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 ?

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.

This is nor open dipole ho need balun. I never use balun for open dipole and all working perfect...

Bton-FM wrote: ↑Sat Jul 25, 2020 6:52 pm 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.

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?

XXL 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 ?

Yea it's a 4:1 balun lamda half coax

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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

I just coil up 3 turns of the coax, about 10cm diameter and cable tie it. I can achieve an SWR of 1.1.

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 ?

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 ?

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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Electronically wrote:

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 ?

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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Instead of. 66 could use. 68 too.

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Electronically wrote:

Electronically wrote:

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 ?

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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Instead of. 66 could use. 68 too.

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So for 4:1 balun at say 100mhz you would do 300÷100×.66×.25×2= 0.99 meters

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Electronically wrote:

Electronically wrote:

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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Instead of. 66 could use. 68 too.

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So for 4:1 balun at say 100mhz you would do 300÷100×.66×.25×2= 0.99 meters

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You can still change. 66 to. 68 for 4:1 balun what ever method you prefer.

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Thanks I’ll give that a go. Am I right in thinking this is converting 50ohm to 200ohm ?

XXL wrote: ↑Mon Jul 27, 2020 4:31 pm Thanks I’ll give that a go. Am I right in thinking this is converting 50ohm to 200ohm ?

Yes but remember the feed-point will be higher.

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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XXL wrote: ↑Mon Jul 27, 2020 12:16 pm 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 ?

VELOCITY RATIO 66%

Electronically wrote: ↑Mon Jul 27, 2020 12:53 pm

Electronically wrote:

Electronically wrote:Instead of. 66 could use. 68 too.

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So for 4:1 balun at say 100mhz you would do 300÷100×.66×.25×2= 0.99 meters

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You can still change. 66 to. 68 for 4:1 balun what ever method you prefer.

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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 ?