ASTROPLANE best vertical antenna ever?

[Marconi]

Thanks Bob.

I don't fully understand these concepts, but I believe a little light went off in my thinking, and it was based on your words above and the video, which helped in the image department. I might've had dyslexia from childhood.

Now, at least I have some image ideas in mind for the transmission mode currents you've been talking about.

 

[bob85]

Eddie,
I hope it gives you something to think about,

you could try this, it may help you understand where im coming from,

take your resonant astroplane model with 1/2wave mast & 1/4wave upper monopole,
look at the feedpoint impedance,

then without messing with the hoop pull the legs in towards the mast as if you were sliding the legs inwards on the fiberglass spreader,

if theres significant transmission-line mode current as i think there is you should see feedpoint impedance go down & resonant frequency go up,

pull the legs outwards away from the mast you should see impedance go up & resonant frequency go down,

how is current & resonance going up and down like that when you have not changed the physical length of any conductor,

imho you are manipulating the characteristic impedance of the 1/4wave lines which increases or decreases
ZT transmission-line mode impedance and IT transmission-line mode current ,

ZT is seen in parallel with ZA antenna mode impedance,
at the feedpoint.

 

[Marconi]

use a 1/4wave upper monopole,
use isolated 1/2wave mast,
choke the coax at the end of the mast 1/4wave below the hoop,
make the space at the hoop as wide as practical for maximum unbalance while still maintaing the correct transmissionline impedance for low vswr by flaring the lines.

what would i avoid,
i would avoid odd 1/4wave masts that screw with vswr & cause the mast to carry significant antenna mode current .

Bob, I would like to attempt to model your idea above and see if I can test the maximizing currents ideas...using your, "I use & I avoid" ideas above. If you don't have these details for your dimensions, then I'll use my Old Top One model and see how it is effected in a similar comparison using the mast length below the loop.

This model has always been more dependable to work on, because I had the real antenna to get the dimesions. Loosecannon recently confirmed my dimensions. He gave me all of his exposed tubing lengths and diameters for his OTO, right down to 6.3125" for the radial spacing between his radials, center to center.

I'm going to use 27.205 MHz at 36' feet in the Real Earth model, and I will not try and fix the match or resonance by changing any length or diameter dimensions.

I've also recently used some old dimensions I got from you. See the PDF file below. There, I had to make some guesstimates for overlap and swagging lengths around the hub area. Overall lengths are not helpful, I needed the better exposed lengths. When I recently built another A/P model, I knew I had guessed wrong somewhere, because I had to do some tuning to the model, changing the Top Hat some for the reactance and making the space between the radials smaller for the resistance. I think I had some length issues in the lower radial area.

I have numerous dimensions in my file for the A/P and they are all different.

Please check out your post below and maybe you can get me closer to the exposed dimensions and the lengths for the swagging sections and the recessed parts. I'm assuming the recessed lengths are 2.5" inches, like the hub height, and the swag lengths are >1" inch.

I took the loop to be a 15" inches radius or 30" diameter.

When you mention above, making the loop as wide as practical, do you mean the stock 30" diameter or do you suggest something wider to try in a test?

I hope this makes sense, I had a spell yesterday, but feeling better now.

 

[bob85]

Leave the hoop stock diameter Eddie,

as i said earlier, when you bring the legs closer to the mast it electrically shortens the mast and sleeve elements,

you will know when the mast and sleeve are resonant when you see the least current below the hoop,
it will be a little longer than it would be on its own without the sleeve around it,

Avanti said 10% shorter than 1/4wave for the upper monopole,

i would start with stock length,
the 10% could be the bracket, and that would be antenna mode current ,

i think its also also excited with high voltage at the hoop as a 1/2wave end fed looking up from the hoop through the bracket shorting the 1/4wave line,

the bracket is invisible to transmission-line mode currents been a short circuit inverted through a 1/4wave transmission-line,

two currents two modes co-inductive.

reread you post, my astro is in the loft assembled minus mast, i can measure it tomorrow if you need assembled lengths,

Eddie,
this wabbit hole is deeper than a dipole, help me out.

 

[Marconi]

as i said earlier, when you bring the legs closer to the mast it electrically shortens the mast and sleeve elements,

I was thinking you were talking about making the hoop larger, but I now think you mean making the space between the radials and the mast in the middle of the radial loop.

 

[Marconi]

reread you post, my astro is in the loft assembled minus mast, i can measure it tomorrow if you need assembled lengths,

By all means, I need the exposed dimensions even if it only confirms some previous dimensions I already have.

I would rather use the A/P than my OTO.

 

[bob85]

No Eddie,
i was talking about the principle of keeping a transmission-lines characteristic impedance for a good match,
while increasing unbalance & decreasing coupling into the mast below the sleeve by tapering the lines to increase distance between mast & hoop,

i think you could use a none flared setup & adjust spacing of the legs to find a good match ( not sure )
that would reduce unbalance & couple more energy into the mast below the hoop imho.

 

[Marconi]

No Eddie,
i was talking about the principle of keeping a transmission-lines characteristic impedance for a good match,
while increasing unbalance & decreasing coupling into the mast below the sleeve by tapering the lines to increase distance between mast & hoop,

i think you could use a none flared setup & adjust spacing of the legs to find a good match ( not sure )
that would reduce unbalance & couple more energy into the mast below the hoop imho.

Sorry Bob, i messed up and posted the wrong link to your comment where you used the word "flared." See post below.

everything about the astroplanes sleeve screams maximum unbalance & minimum mast current from its 1/2wave mast to its flared for wider spacing at the end of the sleeve,

So, I posted the following comment.

I was thinking you were talking about making the hoop larger, but I now think you mean making the space between the radials and the mast in the middle of the radial loop.

I doubt I'm making any sense right now, but I hope this makes a little sense. Again I just selected the wrong post to link.

 

[bob85]

Eddie,
going back to your current logs & red lines not having the typical disitribution you expect which would make you think something broke according to Roy L,

if you resonate the 1/2wave mast and you do see a strange current distribution on the mast as DB described,

don't presume something is wrong with the model just because that red line don't look right for a 1/2wave conductor,

antenna mode current in the mast will have the expected distribution but its very small @resonance and in superposition with the transmission-line mode currents,

you could look at it this way,
its hiding under the transmission-line mode currents with just the part below the hoop been visible, its not exactly like that but you get the idea,

I don't know how NEC handles superposed currents or how it displays the results with the red lines
but a strange distribution ( if that's what you see on the mast ) is not an indicator something broke in the model with this antenna,

A 1/2wave conductor with low current in the upper 1/4wave & high current below the sleeve is what you get with a resonant 1/2wave monopole & 1/4wave sleeve in an open sleeve antenna,

I am hoping NEC displays the currents as DB describes when he tells us he had what looks like a 1/4wave ditribution curve on a 1/2wave conductor which he has never seen in any other antenna he has made a model of,

DB told us he saw the strange 1/4wave distribution @ resonace which is exactly where you get very high antenna mode impedance & low monopole current superposed with transmission-line mode currents.


dare i say co-inductive, two currents in superposition been induced in one conductor.

You may think Avanti were just playing with words to make it sound impressive,
i think the other guys making (coliner lol) 5/8waves, could not lace Lou Martino's boots.

 

[bob85]

Eddie,
about the measurements, IF you really think you need them I will measure mine,

but I don't think you need them to prove or disprove the concept,

all you need do AFAIK is build the model get the legs hoop & bracket in the ball park for dimensions,
then adjust the mast & upper 1/4wave in small increments for least reactance at the feed-point,

resonant frequency does not matter, what matters is that its all resonant.

We are not tuning up on ch20 we are trying to prove or disprove the concept of co-induction, two currents superposed

if all goes to plan as you adjust the mast you will see current below the hoop change, when its lowest that's where the mast is resonant as a 1/2wave,

IM hoping you will see reactance go down at the feed-point & see that strange current distribution on the mast that DB talked about in his models

adjust the upper 1/4wave for least reactance,

there could be some interaction as you tweak the mast & upper monopole,
IM not sure you can simply resonate the mast then resonate the upper 1/4wave with no interaction between them, but you may be able to,

DB may be able to point you in the right direction since he managed to resonate his model & noted the odd current on the mast,

you should use a 1.5" diameter mast not just a wire or you will have to mess with spacing to get back close to 50ohms.
If you mess with spacing you will have to tweak the mast and the upper 1/4wave to restore resonance,

if that works out, extend the mast to resonant 3/4wave,

does current go up below the hoop as your masts antenna mode impedance goes from the very high end impedance of a 1/2wave towards the low end impedance of a resonant 3/4wave causing vswr to rise.

Thanks for helping me out here Eddie

 

[Marconi]

Bob here are two models of the A/P, one with a 197" inch mast and the other at 220" inches.

1. Here is a post from DB that talks about the 220" mast that showed him the best results.

2. Here is my test #3 with a 197" mast. The Average Gain report shows a littile error in the model, so the gain is understated by -0.06 db in gain. I added an image of the currents for wire #1, the radiator. I also added an image for the currents on wire #43 the mast inside the loop and extending below the hoop. Here you can see the currents on these wire per segment. Each segment in these models is very near 3" inches if you want to find a particular current value along a wire somewhere.

3. is the same model with a 220" mast. It too shows to be understated a bit by -0.02 db in gain.

To get the accurate gain for the models in this case...add the (- error) noted for the understatement, to the reported gain.

I do see a little difference in the currents on the mast at segment #1 on each model right below the hub. I did not compare the currents right below the loop however, and maybe that is where you are mostly interested.

I did nothing to the dimensions you sent me in 2014, but I might have figured the overlap and swagging wrong in spots and that may be why these models don't show a better match...even when I set the frequency to the lower end of the CB band, liike when we only had 23 channels. These old A/P were likely cut for the lower frequencies back in the days. Both models suggest this issue in the SWR bandwidth scans I also included.

You're right Bob, this model shows the the 220" mast works better than the model with the 197" mast.

Next I will make the radiator at a full 1/4 wavelength and see what happens.

Do you think I should try a mast that is closer to a length equal to the radial length, and set it below the loop...for what I might presume to be a little short of a true 1/2 wave...maybe due in part to the loop at the bottom.

I also won't to make the radiator a full length resonant 1/4 wave.

 

[bob85]

I just went back to DB's video response to Marconi,

GOOD VIDEO

first point i want to talk about,the source placement difference between DB & Marconi
if its working as i think, the difference between your source placements won't make a heap of difference but there is a difference DB's source is placed more central to the transmission-line,

which gives a little better balance than an offset source, offsetting the source on a balanced line causes some small radiation,
neither of your models are exactly like the real antenna but if you keep it close to the real placement i don't think you will see much difference between models

second point, blue wire,
@ the resonant frequency of the transmission-line that blue line/shorting bracket appears as an open circuit with very low current,
i expect moving away from resonance of the line will see an increase in current on that blue line as the impedance of that blue wire is reduced looking up from the hoop and i expect it to start carrying current,

third point
look at the distribution on the mast, the high voltage at the end of the transmission-lines the low current in the mast below the hoop,
the high current inside the transmission-lines,

that is exactly what i expect the astroplane to look like when the whole shebang is brought to resonance mast transmission-lines & upper 1/4wave


i don't know how i missed the significance of that video.

 

[bob85]

WOW, Eddie thats great,
just as expected look at the mast current, same as DB's

you don't normally get that current distribution with a 1/2wave conductor but you do get that with an open sleeve antenna with a 1/2wave conductor & 1/4wave sleeve brought to resonance,

try extending the mast to resonant 3/4wave,
that will reduce mast antenna mode impedance seen in parallel with the transmission-line mode impedance & maximise mast current & raise vswr if its working how i think it is.

THANK YOU Eddie.

 

[Marconi]

Looks terrible, when I even get close to 3/4 wavelength at 342" inches a littile more than 3/4wl. Match is R = 378.9 X = -1.116 OHMs. SWR = 7.578.

 

[bob85]

vswr is exactly how it should be Eddie, DB saw the same thing,

i don't get the high impedance with the 3/4wave mast yet because antenna mode impedance of the mast is much lower than it is when its 1/2wave long but i will figure it out someday,

i suspect its something to do with the balance at the end of the sleeve upsetting the transmission-lines,
because with the 1/2wave mast you have the low end impedance of a 1/4wave terminating the lines,
with the 3/4wave mast you have the high end impedance of a 1/2wave terminating the lines.

DB was spot on when he told you resonace was the key.

its clear that an astroplane is best isolated about 1/4wave below the hoop where the electrical length of the mast is that of a halfwave,

physically it must be a little longer due to the sleeve raising resonant frequency as seen in open sleeve antennas sigma4's and the like,

yours is low in frequency, did you make the hat wires skinny like the real antenna?
mine is the mk2 version with swaged tube ends, not the old style with expaned tube ends & its tuned for the middle of your cb band with a halfwave 1.5" mast