Any Astro Plane Fans ?

[Riverman]

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 j=0

lets say for example you got very lucky and had an ideal situation for feeding the antenna with 50ohm coaxial cable X=50 with the imaginary reactive part J =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,
X goes down & the imaginary part J 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 j=0,

you have resonance again j=0 but X won't be 50ohms, could be something like X=30 J=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

I know all those terms from this and other discussions. And in my studying. Now it all fits.
Thanks for your help! Great article.

 

[The DB]

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 j=0

lets say for example you got very lucky and had an ideal situation for feeding the antenna with 50ohm coaxial cable X=50 with the imaginary reactive part J =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 J 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 j=0,

you have resonance again j=0 but X won't be 50ohms, could be something like X=30 J=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

Great response, except, change all those X= to R= and the J= to X=... At least that is how most antenna analyzers, as well as many books on the subject, would show that data...

It's whatever is needed to bring feedpoint impedance to 50 ohms or so.

That on the right track?

It depends. I've shown above somewhere (pages ago in this thread, will have to fish it out) that if their is no capacity hat on this antenna, instead you use a 1/4 wavelength vertical and tune to resonance the impedance changes at resonance. Remember, a capacity hat is a load, just like a loading coil. In the case of this antenna and using a different cap hat, I would be prepared to not only change the length of the elements of the cap hat, but the length of the upper vertical element that leads to it as well. With this you can maintain resonance (X), and still have a means of control over impedance (R) as well.

Specifically with this antenna, Oscar said he used no capacity hat on his home made Astroplane. I also remember him saying something about a suggested ring size? I don't know if he made any other adjustments to his antenna design, or even if what I think I am remembering is in response to his lack of a cap hat in his design. It is a change I would likely make if I removed said cap hat from this antenna design...

When it comes to engineering capacity hats, I have some formulas for various shapes of capacity hat in the various versions of the ARRL Antenna Books from over the years. It is possible to calculate how large of a cap hat you need based on the design that you intend to use. If it were me, I would model it and use that as a starting point. HomerBB, on the other hand, would likely build it and make it so it was adjustable in both length of element before the cap hat, and length of the elements on the cap hat, and tune till he got what he wanted. I can't say any one of these methods will yield better results than another, however it is listed in the order from more of an engineering way of doing things to more of a hobby way of doing things.


The DB

 

[bob85]

"change all those X= to R= and the J= to X="

i agree, i don't even know why i typed that, half awake,
i hope riverman can follow what i meant.

i do get things ass backwards

 

[Riverman]

"change all those X= to R= and the J= to X="
i agree.
i hope riverman can follow what i meant.

No worries. I noticed the designations were different but knew what you were talking about.

 

[HomerBB]

...
HomerBB, on the other hand, would likely build it and make it so it was adjustable in both length of element before the cap hat, and length of the elements on the cap hat, and tune till he got what he wanted. I can't say any one of these methods will yield better results than another, however it is listed in the order from more of an engineering way of doing things to more of a hobby way of doing things.

I guess I've become predictable...

 

[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?

Here you go, a four element cap hat AP vs a 3 element cap hat AP. the red line with the additional "tch" label is the three element cap hat.

• As you can see, only a minor difference in performance, the four element cap hat has the edge on gain.
• The angles of radiation are the same.
• The four element cap hat elements are 25.3 inches long, three element cap hat elements are 33 inches long.
• The three element cap hat has the edge here on SWR, but neither are particularly high, and if we really wanted we could adjust the four element cap hat and vertical length to compensate.
• The three element cap hat is slightly more efficient over all, but that efficiency gain is not pointed in an optimal direction hence the lower gain result from above.

Anything Else?


The DB

 

[Riverman]

Here you go, a four element cap hat AP vs a 3 element cap hat AP. the red line with the additional "tch" label is the three element cap hat.

• As you can see, only a minor difference in performance, the four element cap hat has the edge on gain.
• The angles of radiation are the same.
• The four element cap hat elements are 25.3 inches long, three element cap hat elements are 33 inches long.
• The three element cap hat has the edge here on SWR, but neither are particularly high, and if we really wanted we could adjust the four element cap hat and vertical length to compensate.
• The three element cap hat is slightly more efficient over all, but that efficiency gain is not pointed in an optimal direction hence the lower gain result from above.

Anything Else?
The DB

Cool. How about a cap made by someone who forgets
(or doesn't know) to lengthen the radials and simply reduces the number from 4 to 3?

 

[Oscar Goldman]

Here you go, a four element cap hat AP vs a 3 element cap hat AP. the red line with the additional "tch" label is the three element cap hat.

• As you can see, only a minor difference in performance, the four element cap hat has the edge on gain.
• The angles of radiation are the same.
• The four element cap hat elements are 25.3 inches long, three element cap hat elements are 33 inches long.
• The three element cap hat has the edge here on SWR, but neither are particularly high, and if we really wanted we could adjust the four element cap hat and vertical length to compensate.
• The three element cap hat is slightly more efficient over all, but that efficiency gain is not pointed in an optimal direction hence the lower gain result from above.

Anything Else?


The DB

Did you ever model the AP with the straight top section like mine has.

 

[Oscar Goldman]

Cool. How about a cap made by someone who forgets
(or doesn't know) to lengthen the radials and simply reduces the number from 4 to 3?

Then the resonant frequency would shift higher somewhat. Maybe around 100 to 200khz.

 

[Riverman]

Then the resonant frequency would shift higher somewhat. Maybe around 100 to 200khz.

Would shortening the straight top element in your antenna by 25.3" (length of a single element in DB's model) have similar results?

 

[The DB]

Cool. How about a cap made by someone who forgets
(or doesn't know) to lengthen the radials and simply reduces the number from 4 to 3?

The resonant frequency and low SWR point will definitely shift.

When it comes to gain, normally I would expect only a small difference from such a change, however, then I remembered some of the thoughts I had that date to the "blue line" discussion from pages ago, this would effectively cause one of the current nodes to change, and move away from what is an optimal location for it, causing said "blue line" to carry more current, and make the antenna less efficient overall. It still wasn't a big change, but it was more than I would normally expect to see...

Also, SWR on the tuned frequency went from 1.1 to 2.2. Radiation efficiency dropped by over 10%. R went down from about 55 to about 52. X went way down, from near 0 to -41.

Then the resonant frequency would shift higher somewhat. Maybe around 100 to 200khz.

Good guess, although the shift was larger, closer to 800 kHz.

Did you ever model the AP with the straight top section like mine has.

I modeled it at one point, it still worked, although the feed point impedance went up as R went from near 50 to closer to 70. I didn't do anything else to adjust the antenna to compensate. I have since made a change or two and modified the size and height of the cap hat to account for tuning changes just to see what would happen.


The DB

 

[Oscar Goldman]

When I removed the four spoke Hat I substituted a 35 1/2 inch pieces straight tubing to bring it back into resonance.
So the total height of the top section from
The top of the bracket to the tip is 82.75 inches.

 

[Oscar Goldman]

If you need any other measurements off my antenna, I will gladly provide them for you

 

[Needle Bender]

Cool. How about a cap made by someone who forgets
(or doesn't know) to lengthen the radials and simply reduces the number from 4 to 3?

If you forgot to lengthen the 3 cap hat radials but remembered to add a wire around the tip perimeter it might get you back in the ballpark.

 

[Needle Bender]

If someone would like to accept the query/challenge;
I'd be interested in seeing a model of an AP which has both a fully insulated bracket, and an additional horizontal 1/4 wave radial added to the bracket and extending out horizontally & directly away from the cap hat element side of the bracket,
& attached to the bracket near the coax connector.