Any Astro Plane Fans ?

[HomerBB]

You are correct. The long side is not isolated. The short side is. I must have read your post wrong.
I just couldn't see it in the image of the cross brace area.

 

[The DB]

Where the circle is, that is a feed point. For the feed point it shows a wire, but in reality the two sides are isolated from each other. I am not sure why Nec in general does it this way but it does.


The DB

 

[HomerBB]

In that case, I dont get the blue line.

 

[The DB]

I think I have some time, so I'll talk about a few things I noticed with this model. Some of this was mentioned before.

1) Lets start with the capacity hat on this antenna. If I model the antenna with a straight wire instead of a shorter wire with a capacity hat, instead of getting a near 50 ohm impedance I get an impedance in the low 70's. The capacity hat has the effect of lowering the feed point impedance, so I think it is there as a means of tuning.

2) The "blue line" (that is what I am going to call it in this post) I mentioned above. That is something I came across by accident with the original ghz24 model. In that case the current flowing on that wire was so low that you could remove the wire and it made no significant difference to the model's results. SWR, impedance, and gain were all very close to being the same weather it was their or not.

Now with the updated model, adjusted to be the same dimensions of the actual antenna, and converted to aluminum as well, their is a larger, yet still insignificant amount of current flowing in that wire. I also noticed that this wire is 180 degrees out of phase with the feed point that is right next to it. I'm going to show the image I used above, and another with the mast in a worst case length. Lets start with the image I posted above.

And now the worst mast length.

As we can see, the antenna has a different current distribution. The vertical element is carrying much less of the current, and the blue line actually has two to three times the current flow than the image above. In the worst case mast length scenario it appears to be taking current away from the upper vertical element and moving it to the mast.

Now on this antenna, the entire basket area and the upper vertical element are all in phase, the only vertical part of this antenna that my models shows is out phase is the mast. This point also happens to be where their is a dip in the antenna's gain before it spikes back up to normal. Even with this dip in gain, SWR stays within acceptable levels.

While experimenting, I took this wire out and adjust the capacity hat to tune the antenna and initially it worked the same as before. However, when I graphed SWR using various lengths of mast as I did above, SWR peaks higher and for longer periods after the blue line above has been removed. Because of this, I have to say it appears that this wire seems to have a stabilizing effect when it comes to varying mast lengths and this antenna. It doesn't completely eliminate the negative effects of a bad mast length, but it does narrow them to a much smaller range of possible lengths. Unless you happen to have one of these very limited unlucky lengths of mast/feed line, you don't need a choke on this antenna.

The only other thing I have to say about this blue line is even though it directly connects the feed point that is connected to the mast with the basket connection and upper vertical element, the feed point doesn't seem to directly affect said basket element and the upper vertical element through this wire. It is like the two are isolated even with a direct connection. I think this is partially due to when their is current flowing on this blue line, so far it has always been in the opposite direction that current is flowing across the feed point...

3) Taking what I posted above when it comes to the blue line, how this antenna is working in my models is the upper vertical element (with its cap hat) and the basket area on that side are functioning together as a dipole. Because of the physical change in direction where the antenna reaches a current null (the loop at the bottom of the antenna), the other side of the basket is in phase with the "dipole" I already mentioned. Essentially we have 3/4 electrical wavelengths of antenna on that side of the feed point, and because of a physical change in direction, all 3/4 wavelengths is in phase.

The other side of the feed point we have a mast connected. It is physically over 1/2 wavelength long, however, it has the current distribution of a 1/4 wavelength antenna. This is due to the effects of the basket area wich is next to and around this wire. This basket area is having an effect similar to a linear load, which adds capacitance to this wire in series. This capacitance has the opposite effect of a loading coil, and requires this wire be made longer to compensate. That is why the models show a 1/4 wavelength current distribution on such a long wire. It is the same effect as adding a capacitor to the Imax model.

I should also point out at this point that the mast is out of phase with the rest of the antenna.


These are the major parts of what I have noticed about this antenna model.


The DB

 

[Needle Bender]

Needle Bender, if you left your post while creating your text and then came back to the thread and your post was gone...I think you can do a refresh and the text might pop back up in your post reply box.

Well, I hit "Post Reply" and POOF! it was gone.

The AP feed point is direct-drive and therefore must be about 50Ω making it likely a max current node, then it goes down, around & back up where it shorts directly to the grounded mounting bracket so it's rather necessary that point be max current/ MINIMUM voltage.
- For that to be the case it would need to be an electrical 1/2 wavelength from the feed point.

I still believe it's merely a folded 3/4 wave - finished by a cap hat, and having zero mast should make zero difference. I also believe you're getting incorrect SWR values from your model.

...looking forward to trying it.

 

[The DB]

I'm not looking to argue here, I am just making comments and pointing out a concern or two.

I still believe it's merely a folded 3/4 wave - finished by a cap hat

On one side of the feed point I agree with this entirely and have said as much above, electrically that part of the antenna is 3/4 wavelengths long.

I also believe you're getting incorrect SWR values from your model.

OK. All I can say is it seems to match things that people are saying about the patent. I guess I should look it up, anyone happen to have a direct link to said patent for me?

The AP feed point is direct-drive and therefore must be about 50Ω making it likely a max current node

When modeled with the mast this is the case. However, when I remove the mast, even when tuned, the models no longer shows the feed point to be a current node, or perhaps it is just with far less current flow at said feed point. The currents on either side of the feed point are about half of the antenna's peak currents. It also shows both sides of the "basket area" are now out of phase with each other. I think the lower current flow on this part of the antenna is due to the fact that the current nodes you are referring to on either side of the feed point are 180 degrees out of phase with each other, feed points want the current on both sides to be, among other things, in phase with each other. That is just one of the natures of the feed point.

The vertical element, with its cap hat, is still max current, and shows about twice the current flow of the "basket area" of the antenna. Modeling shows this as bringing SWR up to about 3 after tuning the cap hat size to adjust for a lack of a mast/feed line for currents to flow on. I was able to get SWR down to about 1.5 by using a 1 inch vertical above the "basket area" and four wires each about 110 inches long.

This layout (using the original dimensions, not the SWR adjustment version of the model) shows only slightly less gain than the antenna with a mast, less than 0.5 dB difference, so should the model be wrong when it comes to tuning and it does work as you envision, modeling predicts it will perform very close to the same.

I guess my biggest concern with this idea of yours is you are removing a part of the antenna that a significant amount of current is flowing on and expecting it to make little to no difference. If you can get a wire that has little to no current flow removing it is fine, but the more current flowing on a wire the more of a difference it will make over all.

My next biggest concern is this wire with significant currents flowing on it is directly attached to the feed point, and you are expecting no change in the feed point impedance when removing this current carrying wire.

When it comes to your experimenting, I hope it goes well for you. Playing with this antenna's model, I kind of have the urge to make one and play with it myself, if I can find the time this summer anyway...


The DB

 

[Marconi]

Well, I hit "Post Reply" and POOF! it was gone.

I believe you NB. You said the same thing earlier and that is why I posted a suggestion.

The AP feed point is direct-drive and therefore must be about 50Ω making it likely a max current node, then it goes down, around & back up where it shorts directly to the grounded mounting bracket so it's rather necessary that point be max current/ MINIMUM voltage.
- For that to be the case it would need to be an electrical 1/2 wavelength from the feed point.

I still believe it's merely a folded 3/4 wave - finished by a cap hat, and having zero mast should make zero difference. I also believe you're getting incorrect SWR values from your model.

...looking forward to trying it.

I don't think I have posted any models in this thread recently, at least not since Bob anounced his new old A/P install at his location. You must be talking to The DB in your added comments.

Right after Bob made his post I used a A/P model I had saved and I did some work on various mast lengths below the hoop. However I got to feeling bad and stopped. So, I'm far behind in this thread.

The other day DB posted some pattern results and I was very surprised at the antenna gain he reported. So, I got back at it for a couple of days trying to figure out what he was doing to get such high gains.

When he posted his updated model, "adjusted to be the same dimensions of the actual antenna," I could see one noticable difference. He is placing his feed point on the bracket wire, my wire #2 below. I place my FP on the radial that goes down to the hoop, top of wire #3.

I tested his idea but I got a very bad Average Gain result. I suspect this is likely the big reason for his high gain.

NB'r I post this model below. I used my dimensions so that could account for some differences in results, but like DB said of Ghz24's model...maybe it is close enough.

My point here is to help support my idea for Bob's claim that this antenna does not do so well without a mast in the radial area.

Below are three PDF files for my model (Astro P DB's FP 32'), over real Earth. Also note I have added my antenna notes for each model showing my wire length dimensions, and some comments for what I was considering.

1. has the feed point located on the bracket wire at 32' feet. In my model this is wire #2 in the close up view.

2. is the same model as #1 but it is in Free Space and has no supporting mast, only the mast inside the radials and just for consideration for various lengths modeled...it extends below the hoop approx 108" inches.

I also added an image of the bad Average Gain Result I get when I set the feed point on wire #2 as apposed to the top of wire #3...where I think is approximately where the A/P feed point should be located.

3. is the same as above but without any mast at all for the model like I think NB's might be suggesting is possible.

 

[The DB]

Just for kicks, I am comparing the AP at two different heights to my I-10k model.

The green line is the I-10k (actually it is closer to the Shockwave version).

The red line is the Astroplane mounted at the same tip height as the I-10k.

The blue line is the Astroplane's mount as the same height as the I-10k's mount.

I know that their have been claims that the Astroplane was designed to be mounted at a height limit that used to (still does) exist for CB antennas. That is why I modeled a tip height comparison as well as a feed point height comparison.


The DB

 

[Marconi]

DB, it is an amazing antenna on paper. I even realized it in my real world experiences. I was just not very convincing in my claims and I'm sure there were a lots of doubters too. Evidence of doubters is Sirio stopped making their New Top One.

Homer also made good tests and saw good results with his plastic and aluminum tape homemade A/P.

I think Bob even commented it surprised him, but I would like to hear more about his results. I told him I liked the A/P installed up high.

I'm sure he is busy checking this one out...figuring out how if works and why.

 

[The DB]

Hey Marconi, I was hoping you or Bob85 would chime in on my models at some point. I know you two, as well as a few others, have done far more work than I on this antenna in the past. I am the latecomer to this show.

The other day DB posted some pattern results and I was very surprised at the antenna gain he reported. So, I got back at it for a couple of days trying to figure out what he was doing to get such high gains.

When he posted his updated model, "adjusted to be the same dimensions of the actual antenna," I could see one noticable difference. He is placing his feed point on the bracket wire, my wire #2 below. I place my FP on the radial that goes down to the hoop, top of wire #3.

I tested his idea but I got a very bad Average Gain result. I suspect this is likely the big reason for his high gain.

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.

Here is an example of one of those graphs (their are a few posted above).

Here the SWR range is on the left, and the length of the mast in inches is across the bottom. With this I can effectively map out the results of making various changes to an antenna.

where I think is approximately where the A/P feed point should be located.

When it comes to feed points, we definitely have different ideas and different methods. We have had that discussion in the past.


The DB

 

[HomerBB]

I have claimed that my experience with my AP was it performed as well as or better than my 5/8 waves. DBs model overlay lends support to my personal experience.
I do not know the limits of modeling, but I do know that as a common dipole is split in the middle so that the feed point is actually feedpoints, each wire fed at their nearest point to each other. I also know the AP is not fed in the middle of the top bracket, but is fed by the feedline being split at the actual connection point at the top of the short vertical. The SO 239 connector there delivers current to the short vertical via the center coax wire and to the top bracket via the braid.
I think Marconi's placement is nearer the truth in modeling on this point.
However, unless the model shows the bracket isolated from the short vertical at that point except through the loop, and the bracket fed from that END, the model isn't correct at all. Without a correct feedpoint placement and current delivery at the feedpoint (s) it will just be a bent up long wire. Maybe the blue line wont be so blue and hum a happier tune if the feedpoint and isolation setup in the model mimics the real antenna.
My thoughts ...
Homer.

 

[Marconi]

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. Like I've already mentioned, at the time I had no idea we had such a difference in the location of the feed point on our models. I know that ghz24 did a whole slew of CB antenna models using 4Nec2, and he likely did the A/P. Maybe he located his feed point on the mounting hub as well.

But, like I said above...I'm behind in this thread and I could have missed something. You may have made the height and location of your feed point for your A/P clear at some point.

I did try various antenna heights, including 2 wl and I did see somewhat similar patterns on raising, but no height came close to producing the gain you reported.

I also changed the feed point in my model to your position on the hub. The model is set at 32' feet not 72' feet. You'll notice the model set lower still shows a high gain even at normal heights for CB. This tells me that height is not the cause of you higher gain.

IMHO this high gain result is suggestive of the high 2.009 = 3.03 dB AG my model reported.

5.29 - 3.03 = 2.26 adjusted net gain for this model seems to be more correct. According to Eznec manual...this is possibly due to error in the feed point location. See attached section on Average Gain in the manual below.

AGT was in line, actually dead on with a 1. I used a trick that ghz24 showed me to achieve that.

I can't question your resuilts, but I'm trying to figure this all out. So, I sure would be interested in the trick ghz24 showed you.

The higher than normal gains were because I mounted the antenna very high.

That is obvious DB, but there is something else going on here. Check out my FS model above. Any Astro Plane Fans ? It has the Feed Point in the same location you use and it too shows nearly double the gain of a dipole at 0* degrees. In FS, height is not an issue, so for me this rules height out as the sole cause of the high gain.

 

[The DB]

Homer, in modeling, the feed point has a wire that appears to go through it. The reality is, however, that both sides of the feed point are isolated from each other. Where you see a feed point (aka circle) in the model you have to imagine a break. Their is no actual physical connection from one side of that feed point to the other. In my model, the one side of the feed point is as you say, connected only to the side of the basket area.

Marconi's software may be able to show this so the feed point appears with a break, however, the software I use does not do this. Call it a limitation with the software.

Here is my astroplane model in freespace.

First AGT info.

Regular data without AGT turned on.

RF pattern in freespace.

hat is obvious DB, but there is something else going on here. Check out my FS model above. Any Astro Plane Fans ? It has the Feed Point in the same location you use and it too shows nearly double the gain of a dipole at 0* degrees.

I am not sure what is going on with your model, I am seeing slightly more gain than a dipole in freespace, but definitely not twice the gain.

So, I sure would be interested in the trick ghz24 showed you.

As I was telling Homer above, the wire that crosses through the feed point does not actually exist. So if your feed point is on a single segment wire, you can change the diameter of that wire, and by extension have some control over AGT, without affecting the other parts of the antenna.


The DB

 

[Marconi]

Homer, check this model of a center fed dipole. It shows us the gain, angle, and match that one would expect for a 1/2 wave CFD in free space.

2.14 dbi, 0* degrees, R = 72.04 ohms - X = 0.007716 ohms, 1.441 SWR at the feed point. The litertaure is full of this FS result.

You will notice that I use only 1 wire in this example and the feed point is located at the center of this dipole as described. You will see two red (O) here however, because I used the Eznec Split Source feature that gets closer to the ends of the wire segments for two adjacent segments. This is an accuracy tweek only. If I did not use this feature...the model would still show almost identical results, but there would be a small difference to be noted.

This is how the A/P can be fed at the center of the 1/2 wave radiator where we would otherwise expect to see a open feed point, silimiar to what you describe in real life.

I can model very similar results using two wire connected at the center just as well. DB, 4nec2 likely works the same, but DB has told me that he has to physically create two wires in his models...just like I think you used in your A/P depected in this thread where you used PVC in part.

If the NEC idea for a feed point did not work like we see it connect in the real world then this model would not produce the results we see.

If this doesn't make any sense to you...let me know, OK?

Note; I added my Average Gain results for my model as an edit.

 

[HomerBB]

Homer, in modeling, the feed point has a wire that appears to go through it. The reality is, however, that both sides of the feed point are isolated from each other. Where you see a feed point (aka circle) in the model you have to imagine a break. Their is no actual physical connection from one side of that feed point to the other. In my model, the one side of the feed point is as you say, connected only to theside of the basket area.

Marconi's software may be able to show this so the feed point appears with a break, however, the software I use does not do this. Call it a limitation with the software.

In that case, you'll need to move your feedpoint to the top of the short radial to maintain model integrity to the real antenna, to admit Marconi is right on this point, and to see if it makes a difference. Not necessarily in that order.
I'll be watching for the change.