Any Astro Plane Fans ?

[Marconi]

I eliminated most of my local TVI/RFI by using 1.25'' diameter solid fiberglass rod to isolate the mast 105" down from the mounting bracket on my Avanti AP, since the mast is the 1/4 wave counterpoise for this folded 3/4 wave GP. I used 5.5 turns of the coax on a 4" diameter former for the cmc choke around the fiberglass.
Remember, you must have a 50 ohm load at this direct-driven feed point, requiring it be either a 1/4 or 3/4 wave antenna. It must also produce maximum current and minimum voltage at the mounting bracket point of contact or it would short the high voltage node to ground, a very bad idea.
Where the bracket meets the longer side of the radiator is the center of the top 1/2 wave of this folded 3/4 wave, with the bottom end fed 1/4 wave folded 180 degrees back into radiating phase with the top 1/2 wave.
The high voltage node is in the loop around the counterpoise, requiring the extra clearance.

Needle Bender, I think you're right, we will also need a choke of some kind at the feed point. I guess if we lived in the country were nobody was within a mile or so...CMC's would not be a problem, and sometimes CMC's can be constructive. But, if they follow the feedline...that might lead to other problems.

I find that just laying the feed line on the ground can help some with my local issues in the shack. I also think if we're lucky with a feed line length...we might also be able to mitigate such currents enough so as to be of less problem.

Personally I see this antenna being similar to a 1/2 wave J-Pole, and other than some CMC due to a lack of full cancellation...most of these bad currents do not radiate into the far field and thus the feeder stub and the mast work against each other to match and feed the end of a center fed 1/2 wave dipole.

If you look close at the pattern in my model of the A/P you will see the minor skewing caused by the little off-set in the design similar to what we see in the J-Pole.

Thanks for your comments.

 

[bob85]

will you model that eddie, avanti say shortening the mast to less than 1/2wave will cause takeoff angle to tilt upwards,
if it won't it makes isolating easier with less leverage on the isolator, we don't want newton getting mad with us,

im thinking it could cause more current on the mast inside the antenna as the end impedance of the 1/4wave mast would be low compared to the 1/2wave mast,

what does EZNEC say eddie, is a shorter mast more avanti technobull?

 

[Needle Bender]

That was many years ago, and the first time I tried a cmc choke. I saw no degradation in signal nor performance when I went to the isolated 1/4 wave mast, and I lived for the local DX. The farther - the better, without using F layer skip of course.

Marconi, a J-pole is grounded at the base and is 1/2 wave long above the lower 1/4 wave matching stub.
I believe any "skewing" of the AP pattern to be due to the counterpoise effect of the mast radiating opposing currents out the 2 sides. You'll notice they designed a beam utilizing the AP as the driven element, and mounted it so the opposing side currents would assist the side rejection... not a bunch of dummies there

 

[HomerBB]

I have never seen any noticeable CMC issues with my AP, however, the person who obtained my original from me could not run it because of this and resultant TVI. He was parked in a KOA campground.

I merely mounted it not lower than 25' high (usually higher) and ran a coax choke a 1/4 wavelength below the ring.

I told him how to mount it but he stubbornly refused to do it putting his only 8' above his RV roof from the ring.

 

[Marconi]

will you model that eddie, avanti say shortening the mast to less than 1/2wave will cause takeoff angle to tilt upwards,
if it won't it makes isolating easier with less leverage on the isolator, we don't want newton getting mad with us,

im thinking it could cause more current on the mast inside the antenna as the end impedance of the 1/4wave mast would be low compared to the 1/2wave mast,

what does EZNEC say eddie, is a shorter mast more avanti technobull?

Bob, IMHO when Avanti was talking about raising the RF angle...I think they were suggesting: if the bottom of the antenna was closer than 4' feet from the ground or whatever was immediately below the antenna...we would start to see the antenna TOA begin to raise up, again as we got closer.

I base this on the models below. I also did the model at 1' foot above the ground...and the angle raised up another 1* degree going closer. So, this issue had to do only when close to the antenna and the Avanti claim looks to be true.

Bob, in case you were thinking about other longer lengths for the mast at the bottom. The 89" inch mark is right at the hoop, so I would go at least 4' feet and isolated there and follow the Avanti instructions.

If you look close at the red currents mark for the regular grounded model...you will note this current starts 0r terminates into the mast...right near this 4' foot point in the model. I guess the Avanti engineers knew a thing or two back then...in spite of their tendencies to puff in their promotions.

 

[bob85]

that's not how i read what avanti say eddie but i could be wrong,
i think we talked about this in the past,

the whole point of the astroplane is taking advantage of the height restriction you guys had back then,

the test antenna they used was scaled for vhf, i can't imagine anybody would test antennas that low to the ground,

i imagine the mast length and it been grounded or not will effect antenna or common mode end impedance at the hub and effect mast currents,
differential or transmission-line mode currents within the antenna are a different matter and effected by tube diameters and spacing from the mast, that's how it looks to me.

do you think i should stop using EZBOB and try to learn EZNEC lol.

 

[Marconi]

that's not how i read what avanti say eddie but i could be wrong,
i think we talked about this in the past,

I think the warning was suggesting a 4' foot minimum for anything below the antenna as noted on page #8, item #10, and it says clearly that nothing of substance should be within 4' feet below the bottom hoop.

I also see the issue clearly noted in the models I posted for raising the TOA.

the whole point of the astroplane is taking advantage of the height restriction you guys had back then,

I have never tested the idea except by modeling, and you are right...we talked about this before.

the test antenna they used was scaled for vhf, i can't imagine anybody would test antennas that low to the ground,

Maybe that had more to do with requiring a limited space for testing.

i imagine the mast length and it been grounded or not will effect antenna or common mode end impedance at the hub and effect mast currents,
differential or transmission-line mode currents within the antenna are a different matter and effected by tube diameters and spacing from the mast, that's how it looks to me.

I added that matching information to my second attached hand written notes, and I did not see the match (impedance) change much....on making the mast one way or another. I should have added the matching info for the issue Eznec finds when we start to get stuff closer to the antenna than the 4' foot recommendation. See the 48", 24", models I posted earlier again. I added the matching for each and I also included the model 1' foot high that I forgot earlier. Here we clearly see how this small range ill-effects the matching in a dramatic way that we don't see for lengths beyond the 4' foot minimum.

I agree that the lack of symmetry between these currents is probably due in part to the difference in element diameters and spacing used. I've tested the idea with modeling, but I don't recall if I saw what was going on or not. Like most technical results we read about...these guys are dealing with miniscule differences that most hobbyists would likely never consider.

I've tested similar ideas in real life also, when making the bazooka balun for my Starduster...and you and I have talked about that in the past too. In my thinking about CMC's, I tend to consider the issue as a necessary evil sometimes...and about all we can do about it is to try and mitigate the problem as best we can...and sometimes that is just a little nudge we can get using a choke. That said however, I find this to be a very narrow solution.

do you think i should stop using EZBOB and try to learn EZNEC lol.

Nope, EZBob is the standard for you Bob...and without that we would probably be calling you No Comment Bob. EZBob is here to stay, but Eznec would also fit in nicely with your ideas...me thinks.

Be sure and recheck the attached file below.

 

[bob85]

they look bad eddie, cloudwarmers at that height above ground,

where are you looking at page 8, the patent is 7 pages,
i don't see anything that makes me think they are talking about been near the ground,, i recall they mention the hoop should be no less that 4ft above the tower/tripod ect,

i would probably agree if you can put the antenna at a height typical of where they are intended to be used and show that isolating at different distances below the hoop say 1/4wave 4ft & 2ft to simulate different length masts on your chimney with the whole thing well above ground.

thanks for the models.

 

[Marconi]

Sorry Bob, the owner's manual.

I'll get back later.

 

[bob85]

ahh i was looking in the wrong place eddie,
i don't see the 4ft above the top of the tower meaning the same thing as putting the antenna very close to the ground with only 4ft or 2ft of mast below the hoop,
i don't dispute what you say when you do that, the gain and angle at those heights are crap,

imho if we hope to see the claimed benefit of the astroplane it must be mounted in a typical scenario with its tip including the hat equal in height to whatever 1/2wave or 5/8wave you are comparing it to in order to place the current maxima above that of the other antennas,

if you don't do that i see no real advantage over other antennas other than it does not have a cheap and nasty little coil or circuit board matching transformer at the feed-point,

remember the astro was marketed back when you guys had a 60ft to tip or 20ft above the highest point of your property restriction,
the way it reads to me is all claims are based directly on that restriction,
that does not mean its true.

 

[Marconi]

they look bad eddie, cloudwarmers at that height above ground,

where are you looking at page 8, the patent is 7 pages,
i don't see anything that makes me think they are talking about been near the ground,, i recall they mention the hoop should be no less that 4ft above the tower/tripod ect,

i would probably agree if you can put the antenna at a height typical of where they are intended to be used and show that isolating at different distances below the hoop say 1/4wave 4ft & 2ft to simulate different length masts on your chimney with the whole thing well above ground.

thanks for the models.

I did what you suggest Bob, but I was feeling bad and could not concentrate. I'll have to get back on this, because I want to do more trying to understand my thinking.

When you use to talk about tuning and steering the TOA, I was interested, but I did not see how that was likely. As I see things go with modeling, I still don't think it is likely, but I do agree if we sit the A/P close to the ground, less that 4' feet the Eznec models do start to show the match going to heck in a hand basket and it is very obvious. IMO, raising the antenna is the only steering we are likely to do unless we change the antenna construction.

If Martino and Blaese were able to determine that back then using the same science as we use today...then they were right. The only difference is the way they said it one might get the idea that we could steer the TOA to anything desired simply by changing the mast length out the bottom of the hoop. I just don't believe that is the way it works. If these models don't demonstrate this happening like I describe...then sobeit.

I did as you suggested, but I have not published the work yet. I simply do not understand all the nuances on raising the model higher, and then placing something reflective at, near, and less than 4' feet from the hoop. I think I've shown you that the claim that the TOA is affected by the length of the mast inside of the radials, but that looks to be only true if the antenna is near the ground.

What we have always heard about the A/P is that it works best...close to the ground. Now all we have to do is find the guy that claims this and ask him to explain why he thinks this idea is the way it is.

What I see so far tells me that you are correct...this phenomena does not work the same when we raise the antenna up high. Personally, I can't recall that ever being something that I thought about. I do remember setting out one day to prove that the A/P would work up high...and I've been saying that since. So, I haven't changed my mind on that.

My model shows that if you don't do something (choke) minimal to the feed line with your idea of isolation...the currents will just flow on the coax. It looks like a set of 12" radials at the point of isolation, on the mast antenna side will help eliminate most of the passive currents possible across the insulator, but don't hold me to this...I have not yet tried to fully digest this yet.

I think I also see that when you isolate the mast at this magic point of 4' feet below the hoop...this mast shows a boat load of currents on the antenna side, and I don't see and equal amout of out of phase currents to settle that down...so who knows how bad this antenna might act with CMC due to this lack of symmetry. This increase in currents in the out of phase side of the bottom look to have decreased the currents in the top radiator portion also...and that might not be so good. I also added a model at 36' feet to show how the currents on the mast increase when the length of feedline is 36' feet from feed point to ground as a unlucky length idea and how it might work. For me I think 32' feet of coax might be a lucky length or its multiples.

Below are my results so far.

I've already said too much, but I've been thinking about it since I got to feeling better.

 

[bob85]

eddie,
i think if im looking at the models right its showing the effect i talked of years ago, currents on the mast robbing current from the upper 1/4wave,
altering the length of isolated mast would alter the antenna or common mode impedance of the mast changing antenna or common mode current on the mast,

the differential or transmission-line mode currents should not be effected much imho,

when the patent talked about shortening the mast this is what i had in my minds eye as possibly causing the upwards beam tilt they claimed was noted,

the highest impedance/least current should imho be when the mast is electrically 1/2wave or multiples thereof and not connected to ground,
or odd 1/4wave multiples if the mast is connected to ground,

isolating the mast less than 1/4wave below the hoop would cause a lower mast antenna or common mode impedance robbing some current from the upper 1/4wave,

if this idea has any merit then isolating the mast and feed-line about 1/4wave below the hoop would be the best solution from a mechanical & electrical point of view keeping Kirchhoff and Newton happy,

when i say mast i ALWAYS mean mast and/or feed-line as they are the same thing as far as common mode currents are concerned,

i think you need to keep the height above ground constant and just alter the isolation point so any change in the pattern is not due to changing height above ground,

thanks for your efforts in trying to prove it one way or the other.

 

[bob85]

eddie,
i just noticed you have the source in the wrong place, it should be at the top of the opposite leg.

 

[Marconi]

I admit, I was going in circles trying to understand what I was seeing...and when that happens with modeling...it is time to stop for a while.

I think you're on the right track here Bob, and that is why I told you never to drop EzBob...even if you do decide to work with Eznec or some other software. I find it challenging and informative...even though Grampa has already lost it.

Not to change the subject and in this regard, this is why a while back I was trying to get The DB to cooperate with me, him using 4Nec2 and me using Eznec on the Sigma 4. I wanted to see if we could get really close on results...I wanted to confirm first, but I really wanted to consider getting-off into that product...since it is more robust and allows far more segments. My S4 model is right now testing the limits of 500 segments using Eznec, and I would like to test and see if more segments would get some improvement in the model. But, he was off on some tangent he did not want to reveal...and he got pissed because I was asking questions and making statements about what he was saying and doing. Maybe I approached that effort badly, but I had good intentions.

Back to this project on the A/P. I do not understand in my minds eye, like you do on this stuff...if I just depend on words to draw me a picture. I know you fuss at me for that failing, but that is the way I am. You probably see this stuff in your minds eye, and it all makes total sense to you. I don't, and I have to reason my way into all this stuff, and that is a bit more difficult and even worse trying to put words to ideas.

I agree with you about needing some mast below the hoop where you want to isolate. I also consider that Needle Bender tells us he got good results isolating the mast at the bottom of the hoop. His point is well above the sweet spot, for lack of a better word, but I see noticeably less currents flowing on the mast with isolation rather than no isolation, and I also see this sweet spot looks to be right at the 4' foot level at 27 mhz, so I would consider that area too. I guess the spot will have to be tested, but first you have to see the need and see if I'm right about this antenna having the potential to be really bad. Homer has confirmed this idea from the guy that bought his original A/P, so maybe this bugger is bad in this regard.

Bob, in my modeling so far, I don't see mitigating the CMC's issue to be productive in performance, but it could make your neighbors happy if your idea works.

If you are worried about the lack of support in you windy area...then I've recently seen an idea that might have merit in adding strength to an isolation area. I think it was a new German antenna.

I know you understand the idea of isolation and that relationship with the feed line Bob, but other readers may miss that distinction so I think it needs to be discussed, and that is why I mentioned it.

I will also try your suggestion later today if I'm feeling better.

eddie,
i just noticed you have the source in the wrong place, it should be at the top of the opposite leg.

Bob, I wrestled with the source location, back and forth, for days on this antenna. I realize the physical FP is attached to the top leg on the other side, but the model works better if I put it near the center of the dipole. This is also why I tend to refer to this antenna as a center fed dipole rather than an EF dipole. The Average Gain test also shows better results with the FP on the base of the radiator.

I will check out your issues later, about isolation with regards to putting the feed point on the leg like you've noted here. You are pretty sharp to pickup on that Bob, I doubt many of the modeling pros on the forum would have noticed that.

 

[HomerBB]

A Question:

On your models with the source attached to the center of the long radiator, is the top of the short side isolated from the top bracket or fully connected?

On the real antenna the short side is isolated from the bracket, The center coax conductor feeds the isolated leg, the coax shield attaches to the bracket and shorted to each other by way of the route through the hoop and both legs.

Would this make a difference in the models?

Feeding the long radiator side leads me to think the model may be producing results for a off-center fed dipole with cap hats on both ends.