Any Astro Plane Fans ?

[The DB]

After some playing I think I got it... One simple little mistake on a feature I am not used to using throws everything off...

This what you want?


The DB

 

[HomerBB]

DB, I think Bob is asking you to model a 1/4^ line, no antenna at all. Feed it at the bottom, short it at the top. He wants to test a theory regarding the blue line and what creates it.

 

[The DB]

So like this?

The DB

 

[bob85]

its rather skinny compared to the spacing of the astroplanes radials to the mast DB,
what color is that transmission-line? my eyes cant tell if theres much current flowing or not

 

[The DB]

The transmission line always shows up as the same color as minimum current on the chart. 4Nec2 doesn't calculate current for the transmission line itself, but it does calculate it at the connection on each end. I think it is done this way so the transmission line itself doesn't distort the results of the model. Do you want me to zoom in on one of the ends? The feed point is peak current, the other side has almost no current.

Mind you, I'm talking about the image of just the feed line, not the astroplane that includes a feed line. In that one the antenna side of the feed line is peak current, and their is a current null on the other side of the feed line.


The DB

 

[bob85]

yes just the feedline DB, is the far end blue?,

 

[The DB]

Far end from the feed point is blue.


The DB

 

[The DB]

@The DB
Do you have a graph/plot of the SWR curve for the model. Real world experience has found resonance below the Cb band from the facotory antenna. Might explain the SWR going to 1.2 wgen the feedpoint was adjusted to the corner.

Forgot about this, so here it is.

This is the SWR curve for the model with the feed point where you suggest it should be.

The DB

 

[Needle Bender]

Isn't the feed point (center of coax) connected to the insulated top of the short side downward leg?

- And DB, I'm wondering why, in your earlier graph of modeling it against the I-10Kwave at two different heights, it shows differing patterns?
- Change in elevation above ground? ...mast length?

I'd like to know the details of those variables, if possible.

Enjoying the thread!

 

[The DB]

Isn't the feed point (center of coax) connected to the insulated top of the short side downward leg?

Yes. When it comes to modeling I can put the feed point on either side of that point but not directly on it, and as I have shown in post 287 in this thread the differences are very minor.

- And DB, I'm wondering why, in your earlier graph of modeling it against the I-10Kwave at two different heights, it shows differing patterns?
- Change in elevation above ground? ...mast length?

I'd like to know the details of those variables, if possible.

Enjoying the thread!

The two astroplane models were exactly the same except for height. They used the same mast length, the one that modeling has shown me is optimal. The red pattern was the same tip height to the I-10k, the blue one the same mounting point height. Their were no other changes between these two models. I was wondering about a comparison with another well known antenna model that was, like the astroplane model I was working with, complete including a matching system. The I-10k is one of the few antenna models I have that meets that requirement for various reasons.

Many people are surprised at the how much a small change in height can change an antenna's radiation pattern.. Every half wavelength in height above ground causes a new higher angle lobe to form, effectively pushing the other lobes closer to horizontal. I have seen this with every model I have modeled at different heights. When you have seen enough of these you can very accurately estimate a models height above ground just by looking at the lobes. An example of this is the first line in post 282 where Marconi asked:

DB I could tell from the patterns you posted that the antenna was high...maybe even 2 wavelengths at 27 mhz, but I did wonder why.

I could model the same comparison with the masts going all the way to the earth, based on what I have seen I don't think it will make that much of a difference unless one of those heights happens to be one of the few bad mast lengths that I have been mentioning above.


The DB

 

[Riverman]

I remember it when it first came out forty-five years ago.

http://www.cbtricks.com/ant_manuals/avanti/av101/ad/index.htm

Here's my question:

What effect does removing the top hat have?

Thanks.

 

[The DB]

Here's my question:

What effect does removing the top hat have?

Thanks.

When I remove the cat (top) hat in my models and adjusted the upper vertical element to compensate, the antenna performed about the same, but didn't tune as well. The R variable was in the low 70's instead of near 50 ohms.

Plus, according to the patent the top of the antenna was meant to meet certain height requirements (the patent speaks of government limit of 20 feet above the building the antenna is mounted on), so when mounting for said specific maximum height allowed, the cap hat allows the rest of the antenna to be mounted higher, which increases performance some (I doubt so much that you would notice). Mounting this antenna this way, the patent also talks about the antenna being more likely to be above other nearby obstructions, such as other houses, effectively putting this antenna's signal in the free and clear.


The DB

 

[Riverman]

When I remove the cat (top) hat in my models . . .
The DB

Thank you!

 

[Marconi]

AGT was in line, actually dead on with a 1. I used a trick that ghz24 showed me to achieve that. The higher than normal gains were because I mounted the antenna very high. I did this because I was playing with a feature I knew about in 4Nec2 that I hadn't played with before. Essentially I can change the length of a wire through a range of lengths, and can map the results of said changes, like SWR gain and impedance, on a graph. What I was doing was graphing the changes in the mast length, and the upper edge of those lengths was about two wavelength high, which means I needed to have the feed point of the antenna at least that high. When I lowered the antenna back down for the comparison to the I-10k antenna the gain numbers should be more in line with what you expect.

DB, do your words above explain the trick that ghz24 showed you in order to get your Average Gain dead on 1?

Below is my latest model of an Old Top One made by CTE. I have all the parts for this antenna so I decided to use it to model. This is also the antenna that I have installed at my location several times, and each time I did my usual Antenna Work Sheet reports along with Signal Reports over time so I have some records.

I also included my Free Space model to show the Average Gain results for the model over Eznec's idea of real Earth.

 

[The DB]

DB, do your words above explain the trick that ghz24 showed you in order to get your Average Gain dead on 1?

Yes.


The DB