Any Astro Plane Fans ?

[The DB]

is that adjusted for resonance of the 1/4wave line DB,

Do you mean the loop part? No, but I can. To do that I would need to remove the upper vertical element.

seems odd that adding the upper 1/4wave caused significant current to flow in the short at the top of the 1/4wave line when theres nothing flowing in the upper 1/4wave,

In my experience you are looking at this backwards, it isn't the upper 1/4 wave line that is causing the significant current flow in the short, the short is causing significant current flow in the short. The current flow in the loop won't change much, if at all, with the removal of the upper vertical element

ps,
i meant the model with optimised 1/2wave mast since we know the astro does not like the mast ending up next to the hoop,

I don't think these changes are going to do what you think they are going to do. I can tune the loop by itself to resonance fairly easily, that isn't a problem. However, if the mast is attached, either the loop will be a drastically different size from the AP, or the mast will be a drastically different size. Which one do I use to tune for.

I should also point out that the "basket" really no longer exists. The hoop is gone, as well as one of the sides. The feed point is also in a very different location now. The mast, which on the AP has a 1/4 wavelength pattern, so even though physically longer was electrically only 1/4 wavelength in length. These need to be factored in to the changes you are suggesting. You can't just remove a significant part of something like the basket of the AP antenna, something that is causing a pretty significant effect elsewhere on the antenna, and expect the same effect to still happen to the same degree on the same affected wires.

I'm happy to try, this afternoon or later this evening when I get some time, but I am sure it isn't going to do what you think it is going to do.

That being said, I have applied what you said above slightly differently than you are here, and I think the idea that part of the antenna is acting like a feed line impedance transformer has merit as an explanation of that is happening. However, I think you are either applying this to the antenna improperly, or you are holding on to assumptions that are throwing things off. That is going to take some time to explore on my part, however.

Seriously, have you considered learning to model? I think it would benefit you greatly, and I highly recommend it in your case. You have a good theory background already, and that will only benefit you should you start modeling. Should you decide to learn, I am willing to help in any way I can, and I am sure Marconi is as well.


The DB

 

[Marconi]

I'll do what I can with Eznec and I have time when I feel OK.

I could email some models to start. But Eznec cost money so 4Nec2 might be a better choice to learn.

DB can even do videos. DB did you ever fix the screen view problem where it is hard to read the details or is it just my bad eyes?

Click example here:

 

[HomerBB]

I don't recall this experiment.

Do you mean you and I experimented with radial lengths on the A/P?

Did I do models or what?

OK. you never changed the cap hat, but did you have to change anything to effect the tune after it was built?

If you used your analyzer, did you keep any notes about the antenna match?

Since you were working with PVC which might have been larger diameter than the spec dimensions on the work sheet...did you compensate any?

Did your PVC A/P have a near perfect match near the middle of the CB band?

Do you have a real A/P?

I want to model using the metric spec sheet you used. But, I have hesitated on this one because the likely very wide bandwidth could allow a good match over a wide range in the real world. I'm not even sure this spec sheet was cut for the CB band in America.



Thanks again for your heads up.

I kept notes, but that PC crashed with everything on it.
I had an original AP, but traded it away. Afterwards I built my homebrew using CPVC and PEX.
The homebrew tuned within CB.
I did not have an analyzer until later.
We only tried different skirt spreads.

 

[bob85]

DB
in your model with the blue line theres minimal current in the short circuit, also in your 1/4wave shorted line model theres minimal current in the short,
a 1/4wave inverts a short circuit to look like an open circuit,

so why do you have current in the last model with the upper 1/4wave & no mast below the hoop ?

is it that the source wires screw the line length up,
the upper 1/4wave which has no current on it
no mast below the hoop, what are your thoughts,

im not trying to make the model work with half of it missing,
it won't work but i may get to see if its possible that its acting as i described,, its also possible it won't show us.

 

[The DB]

in your model with the blue line theres minimal current in the short circuit

Yes, although for a reason other than what I think you are trying to go. Many yagi's, for example, are directly shorted to the boom, their is no current flow in the boom either inspite of a direct short connection. A big part of it has to do with what is happening electrically on both sides of the wire, or in the case of the Yagi antennas where the elements meet the boom. The fact that the wire in the blue line question/discussion is very short I think is also playing a part. If that connecting was a half wavelength long, I think we would have seen a full half wavelength current distribution on it.

also in your 1/4wave shorted line model theres minimal current in the short

I don't trust anything I did with the feed line yet as like I said, it wasn't something I had done in the past. It isn't something I need for modeling in general, and doing a few things it does things I don't expect and don't understand. Honestly I need to play with this some more. More than once I set it up completely wrong thinking I had it done right.

a 1/4wave inverts a short circuit to look like an open circuit

No argument here.

so why do you have current in the last model with the upper 1/4wave & no mast below the hoop ?

Ignore the upper half wavelength portion of this model and you end up with a half wavelength loop. In a model that is based on a complete loop from one side of the feed point to the other, the point along the loop that is electrically the farthest away from the feed point will have a current peak. In a half wavelength loop, the feed point being electrically 1/4 wavelength away from the feed point (both directions) it is then at a current peak. This is in opposition to the tip of a dipole, for example, where their is no current flow. These two situations are opposites, an open circuit (tip) can have no current, and a short circuit (loop) will have maximum current. This is how every model I have ever made using these patterns has been.

Now, factoring in the upper vertical element, I don't know if I am supposed to be surprised that the loop with that element shows no current in that element or not. Being that the upper vertical element is attached at a current node, I kind of am surprised that their are very few to none though, I figured some would make their way their. The upper vertical element will present a low impedance just like the other half of the loop... This isn't something I have done before so its knew to me.

If I add a second 1/4 wavelength section below the loop, essentially making the mast as long as you are thinking it should be by attaching the rest below the height of the removed hoop, it changes the current distribution entirely. Their is noticeably more current flowing on one side of the loop than the other.


The DB

 

[bob85]

"If I add a second 1/4 wavelength section below the loop, essentially making the mast as long as you are thinking it should be by attaching the rest below the height of the removed hoop, it changes the current distribution entirely. Their is noticeably more current flowing on one side of the loop than the other."

That's the part i was interested in DB,
can we have a 1/4wave line in parallel with the 1/2wave radiator with most of the current flowing in the 1/2wave radiator & a blue line at the top of the 1/4wave as seen in your blue line model
when its fed at the bottom,

it still looks like an upside down modified j-pole to me,

the loop impedance is high until you add a mast or feedline inside the aperture creating two parallel tapered 1/4wave transmission-lines,

when you place radials around a monopole you form a transmission-line & transmission-line currents flow,
impedance drops & resonant point rises, the whole shebang is electrically shorter than the individual elements alone,

i hope we figure it out eventually.
nothing you have shown us so far has made me think it can't work like that but i could be wrong.
i will message you with how i got to thinking this way.

 

[HomerBB]

DB, considering this little short blue line in the mounting bracket has reduced the thread to the "Blue Line" thread, perhaps if you posted a model lacking one half the metal bracket, that is, as if the bracket were acrylic and nonconducting on the short skirt side, we could see what the whole antenna is like. As it is, I am having dreams of blue. Even the milk in my cereal this morning was blue. . . Well, the milk was just outdated.

 

[The DB]

Hey bob85, I will get back to your post later, I am not ignoring it or your pm, I swear.

DB, considering this little short blue line in the mounting bracket has reduced the thread to the "Blue Line" thread, perhaps if you posted a model lacking one half the metal bracket, that is, as if the bracket were acrylic and nonconducting on the short skirt side, we could see what the whole antenna is like. As it is, I am having dreams of blue. Even the milk in my cereal this morning was blue. . . Well, the milk was just outdated.

So essentially remove the blue line and see what we get? I can do that.

Here we have the current distribution of the original for reference.

Then I removed the "blue line" that we have been talking about and adjusted only the mast length to tune the antenna (8 inch longer mast length, thats the only other difference).

As we can see, the current distribution is very close to being the same weather or not that line is present. Two things to note on the model with the "blue line" removed, the currents in the basket are not in balance, which would require making adjustments to the basket and cap hat to fix should the need arise. Also, the currents are noticeably stronger much further down the mast than with the original model. These are the biggest changes I have noticed.

So shall we compare the gain between these two models?

Here we see the gain between the two antennas is very similar, the model with the "blue line" removed (the blue line in the plot) does have slightly more gain.

And finally the model data for both models, the original model on the left and the model without the "blue line" is on the right.

Looking at the differences between these models, their really isn't much. SWR is slightly higher without the "blue line", although that can be adjusted with a small adjustment to the cap hat. Looking at the Radiat-eff (radiation efficiency) we see that the model without the "blue line" is about 13% more efficient, which explains why that model has a higher gain.

At this point I thought I was done. I decided to double check and see what other differences I could find with these models to be sure, and it is a good thing I did. I did not expect the next thing I found.

Here we will look at SWR bandwidth between 25 and 29 MHz. Like above, the left is the original reference model, the one on the right is the model with the "blue line" removed.

I did not expect this difference. Removing the blue line seems to widen the bandwidth of the antenna quite a bit.

And for completeness sake, this is something I mentioned earlier when I first noticed the lack of currents on the "blue line". This is a comparison of SWR over a range of mast lengths. As I stated above, the model with the "blue line" has a much more stable SWR over a range of mast lengths, in this case 100 inches to 850 inches. I didn't actually post a comparison above, so here is what I was talking about when I first brought it up.

As we can see, in the case of the original model, after an initial spike SWR barely makes it above 1.5 on the original model. The model without the "blue line" is much less stable over the range of mast lengths. After the initial peak, we get much larger swings in SWR, several times going above 2.

So, in conclusion, as we can see that the "blue wire" is not necessary for this antenna design to work. Their are advantages to it not being included. However, with the mast length graphs, their is also an advantage to keeping it as well, namely pretty much any mast/feed line lengths you want to use.

Any thoughts, questions, other aspects, ect. I should compare between these models?

Oh, and Homer, careful with that expired milk...


The DB

 

[HomerBB]

Thanks!
I think the reasons to use a nonconducting bracket section at the blue line out weigh those to leave it in. To tune for optimum SWR just set the mast length. Unless you adjusted the upper vertical and cap hat for the model, I see the radiation pattern as more even than witg the "blue line" section in.
Pros:
More gain.
Better radiation pattern.
More bandwidth.
Better radiator efficiency.

Con:
Mast length sensitivity.

Did I miss anything?

Got my eye on the milk.

 

[The DB]

Unless their is something I missed, I think you got it Homer.

If I were a manufacturer making this antenna for the masses I would leave the "blue line" in as it is easier for those who don't pay attention to such things, however, if I were a hobbyist making one of these myself and I wanted every little benefit out of this antenna, I would use a non-conductive bracket.


The DB

 

[HomerBB]

Unless their is something I missed, I think you got it Homer.

If I were a manufacturer making this antenna for the masses I would leave the "blue line" in as it is easier for those who don't pay attention to such things, however, if I were a hobbyist making one of these myself and I wanted every little benefit out of this antenna, I would use a non-conductive bracket.


The DB

cool

 

[HomerBB]

@The DB, @bob85, @Marconi
From a practical, and empirical, standpoint, what would the actual feedpoint of the antenna with the blue line segment of the top bracket look like?
It looks as if the bracket will be attached conductively only to the mast. It is isolated from the short skirt radial. Apparently the antenna has a significant capacitive component with the blue line suggesting rg11 hat all along..?
So, how would you fellows mount the feedpoint?

 

[The DB]

From a practical, and empirical, standpoint, what would the actual feedpoint of the antenna with the blue line segment of the top bracket look like?
It looks as if the bracket will be attached conductively only to the mast. It is isolated from the short skirt radial. Apparently the antenna has a significant capacitive component with the blue line suggesting rg11 hat all along..?
So, how would you fellows mount the feedpoint?

If you remove the "blue line" segment in the model, when it comes to actually making the antenna the only part of a metal mounting bracket that would be attached would be the mast. You could simply isolate both sides of the basket from the mounting bracket like you did with the feed point side in your last build. If you used a non-conducting bracket, you could simply put the two sides of the feed line between the mast and the side of the basket that does not contain the upper vertical element.

I'm curious to the "significant capacitive component" you are referring to, specifically related to the "rg11 hat". My models do tent to show that this antenna's feed point does tend to be capacitive most of the time, but you lost me on the "rg11 hat".

 

[HomerBB]

That has me baffled, too, except I know what you don't - that it is a stupid spell check typo. I wrote the word "that". Spell check changed it to "rg11 hat".
Way too funny. Did you model it?

 

[Marconi]

Homer I was afraid to ask.

But I will ask, does the blue line show capacitance due to it being short?