Base Marconi's1/2 wave J-Pole an alternative viewpoint

[The DB]

I like how you built this antenna model, including the tapped stub. You even matched it, although that went out the window in your second model and its common mode currents.

I can confirm that j-pole antennas tend to have a lot of CMC's, this is both from modeling and experience. A lot of people simply throw them up and assume they are fine. Most of the directions I have seen for them don't include any form of choke. If anyone decides to make one, you should be aware of this.

Good to see how critical things can be.
Perhaps u realise...but just to have it said... isolated doesnt always mean no wanted or unwanted currents...

Imagine a yagi... with isolated elements..
That thing works... ...just a couple metal pieces isolated from each other....
The same is true for other antennas.
(That influence while isolated)

It is possible to "isolated" the j pole from the mast while the mast will still have "currents" and skewing.. keep up the good work..
Kind regards H.

This is true, however, the isolated elements of a yagi are in the primary field of the antenna, while the isolated mast in the j-pole model above is not. Their will be orders of magnitude of difference in induced current in these two situations. In one case you have elements that are designed to react to the RF field of the antenna, and in the other you have an isolated element that is designed to isolate RF currents as much as possible. Sure currents will still flow on the mast in Marconi's isolated mast model above, but their will be so little current flowing on it compared to the elements on the Yagi antenna, Their simply won't be enough to make much of a difference.


The DB

 

[Henry HPSD]

Sure...it was an example to visualize DB.
Many find it hard to understand how something isolated can still hold currents.

And yes.... while the 'difference' between the two are large...the "impact" of it can still be of large influence perhaps we diagree on that.

And i would state...
It still is in the "primairy" field ...but has a much different effect.
But hey...thats "words"...i know what u mean..and oke with it !

The thing debatable would be :
1 what is large..
2 at which situation...

A never ending story..

@ marconi..
Why did you quote me in your start ?
I fail to see what your "yellow" part has to contribute... could u clarify ?

Kind regards,

 

[Marconi]

I like how you built this antenna model, including the tapped stub. You even matched it, although that went out the window in your second model and its common mode currents.

Stever, I recall recently telling Bob the story of the source for this J-Pole model. If I can find the words...I'll post a link.

It's true the match went into the weeds on removing the choke from the mast that was directly connected to the antenna. This was my point in posting the model. My hope was to show the good and bad side of the J-Pole. instead of folks just hearing words about how bad the J-Pole is and never a word for "why."

There is other evidence, from Sirio, that supports the merits of the J-Pole design...but the old stories seem to still hang on. I was just offering a different viewpoint and reminding folks...when I got into the issue of why there was ever a battle for the truth on this issue of the J-Pole relative to the S4/NV4KK.

 

[Marconi]

I can confirm that j-pole antennas tend to have a lot of CMC's, this is both from modeling and experience. A lot of people simply throw them up and assume they are fine. Most of the directions I have seen for them don't include any form of choke. If anyone decides to make one, you should be aware of this.

Here are some other cogent comments for why the J-Pole gets a bad wrap...due to limited information on solutions. And instead, just believing the old hype.

Thanks Steve.

 

[Marconi]

@ marconi..
Why did you quote me in your start ?
I fail to see what your "yellow" part has to contribute... could u clarify ?

I just got lazy and didn't ask for permission as I looked for references that supported my my conclusions for the J-Pole. I guess you missed my point, but my idea was to help show the J-Pole can be effective if one just takes due consideration instead of making a bad installation.

If I could, I would remove it.

 

[Henry HPSD]

Owh no ...dont worry...Im more than happy...and you dont need to ask permission...its on the internet ...do with it what you want. I was just wondering.
You are spot on that some antennas can perform "oke" in one situation and in another "its terrible' ..I think i have made those remarks in that sigma 4 article as well.

Kind regards, H>

 

[Marconi]

Henry, I also have a series of models that were planned to post later and they were intended to show the "oke" and 'it's terrible" aspects similar to your suggestion. But I don't want to be accused of spoiling someone's imagination.

BTW, I didn't use those same words in "qoutes" above.

 

[Henry HPSD]

For me....it is not of interest if you did or did not use the same words....neither the option of being accused of something...

It only fills up loads of "comment space".

What is of interest...
Is to show that what you are saying is very valid, and you can back it up with examples. And discuss those (if needed) and If people see those examples perhaps they will "test" en draw a conclusion

 

[Marconi]

Henry and possibly a few others, I would appreciate not being preached-at about what I post and say when I recount some history as I recall it. If you don't like some of my comments...you can just ignore what you don't like and discuss with me what you find of interest in my posts here.

Does that sound fair?

Here are two models of an antenna I have compared to the J-Pole that were isolated vs. no isolation on a NV4K model. I notice quite different results when the mast is attached or not on the J-Pole at the beginning of this thread and compared to the NV4K below.

 

[HomerBB]

Amazing difference between the non isolated and isolated versions. Better over all radiation pattern and more gain on the one isolated from the mast.

 

[Henry HPSD]

@ Marconi,
I really had a large reply here... ( i typed for 30 minutes and it was a bit "hard" every now and then..but honest...)

Reading back...
Im not sure if it contributes, or would provide the outcome i hoped for...
Instead...please allow me to say..


Sure marconi...
Ill Try.... Im hoping you will try to minimize your "things" as im a bit on facts and have difficulties reading or understanding all the besides remarks. ( i really do...very irritating)
(perhaps that is a foundation for some confusion every now and then)

H>

 

[Marconi]

Fine with me brother. If I slip up...just let me know. I cherish the opportunity to be your friend. This old thing between us is forgotten.

 

[Marconi]

Amazing difference between the non isolated and isolated versions. Better over all radiation pattern and more gain on the one isolated from the mast.

I was amazed too.

 

[Henry HPSD]

Their simply won't be enough to make much of a difference.

Oke...what about a simple EFHW antenna roughly a half wave above ground...
Blue is without a mast....black is with a mast and its isolated from the antenna. I would say that is a difference .....

Or the t2tl this time black is without a mast...again...its isolated... the mast is 10cm away from the antenna ...ehm..4 inches and they "overlap" 4 inches (you need to attach the antenna to it)



And i didnt try to make things "worse case".... just a fast impression...I didnt verify the models...but im fairly confident it is of "influence" Could you agree DB ?

 

[The DB]

4 inches and they "overlap" 4 inches

Actually, I was referring to no overlap. I have installed my share of a99/imax antennas using 2 foot long fiberglass rods as a separator in the last few years (their are previous posts on this forum where I talk about this). This put the antenna a few inches directly above the mast, and this is specifically what I was thinking about when I made that statement. Mounting these antennas in a way that they overlap the mast is simply creating a capacitor between the mounting bracket and mast, which, to me, is not the same as isolating the antenna from the mast. Using something like a fiberglass rod to elevate the mounting bracket above the mast would be far better at minimizing said capacitance, and thus better at "isolating" said antenna from the mast (even if its not complete isolation electrically it is very close). Am I wrong?

And t2lt antennas, in my experience most are hung in a tree or on an extendable fiberglass pole, so in those cases their will generally be no overlap, and no metal mast.

In your models, what ground quality are you using? It appears that it is a better ground than I typically use...


The DB