DIY J-poles

[KR4NIV]

I made two j-poles yesterday. Pretty easy to build and tune.
I used this j-pole slim jim calculator:
Slim Jim and J Pole calculator – M0UKD – Amateur Radio Blog
As soon as weather permits, I'll get these up on the mast and see how they perform.

2 meter



70 cm

 

[w2lie]

I remember building a dual band copper J-Pole as a kid with my dad. Great project and helped me learn some basic plumbing techniques as well as how to tune an antenna at a young age. Seeing your project takes me back.

J-Poles are great antennas.
Somewhere I also have a home brew twin lead antenna that we built too.

 

[AK9R]

Very nice. Now make one for 20m.

 

[EAFrizzle]

You got a plane?

 

[KR4NIV]

Very nice. Now make one for 20m.

Sure why not. It would only be 15.2 meters long!! 🤣

 

[mrweather]

I made one for 6m once. It wasn't too long. Had to use 3/4" copper pipe.

 

[KR4NIV]

I made one for 6m once. It wasn't too long. Had to use 3/4" copper pipe.

I thought about that but, made a 6m dipole instead. Used 1/2 copper pipe.

 

[Ubbe]

The j-pole are a balanced antenna and the coax are unbalanced so you will need a balun to properly connect to it.

The coax outer braid will otherwise be hot with RF and in many cases needs to be decoupled by winding up the coax in several turns as close as possible to the antenna and that will short circuit the RF to prevent the coax to be part of the antenna but will also loose half of the signal level.

/Ubbe

 

[KR4NIV]

The j-pole are a balanced antenna and the coax are unbalanced so you will need a balun to properly connect to it.

The coax outer braid will otherwise be hot with RF and in many cases needs to be decoupled by winding up the coax in several turns as close as possible to the antenna and that will short circuit the RF to prevent the coax to be part of the antenna but will also loose half of the signal level.

/Ubbe

Thanks for that info. I plan on following this instruction on that calculator page:

"It is recommended to use some sort of choke at the feedpoint. Three turns (for 145MHz) of the coaxial cable around a 40mm former (PVC pipe etc), or taped up and hung freely is adequate. Five turns, 6cm diameter for 70MHz. I have also used a clip on ferrite or two for VHF. As with any balanced feed antenna, this will help prevent the braid of the coaxial cable from radiating, and becoming part of the antenna, and therefore affecting SWR and performance. You can check the effectiveness of the choke by touching the coax below the choke and if the SWR changes significantly, your choke is inadequate."

 

[AK9R]

J-poles are an end-fed half-wave antenna. The feed point, being at the end, has a high impedance. The parallel elements at the end constitute a transformer, or matching stub, to convert the 50 ohm impedance of the feedline to the high impedance of the antenna. J-poles do not need a balun because they already have one. J-poles can benefit from common mode chokes to keep RF off of the feedline.

 

[EAFrizzle]

Speaking of J-poles, I miss cheap 300z twin-lead being available everywhere. With the right kite or balloon, HF J-poles ought to be doable. 😆

 

[W4AXW]

I thought about that but, made a 6m dipole instead. Used 1/2 copper pipe.

Much better choice.....

 

[Ubbe]

J-poles do not need a balun because they already have one. J-poles can benefit from common mode chokes to keep RF off of the feedline.

So you are telling me that where the arrow points are a "cold" point carrying no RF and can be used as a ground by the coax? So why could it benefit from a common mode choke?



/Ubbe

 

[Ubbe]

If I google j-pole and only filter on pictures I can see that perhaps 75% have the coax braid connected to the longer 3/4 wave element and the rest has it connected to the 1/4 wave element. Either will work as the two elements works in opposite phases. The end stub design only works as an impedance match and not as a balun. You can see on its design principal that both elements are "hot" and carries the signal but in opposite phase and will require a transformer balun to make use of the whole signal level.

/Ubbe

 

[nd5y]

If J-poles are so good then why do only hams use them? Why is it that you never see them on any commercial or government tower?

 

[KR4NIV]

If J-poles are so good then why do only hams use them? Why is it that you never see them on any commercial or government tower?

Maybe because Hams like to build stuff. These are pretty easy to throw together. It's a thing in every hobby I guess.

You can see on its design principal that both elements are "hot" and carries the signal but in opposite phase and will require a transformer balun to make use of the whole signal level. /Ubbe

Interesting point. Time for more research and learning. Antennas!

 

[popnokick]

J-poles are very sensitive to stray RF fields and influence from nearby metal objects, including feed lines. This makes them sensitive to other nearby antennas and PIM (Passive Intermod) found on tower locations. They also are often themselves emitters of stray RF from their feed lines. All of these characteristics may not be a problem for ham radio home stations, but make J-poles inappropriate for tower and other professional installations.

 

[mrweather]

Not to mention j-poles have no gain over an equivalent dipole. Most commercial antennas (4-bay, etc.) have much more gain.

One advantage they do have is they're DC grounded.

 

[KR4NIV]

This is good info.

 

[W4AXW]

Maybe because Hams like to build stuff

Indeed some amateurs like to build stuff, especially antennae. It might be my favorite aspect of the hobby, but I try to build efficient radiators with minimal reactance and CMC issues. Unfortunately J-poles don't qualify.....