you need a balun to prevent RF coming back down the braid of the coax - this can be a Pawsey stub, a 4:1 coax balun (thet's what the picture shows) or a simple choke balun. Using the 4:1 balun does not change the physical length of the antenna, it changes how high up the coax is connected on the matching stub. Personally I have used the choke balun of 5 or 6 turns of coax 6 inches across cable tied up at the tail of the antenna - it's simplest and it works. More info is here - this is for the slim-jim type antenna, but the matching and feeding for this is in general the same as the j-pole.
http://www.hamuniverse.com/m0uxbslimjimnotes.html
https://www.ebay.co.uk/itm/4-Metre-J-Po ... 3430568937
https://www.facebook.com/Ceecom-Amateur ... 186974066/
The j-pole is a balanced antenna, to feed it with coax you need a balun to prevent RF coming back down the braid of the coax - this can be a Pawsey stub, a 4:1 coax balun (thet's what the picture shows) or a simple choke balun. Using the 4:1 balun does not change the physical length of the antenna, it changes how high up the coax is connected on the matching stub. Personally I have used the choke balun of 5 or 6 turns of coax 6 inches across cable tied up at the tail of the antenna - it's simplest and it works. More info is here - this is for the slim-jim type antenna, but the matching and feeding for this is in general the same as the j-pole.
http://www.hamuniverse.com/m0uxbslimjimnotes.html
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- Neckmin
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Re: J Pole Help !!
I just coil up 3 turns of the coax, about 10cm diameter and cable tie it. I can achieve an SWR of 1.1.
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- proppa neck!
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Re: J Pole Help !!
Im going to try the 4:1 Balun, but I cant find the velocity factor of RG213/U
Is there a calculator i can use to work out the coax length or is there some kind of equation I can use ?
Is there a calculator i can use to work out the coax length or is there some kind of equation I can use ?
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Re: RE: Re: J Pole Help !!
Just use this 300 divide by frequency x . 66x.25 then there's your answer.XXL wrote:Im going to try the 4:1 Balun, but I cant find the velocity factor of RG213/U
Is there a calculator i can use to work out the coax length or is there some kind of equation I can use ?
So for 100mhz you would do this
300÷100×.66×.25= 0.49meters
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- Electronically
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Re: RE: Re: RE: Re: J Pole Help !!
Instead of. 66 could use. 68 too.Electronically wrote:Just use this 300 divide by frequency x . 66x.25 then there's your answer.XXL wrote:Im going to try the 4:1 Balun, but I cant find the velocity factor of RG213/U
Is there a calculator i can use to work out the coax length or is there some kind of equation I can use ?
So for 100mhz you would do this
300÷100×.66×.25= 0.49meters
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Re: RE: Re: RE: Re: RE: Re: J Pole Help !!
So for 4:1 balun at say 100mhz you would do 300÷100×.66×.25×2= 0.99 metersElectronically wrote:Instead of. 66 could use. 68 too.Electronically wrote:Just use this 300 divide by frequency x . 66x.25 then there's your answer.XXL wrote:Im going to try the 4:1 Balun, but I cant find the velocity factor of RG213/U
Is there a calculator i can use to work out the coax length or is there some kind of equation I can use ?
So for 100mhz you would do this
300÷100×.66×.25= 0.49meters
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- Electronically
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Re: RE: Re: RE: Re: RE: Re: RE: Re: J Pole Help !!
You can still change. 66 to. 68 for 4:1 balun what ever method you prefer.Electronically wrote:So for 4:1 balun at say 100mhz you would do 300÷100×.66×.25×2= 0.99 metersElectronically wrote:Instead of. 66 could use. 68 too.Electronically wrote:Just use this 300 divide by frequency x . 66x.25 then there's your answer.
So for 100mhz you would do this
300÷100×.66×.25= 0.49meters
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Re: J Pole Help !!
Thanks I’ll give that a go. Am I right in thinking this is converting 50ohm to 200ohm ?
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- Electronically
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Re: J Pole Help !!
For something like a j pole antenna, that has a resonant impedance of about 200 ohms, you might use a 4:1 ratio balun and a 50 ohm feedline. In this case, the balun does both impedance matching and balanced to unbalanced conversion.
The reason you want to use a balun is that when coax is working right, all the currents are on the inside of the shield, and the outside of the shield stays at ground potential. If you connect the coax directly to a balanced antenna, then currents will flow on the outside of the coax and this will be a problem if you touch it or if it travels near other metal on its way to the shack. The feedline will also radiate and can cause interference with sensitive equipment it passes near. So a balun is a good thing, especially at high power levels.
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The reason you want to use a balun is that when coax is working right, all the currents are on the inside of the shield, and the outside of the shield stays at ground potential. If you connect the coax directly to a balanced antenna, then currents will flow on the outside of the coax and this will be a problem if you touch it or if it travels near other metal on its way to the shack. The feedline will also radiate and can cause interference with sensitive equipment it passes near. So a balun is a good thing, especially at high power levels.
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- rigmo
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Re: RE: Re: RE: Re: RE: Re: RE: Re: J Pole Help !!
Im baffled as to why youve done the same calculation but then times it by 2 ? is that so you have a half wave length instead of a 1/4 wave length ?Electronically wrote: ↑Mon Jul 27, 2020 12:53 pmYou can still change. 66 to. 68 for 4:1 balun what ever method you prefer.Electronically wrote:So for 4:1 balun at say 100mhz you would do 300÷100×.66×.25×2= 0.99 metersElectronically wrote:Instead of. 66 could use. 68 too.
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