Did you mean a New Top One model?
I just went back to pg1 and realized that I missed reading posts 11 - 40!
No NB, we've been discussing the A/P since Homer has a home made A/P. I have work to do on the New Top One.
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Difference in AstroPlane vs. New Top One per Eznec5
- Thread starter Marconi
- Start date
No NB, we've been discussing the A/P since Homer has a home made A/P. I have work to do on the New Top One.
Yep, I caught that and fixed it while you were posting I think. Thanks for watching close and watching this old man's back my friend.
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i tend to agree the astro is 1/2wave,
i think the lower 1/4wave radiates in the same manner as the sigma lower 1/4wave or the short leg of a j-pole but inverted making the astroplane a no radial endfed hat loaded 1/2wave,
heres how i imagine it works untill i learn otherwise,
the distance around the loop formed by the radials & hoop is 5/8wave so antenna mode impedance is high with little antenna mode current flow,
in parallel with the antenna mode impedance is the impedance of the transmissionline formed by the mast/radials/coax which is the high end impedance inverted through the 1/4wave transmissionline,
avanti tell us the 50ohm impedance is determined by the spacing/flare of the radials to mast so we have low 50ohm feedpoint impedance high transmissionline mode current,
looking at the open sleeve article and turning it upside down, when the mast is 1/2wave electrical ( 1/4wave below the hoop ) mast impedance is high so little current flows on the mast below the hoop, when mast length is odd multiples of 1/4wave, mast impedance is low so significant current flows on the mast,
avanti claim removing the flare in the radials causes downtilt of the pattern below the horizon and that the chosen flare gave a takeoff about 4 degrees above the horizon,
they also note that when the mast is shortened to less than 1/4wave below the hoop takeoff angle rises,
i imagine a similar scenario to other antennas, mast currents been determined by mast length and it been grounded or not,
the article eddie ordered told us avanti engineers admitted they did not understand exactly how the astroplane/astrobeam worked.
i think the lower 1/4wave radiates in the same manner as the sigma lower 1/4wave or the short leg of a j-pole but inverted making the astroplane a no radial endfed hat loaded 1/2wave,
heres how i imagine it works untill i learn otherwise,
the distance around the loop formed by the radials & hoop is 5/8wave so antenna mode impedance is high with little antenna mode current flow,
in parallel with the antenna mode impedance is the impedance of the transmissionline formed by the mast/radials/coax which is the high end impedance inverted through the 1/4wave transmissionline,
avanti tell us the 50ohm impedance is determined by the spacing/flare of the radials to mast so we have low 50ohm feedpoint impedance high transmissionline mode current,
looking at the open sleeve article and turning it upside down, when the mast is 1/2wave electrical ( 1/4wave below the hoop ) mast impedance is high so little current flows on the mast below the hoop, when mast length is odd multiples of 1/4wave, mast impedance is low so significant current flows on the mast,
avanti claim removing the flare in the radials causes downtilt of the pattern below the horizon and that the chosen flare gave a takeoff about 4 degrees above the horizon,
they also note that when the mast is shortened to less than 1/4wave below the hoop takeoff angle rises,
i imagine a similar scenario to other antennas, mast currents been determined by mast length and it been grounded or not,
the article eddie ordered told us avanti engineers admitted they did not understand exactly how the astroplane/astrobeam worked.
"Hat loaded"? - does that mean you're getting a big head? :tongue:
My vote is for a lot simpler explanation: A folded 3/4 wave operating as a capacity hat topped 1/2 wave.
I imagine that removing 1/2 the ring, or one side, would not change the antenna tuning or performance more than negligibly. I believe the minimal current at that point is simply splitting and taking two identical paths around the mast/counterpoise.
i think your over analyzing
Avanti already had the 3/4sigma4 and other big 5/8ths waves out there. Their intent was to get a small antenna up to the 20' max rule that would allow the city dweller with limited space to maximize signal "where nearby buildings block signals from ordinary antennas"
1/2wave electrically with 5/8nomenclature(only in name because of history of calling an 1/2wave end fed so)
^^ ab v c^^
Avanti already had the 3/4sigma4 and other big 5/8ths waves out there. Their intent was to get a small antenna up to the 20' max rule that would allow the city dweller with limited space to maximize signal "where nearby buildings block signals from ordinary antennas"
1/2wave electrically with 5/8nomenclature(only in name because of history of calling an 1/2wave end fed so)
^^ ab v c^^
"Hat loaded"? - does that mean you're getting a big head? :tongue:
My vote is for a lot simpler explanation: A folded 3/4 wave operating as a capacity hat topped 1/2 wave.
I imagine that removing 1/2 the ring, or one side, would not change the antenna tuning or performance more than negligibly. I believe the minimal current at that point is simply splitting and taking two identical paths around the mast/counterpoise.
NB, I'm not your best source for circuits or currents, but this is my best thinking, right or wrong, based on what I imagine these models are indicating in the attached tabular current reports. These reports indicate the current magnitude, and the phase degree with sign (+ -) of phase for each segment in each wire.
I go by the understanding that in-phase current between wires occurs when the signs are equal, and out-of-phase where the currents are with an opposite sign, similar to what we see with coax being opposite and not radiating vs. a half wave radiator showing equal currents and radiating.
NB, the problem with using only 1/2 the loop would be: the whole loop creates a very high impedance area to help choke currents on the mast below the hoop. More importantly the full loop establishes a phase reversal for the left side radial, opposite the feed point, and a shift to an out-of-phase condition with the mast and/or the feeder radial under the feed point. Thus there is cancellation in this area, but I'm not sure if cancellation is between the two radials, leaving the mast in-phase with the top element and radiating, or the left radial and the mast out-of-phase and not radiating, leaving the right radial and the top element in-phase, and radiating. Whatever controls how and where these out-of-phase currents responds...may depend on the magnitude of those currents including the degree.
I've posted models about currents before, but me thinks the subject doesn't seem to be of interest to most.
Here is what I see regarding the phase of the wires and this is what the model's current data shows with an even multiple model 6 x 1/4 wave, and the another odd multiple model 5 x 1/4 wave.
I'm not sure if I'm right, because I think this disagrees with what Bob tells us on the subject, but maybe not. I just used the same model of my A/P over real earth at these two specific heights in order to show the effects of currents on the mast as an example for the likely importance of mast height, the same thing Bob mentions.
I added the gain and angle patterns in order to see the results of such effects. This is a lot of detail, but I also added the current logs so you can see the magnitude and phase for every segment in every wire in the model for yourself without trying to decipher the red lines. Even so, I think the red lines are close to showing the true nature of the currents. I also noted the (+ -) on the antenna view by hand beside the segments with the sign of phase results. This is the basis for my thinking and if it wasn't for this log and modeling, I wouldn't even speculate on the subject of currents and phase.
View attachment AstroPlane model currents.pdf
Note that there is very little current flowing on the mast for the odd multiple 1/4 wave model and much current flowing on the even multiple model.
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nb, where do you see 3/4wave folded?, i have heared this idea before but never seen an explanation of where the 3/4wave is hiding,
if you go with a simple explanation ignoring transmissionline mode currents where do the antenna mode currents flow?
you could try feeding the antenna without the mast/feedline to test avanti's claim that the 50ohm comes from the transmissionline mode impedance.
if you go with a simple explanation ignoring transmissionline mode currents where do the antenna mode currents flow?
you could try feeding the antenna without the mast/feedline to test avanti's claim that the 50ohm comes from the transmissionline mode impedance.
The mast in my models are always grounded unless I talk about it not being grounded and then I usually isolate at the antenna not the ground.
In order to show an antenna with a tall mast, large enough to see detail, the ground symbol at the base of the mast is generally below the bottom of the image.
Maybe I misunderstood you are the idea, and that is why I bother to try and show the point I'm trying to make, right or wrong.
Did you get a better handle on what I'm trying to show with the current log?
You know, besides location and among others, this factor may help explain why were hear so many differences in results with the same antennas. Until I read W8JI talking about worst case scenario's, I never gave currents much thought.
BTW, I removed the coax choke I had at the 9' below the ring/hoop placement with straight coax. The analyzer detected no difference in is readings for SWR/R=/X=
Just a note.
i understand what you are showing us with the mast currents eddie,
im not sure what the + & - signs are meant to indicate since i see current on the outside of the radials in phase with the upper 1/4wave like a 1/2wave,
does a dipole have the same + & - either side of a dipole feedpoint in your current logs?
what happens to the astroplane when the mast/coax are removed from the model?
im not sure what the + & - signs are meant to indicate since i see current on the outside of the radials in phase with the upper 1/4wave like a 1/2wave,
does a dipole have the same + & - either side of a dipole feedpoint in your current logs?
what happens to the astroplane when the mast/coax are removed from the model?
Being a 'no-matching network' direct-fed aerial, I suppose it would need to be 50 ohms at the coax feed, making that point max positive current (beginning of 1st 1/4 wave) radiating down from that point to minimum current/max voltage at the point 1/2 way 'round the ring (beginning of 2nd 1/4 wave) on around the 2nd half of the ring back up with inverted negative current (providing in-phase positive radiating current) going max current/min voltage at the point of the beginning of the top hat loaded 1/4 wave (beginning of 3rd 1/4 wave).
The point at which the radiator is grounded to the mounting plate, at the base of the top cap hat loaded 1/4 wave (#3 1/4 wave) would have to be minimum voltage/max current.
Simply by removing 1/2 the ring, the performance should reveal it to be one or the other(s).
That's my best guess anyway.
That is good to know Homer. You were reading my mind. I was just about to look up where exactly you put your choke below the hoop, seriously. I was going to isolate the mast in my A/P model at the same point, and see what difference it made in my model that is working pretty good now that I've adjusted some mistakes and omissions. BTW, I think I'm about to give up on the idea of isolating the mast on the A/P, based on what I see happening to the match. However, there may hopefully be another way to improve stopping the RF on the mast/feed line.
Because of your testing noted above, and to confirm my suspicions that isolation is not the way to go, I think I realize now that my idea that isolation, using an insulator, would respond similar to the affects of using a feed line choke, maybe no-way-no. Now, with your information, I believe common sense would suggest that is wrong or it needs more consideration. Sorry for any confusion here, there is a distinction in there...that I'm really trying to make, believe me.
The feed line choke should be transparent to the system, but my models show that isolation affects the match, just like adding a mast to a model affects the match. So, I figure in the real world the same might be true. I'm just going to have to learn how to model a choke. I think it is possible with Eznec and maybe even effective at doing what is commonly believed.
I've just been looking at an old 2007, thread to refresh my memory of what has previously been said on the A/P subject. Look here, if you haven't seen it, you will be interested I think: http://www.worldwidedx.com/cb-antennas/26370-new-style-astro-plane.html
I'm testing some of the wisdom suggested in this thread. Based on my model and real world experience with the A/P, I believe some of it might have been off the mark a little bit.
I also have an old thread from 2010, noted below that is not too long back, but I can't find the thread still on the forum, and that is too bad. I found the older thread noted above, back in 2007 sometime, but this more recent one entitled "AstroPlane up and Testing" is not to be found. You will find this one interesting also. This guy, Starduster, came on for a while and could model, and was building a really nice and durable A/P styled antenna, and was experimenting on the best design. He is the one that really got me interested in learning to model, but he disappeared and I hated that. He was one of the few that didn't just give us words. Like you, he really did some work and showed us. Check it out, he was doing some of the very same things that you just did with your A/P, and that is remarkable. Too bad it's such small print, and ignore my notes. I have no idea what most of that means any more. If you can read his bandwidth charts, you will note there are frequency mistakes in his list. I just fix them in my mind.
View attachment Starduster and his AstroPlane.pdf
I just got through remodeling my working model of the A/P with a full 1/4 wave tip, and I'm in the process of considering how it responds...considering what has been said about it in the attachments above. More to come.