J-pole feed points.

[Otis413]

I have noticed recently that different websites on how to build J-pole antennas say to connect them in different ways. Some say to connect the coax center to the long side, others the short.
Does it matter?
I built one back last summer (I'v only had my license for about 10 months) with the coax center connected on the long side, seems to work good, low SWR anyway.

Otis
KD8VBV

 

[AK9R]

I have noticed recently that different websites on how to build J-pole antennas say to connect them in different ways. Some say to connect the coax center to the long side, others the short.
Does it matter?

No.

BTW, the railfan in me likes your avatar.

 

[Otis413]

Thanks!

No.

BTW, the railfan in me likes your avatar.

If you're ever in the area, drop by the shop, I'll give you the nickle tour for free!!

Otis, KD8VBV

Cass Scenic Railroad State Park

 

[nd5y]

It does make a difference.
See End-fed Vertical and J-pole

 

[Otis413]

So if I read that right, the SWR is not necessarily affected by which side the coax center is connected too, but the radiation pattern is, and is better if the center is attached to the long side?

 

[LtDoc]

I think you read it right. The radiation pattern is where the biggest change is, and that's because of a number of things (as mentioned). One alternative to any 'skew' in that radiation pattern is to add another 'stub' or two, or three to that 'J'-pole. Of course that also tends to change it's name to something like a 'skeleton sleeve' antenna, sort of. It would 'even-out' that pattern skewing, but there will still be some because of how/where the antenna is mounted (as also mentioned).
If you stop and think about it, SWR just isn't that big'a deal, it can always be dealt with/changed. Where an antenna puts a signal is a big deal, right? It's just a different way of looking at it and setting priorities. It's all relative. You can have a perfect SWR and a terrible antenna. Or a really good antenna and a terrible SWR. I know which I'd rather have...
- 'Doc

 

[nanZor]

So if I read that right, the SWR is not necessarily affected by which side the coax center is connected too, but the radiation pattern is, and is better if the center is attached to the long side?

Correct! Part of the radiation pattern problem with a J-pole is that there are multiple balance problems:

1) Unbalanced coax attached to a balanced quarter-wave transformer.
2) Balanced quarter-wave transformer attached to the very end of a half-wave radiator which at that point is an unbalanced connection.

Common-mode issues (the outside of the coax braid acting as part of the antenna) are what distorts this pattern, and is one reason you see most of them choked near the feedpoint. While not perfect, the center conductor of the coax should go to the wire of the balanced transformer that ends up connecting to the half-wave radiator (basically the long side).

The radio won't care which way you connect the coax, but if the coax center conductor is connected to the short side, the common-mode current in that configuration tends to make the radiation pattern point downwards.

For those with repeaters close by, or casual simplex operations this may not matter much, but most want to do it right. Still, the common-mode braid radiation issue is one reason you never see J-poles in the commercial world, unless perhaps the guy running the place is a ham and decides to put one into service anyway.

 

[zz0468]

Still, the common-mode braid radiation issue is one reason you never see J-poles in the commercial world, unless perhaps the guy running the place is a ham and decides to put one into service anyway.

The fact that these things aren't seen in commercial service should give one pause to wonder. They are very mediocre antennas, and about the only advantage I could see is the lack of horizontal radials for a counterpoise. I've never understood why these are so popular on the ham bands.

 

[AK9R]

I've never understood why these are so popular on the ham bands.

Because they are easy and inexpensive to build. Hams are traditionally tight with their money and I think some of them would rather build a mediocre antenna, thus giving them a sense of accomplishment, rather than buy a better antenna at a higher price.

You don't see them in commercial service because most of those system owners or operators are looking for an antenna with some real gain, which a J-pole does not have, and they have they money to spend on a "real" antenna.

 

[PrimeNumber]

The fact that these things aren't seen in commercial service should give one pause to wonder. They are very mediocre antennas, and about the only advantage I could see is the lack of horizontal radials for a counterpoise. I've never understood why these are so popular on the ham bands.

It's the easiest end-fed vertical antenna to build, and the easiest to find parts for. No loading coils or input transformers, you can get parts down at Radio Shack or the nearest hardware store.

But I do see all kinds of weird groundings on these things, usually at the bottom of the stub directly to the mast. That's got to make a major opportunity for ground loop noise.

 

[nanZor]

But I do see all kinds of weird groundings on these things, usually at the bottom of the stub directly to the mast. That's got to make a major opportunity for ground loop noise.

That's exactly right - the mast if conductive, is now part of the antenna and contributes further to the pattern distortion. One should try to choke the coax feedpoint, but some don't realize just how much ferrite material that takes to do much good, or wind an air-core coax rf-choke with the wrong dimensions, or so tight that they damage the coax internally - or the sun will do it slowly for them.

For many, this antenna, even with it's common-mode pattern-distortion problems, is better than a rubber duck usually, and that alone makes it a winner in most circles.

It is an adaptation of one of the earliest HF antennas, the zepp, where the halfwave radiator above the quarter-wave transformer was/is usually horizontal, and common-mode distortion wasn't too big of an issue for those not too concerned about a clean pattern.