Any Astro Plane Fans ?

[bob85]

Riverman,

somebody tried in the past,

Lou Martino passed away some years ago,
Herbert Blaese was still around but not interested in talking about CB antennas,
Dale Parfitt was still around & talking about antennas on Eham last time i looked,

i think Dale said that Herb designed the astroplane,

it would be interesting to hear what Herb had to say about the astroplane & sigma4 and the claims of superior performance over 5/8waves,

my current ( no pun intended ) opinion is they both achieve better performance because they put current maxima higher above ground and no out of phase radiation like a 5/8 has.

 

[bob85]

Eddie or DB

any chance one of you could model a hat loaded starduster with a 4ft upper 1/4wave and compare it to a 5/8 at the same tip height.

im not a fan of none dc ground antennas,
you could probably add a shunt across the feed-point & shorten the radiator or hat a little to compensate.

 

[Riverman]

Riverman,

somebody tried in the past,

Lou Martino passed away some years ago,
Herbert Blaese was still around but not interested in talking about CB antennas,
Dale Parfitt was still around & talking about antennas on Eham last time i looked,

i think Dale said that Herb designed the astroplane,

it would be interesting to hear what Herb had to say about the astroplane & sigma4 and the claims of superior performance over 5/8waves,

my current ( no pun intended ) opinion is they both achieve better performance because they put current maxima higher above ground and no out of phase radiation like a 5/8 has.

Well, darn.
w9cll probably won't have much luck.
Didn't know Dale worked at Avanti. Since leaving them he's done great things at Par Electronics with his Omniangles, HF Rectangles, and Endfedz (which he sold to LNR Precision I believe). Very nice guy as well.

 

[bob85]

Heres what avanti said about comparing antennas in the owners manuals,
i think its clear that height means the tip of the antennas,
and we have a height to model them at, 60feet above the ground.

"ANTENNA HEIGHT"

"Whenever antennas are been compared, they should be installed at the proper distance above the ground and preferably in an open field,
This operation is not even legal for CB'ing when using a horizontal beam because the proper distance above ground for this mode of transmissionis 36 feet or more,
Only the test of a vertical antenna may be run at a legal height of 60 feet in an open field"

&

"ideally antennas should be one wavelength
(36' at cb frequency ) or more above the ground,
For comparison testing always install test antennas at the same height"

They also say that the sigma 5/8 is 22ft & legally installable (must be installed 2ft below highest point on house or building)

 

[The DB]

Eddie or DB

any chance one of you could model a hat loaded starduster with a 4ft upper 1/4wave and compare it to a 5/8 at the same tip height.

im not a fan of none dc ground antennas,
you could probably add a shunt across the feed-point & shorten the radiator or hat a little to compensate.

Starduster with a cap hat? Sure I can do that when I get a chance. Their is actually something I have been meaning to play with with the Starduster design that I haven't played with because I have been playing with the Astroplane antenna models. I am actually curious how such an antenna compares to said Astroplane models I have been playing with lately.

You can put a shorted 1/4 wavelength stub at the feed point or elsewhere on the feed line, that would provide said DC grounding that a non DC grounded antenna is missing, would it not? A certain video I posted in some other thread on this forum explains and shows it at a little past 32 minutes in... I wonder how many references I am going to make to various parts of that video to explain things now...


The DB

 

[Riverman]

The A/P has 4 top hat radials @ 90 degrees.
Would 3 @ 120 degrees (like on the old Hy-Gain CLR-2) work equally well?

What determines how many are used? Their length?

 

[The DB]

The A/P has 4 top hat radials @ 90 degrees.
Would 3 @ 120 degrees (like on the old Hy-Gain CLR-2) work equally well?

What determines how many are used? Their length?

Three would work very similar to the four, however, they would need to be longer to compensate for the missing one. Maybe I'll get a chance to show that one at some point as well.


The DB

 

[Riverman]

Three would work very similar to the four, however, they would need to be longer to compensate for the missing one. Maybe I'll get a chance to show that one at some point as well.
The DB

Sounds like their total length is what matters, not the number of them. What determines how much length is needed? And is that number dependent on their diameter (i.e. 1/8")?

 

[Oscar Goldman]

The more you have the shorter they can be to have the same capacitance.
You can also use a perimeter wire to build more capacitance.
The diameter of the conductor does make a difference.

 

[The DB]

Sounds like their total length is what matters, not the number of them. What determines how much length is needed? And is that number dependent on their diameter (i.e. 1/8")?

The importance is the capacitance (more specifically capacitive impedance) they add to the antenna. Four elements of a given length will provide a given amount of capacitance, if you only use three elements, you need more metal in those elements, either making them wider, or longer (or perhaps both depending on the situation).

Remember, with a cap hat, how much capacitance is key. It will also be interesting to see if and how much it changes the feed point impedance of the antenna. I don't expect a big change, but at the same time I will be surprised if their is no change to the feed point impedance as well. We will just have to see what the modeling tells us when I get the chance.


The DB

 

[Riverman]

The more you have the shorter they can be to have the same capacitance.
You can also use a perimeter wire to build more capacitance.
The diameter of the conductor does make a difference.

Very good. One last question. Am studying for my General and trying to understand this.

How does the antenna designer know when he has the correct amount of capacitance in the top hat?

 

[Oscar Goldman]

The importance is the capacitance (more specifically capacitive impedance) they add to the antenna. Four elements of a given length will provide a given amount of capacitance, if you only use three elements, you need more metal in those elements, either making them wider, or longer (or perhaps both depending on the situation).

Remember, with a cap hat, how much capacitance is key. It will also be interesting to see if and how much it changes the feed point impedance of the antenna. I don't expect a big change, but at the same time I will be surprised if their is no change to the feed point impedance as well. We will just have to see what the modeling tells us when I get the chance.


The DB

When I built a 40m monoband vertical with top loading. I used 6 spokes 3 foot long each, out of 3/8 tubing. The antenna ended up being 25 foot tall, and the feed point impedance did drop down. Not enough to use a shunt coil at the feed point, but it did drop down some.

 

[bob85]

DB
yes a shorted 1/4wave stub would do it.

i think the video will be very usefull from time to time

 

[Riverman]

I think DB just answered my question about top hat capacitance. It's whatever is needed to bring feedpoint impedance to 50 ohms or so.

That on the right track?

 

[bob85]

On the right track but off a little, keep going.

if you had a resonant antenna the feedpoint impedance would be resistive with no reactance x=0

lets say for example you got very lucky and had an ideal situation for feeding the antenna with 50ohm coaxial cable r=50 with the imaginary reactive part x =0, vswr= 1:1

for whatever reason you can't have an antenna that tall and must make it shorter without ruining its performance,

you shorten the antenna it becomes reactive and is said to be electrically short or capacitive, & the imaginary part x is no longer zero, vswr is no longer 1:1,

you add the hat to electrically lengthen the antenna and adjust it to restore resonance x=0,

you have resonance again x=0 but r won't be 50ohms, could be something like r=30 x=0 depending on how big your hat needs to be & vswr won't be 1:1

its important to remember that resonance and vswr are NOT the same thing.

https://www.nonstopsystems.com/radio/pdf-ant/article-vrt-httd-dip-10m.pdf

edited because i typed x not r & j not x., must read what i type.