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

[Marconi]

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?

DB, no I don't think you are wrong. My thinking is based on the attached models and overlay.

I do see a difference but as usual...I don't see much difference among the four models noted as an A99 with 4 horizontal radials.
1. with no mast at all,
2. a mast directly below the antenna is isolated by 4" inches from the antenna, 3. a mast offset from the antenna and set 2" inches away,
4. and another set 12" away beside the mounting bracket and between two radials, with a 0.50" inch head-room clearance below the radials.

I also included two images of the tabular currents magnitudes per segment to compare for the models with a mast.

EDIT: After Bob gave me a heads-up that my models did not show CMC, I went back and checked and I get similar results to what Henry posted. Right now I can't explain why my old EFHW with 4 72" or 108" radials showed no obvious CMC's, but it could be due to the radials.

Sorry Henry and DB, I should have looked before I leaped.

Thanks, Bob.

 

[Henry HPSD]

Hi DB !

Thanks for your reply.
Well...I like to "reconstruct" a real situation...and i see most vertical antennas are attached to the mast instead of "on" it.
Some antennas "depend" on it ....for that "C".

It would indeed be better to set it "on top"...preferably with some radials chokes etc
Im a big fan of that....That is better for the J pole as well.
Although i would have wanted the "main radiator" to "blend on top" of the mast...
For most people that provides an "issue" (antenna mast diameter)....
Ill have a look to see what it does if it is "precise on top" of the mast or perhaps u already did ?
(on beeper duty now...so that ill have to wait)

hmmm...thinking about that "unwanted"....Perhaps a point to investigate...:
We know radials can help...to reduce those unwanted currents.
Maybe...there is a "optimum" angle...for the V4K that, that cone...
Can do its job as matching device and still be able to reduce that what is unwanted ?

It would be very plausible if used "good ground" had to help out a friend a few days ago who was convinced "clay" was so much better then his "village" ground it was the reason for his -16dB difference each time. (it wasn't....height was the cause btw)
Perhaps its still on "clay"...Ill have a look, happy i already said i didn't look to close at those models lol....

Enjoy easter weekend !
Kind regards, H>

 

[bob85]

Eddie,
did you pick an optimal mast length for minimum cmc or are radials just doing a good job,

you did pick an antenna that has little current at its base, the opposite situation to the longer antennas, how does that effect what you are demonstrating,

 

[Marconi]

Eddie,
did you pick an optimal mast length for minimum cmc or are radials just doing a good job,

you did pick an antenna that has little current at its base, the opposite situation to the longer antennas, how does that effect what you are demonstrating,

No Bob, I just picked an old model of an A99. I just now noticed, that the model I picked for my demonstration was in free space(FS). I thought Henry mentioned and EFHW and that is why I picked that old A99 model. Henry also posted a pattern of a t2tl, but I didn't have such a model.

Honestly, Bob, I didn't even consider if my model was free or at a minimum with CMC. Obviously, I was not thinking...I was just focused on making a model that had the various configurations I used in the post and if I could see the effects of capacitance that DB mentioned. Also, I didn't consider to change the title designation that showed my A99 model was in FS - Free Space. I just have added a mast to all but one model and the results I got showed very little effects no matter how I configured the mast relative to the antenna in this case. How silly of me.

I will find a model that is showing us lots of CMC instead of looking at the capacitance effects that Steve mentioned...and maybe do this over so I can see the effects.

 

[Marconi]

Thanks for understanding Henry.

Bob, here is why I did not see the CMC you noted should be all over the mast for such a model like I used. Obviously Henry's model was not isolated and I just missed the fact in reading.

1. shows the old model I started with. It was in FS and I never noticed it.

2. shows the model where I added a mast that was ISO 4" inches from the antenna. There are not any CMC to be noted for this FS model. This model is included in my post above and shows the pattern with gain and angle for comparison.

3. shows model #2 with the mast connected directly to the antenna...no ISO. Here we see the CMC. So no, I did not pick a model that had radials that worked really well. What happened is the ISO always tends to cover up to some degree these very bad CMC...just like I think Henry has suggested. This is also why, long ago, you told us about isolating the mast, remember?

You've brainwashed me.

 

[HomerBB]

@Marconi
Something I thought you might like.

 

[The DB]

I think that the "poorly" and "good" mattes far less than the person who made that realizes. I should also note that the coax is the same on the "poorly" as it is on both "ideally" options. In fact, the only difference in the "poorly" and two "ideally" options is you are giving the cmc currents a different option than just flowing down the mast, options that are potentially equivalent to a full set of horizontal radials.

I do like the second "ideally" option (the one using a sleeve but a stub would also work here) for controlling cmc's on the mast. the fact that it is angled up compared to the other "ideally" option is also a bonus in my eyes. I have yet to model/test it such a system, but I am reasonably sure that it will be very effective way to do the job. More so than the other "ideally" option, which is little more than using several angled down radials, which is fine, but angled down radials don't block cmc's as well as horizontal radials, and may themselves induce currents to flow on the mast, which kind of defeats the point...

Also, in none of these cases does it show anything that is limiting to preventing currents from flowing on the feed line, so don't forget about a proper choke for the frequency of use as well.


The DB

 

[HomerBB]

In the first two his point was (in his opinion) that the shield is best connected to the 1/4^ stub side of the jpole and center to the longer side. The others were about cmc reduction. I think his point, valid or not, was that the basket style radials were equally effective as a sleeve... now where have we heard that before...

Thought the modelers might want a crack at the sleeve and radials things.

 

[Marconi]

Back in the days we use to use a form of this idea on our Stardusters.

http://www.comsistel.com/HAM documents/Good Analysis of a Balun .pdf

 

[Marconi]

DB, I posted a model of an A99 demonstrating the lack of difference with several install ideas on talking about what works and what does not work using Isolation (ISO) of a mast to ground.

See my post #31 and the PDF file attached: Marconi's1/2 wave J-Pole an alternative viewpoint

Maybe I could have considered using High Accuracy Ground type here instead of MiniNec in that situation for the A99 that had no mast. But, as I mentioned to you earlier, were it not for the warning message I get in Eznec...I had no clue as to the distinctions being made with Eznec/4NEC2 on "ground."

 

[The DB]

DB, I posted a model of an A99 demonstrating the lack of difference with several install ideas on talking about what works and what does not work using Isolation (ISO) of a mast to ground.

I once did some modeling examining choke locations and cmc's flowing on feed lines/masts, I think it was for Homerbb. It was far from complete, and I have gotten some new ideas to try since then, such as the 1/4 wavelength stub on the mast idea mentioned above, and shown in one form or another on this forum recently. I just haven't had the time lately, but it is something I intend to do at some point.


The DB

 

[Henry HPSD]

If of use...i remember some measurements....its not that hard to measure the amount of choke. And i was astonished (?) by the measurements provided. I always said to the "clients" tape it to the boom ...you will be fine.. only to find out..that is not the case (im a bit "wants best...oversecured...need to have that said hihi)
... but i know taping a choke to the boom will have influence. It can be that bad the the amount of choke is negligible...I know placing a choke around a boom will have influence...I know 1 turn (extra or less) does have a dramatic impact. A "optimised choke...(measured) will have 4..5 turns RG213 with 10cm diameter (2 inch) and a way from "objects"... that how it should be done. That way ....you will have "maximum" choke on 27 MHz.. Im not saying anytihing else wont work....im saying what is best...

 

[Marconi]

Henry, I've heard similar advice before, but here is a strange story I personally experienced with chokes. It makes me scratch my head and wonder.

Before I had to stop working with my real antennas, I put up my Sirio New Top One on a mount right outside of my shack. I had it installed at 27' feet to the hub.

Almost right away I found out this setup was producing TVI. So bad in fact that it would disconnect a TV in my family room that worked off as a remote, hard wired to a Comcast modem. I had to pull the power plug on the modem for it to reset.

I had never noticed any problems before with this particular antenna, but I always had it up about 40' before. I let it down for some bad weather.

Well, the story is I had the excess coax on the ground and one day I went out to secure the coax as the lawn guy was here to cut the grass. I rolled it up in a 12" roll as best and as net as possible, and hung the loop to a guy bracket on the PU pole and let it just hang there.

Later I discovered the TV worked fine while I worked my radio. I took the coil down and put in back on the ground after the lawn guy left...and the TVI was back again, worse than ever it seemed.

Two things I thought about.
1. I had it laying on the ground figuring that would help if I had RFI/TVI.
2. I strapped the choke, as it were, directly to the mast, a serious no-no as I understood at the time.

I've always had unpredictable results with chokes...and for the most part I never use one.

 

[HomerBB]

There's no two ways to sundown about it, Marconi, you're just contrary.

 

[Henry HPSD]

Very plausible marconi....Heck...just moving coax 1 meter left or right can have a hugh impact.

Thats what i said... the issues with "chokes"....
Read the last line from my previous post again

Im not saying anytihing else wont work....im saying what is best...