I Qs re aspects of the Holmdel Horn Antenna used to find the CMB

[sophiecentaur]

Hi Sophie:

You may not believe this, but in my long technical career before retiring, I actually did come to know what works for me, and what is a waste of my time. I also came to know colleagues who had different roads to success. A principle I feel is generally true is that there is more than one right answer to just about everything, and especially about the right way to do things.

Regards,
Buzz

Yet again you have proved me right about your lack of the essential basics. That diagram of yours is total nonsense. It is an offset paraboloid. (Look it up!) What good would a focus outside the horn do? The term Offset Paraboloid was mentioned earlier (also in that article iirc). The focus (of course) is at the feed antenna. Where else would it be?
Perhaps, by the end of your "long technical career" you have managed to assemble some useful stuff but I would guess it took you a long time to assemble what you (think you) know.
I will stop answering this thread because you clearly haven't taken much on board and you seem to be determined to do things without help.

 

[davenn]

On the figure below the red line is the axis of the parabola. The top end of the red line is the focus of the parabola with respect to rays moving parallel to the red axis. My question is a mathematical one about a property of parabolas. Is the distance (a) from the focus at the end of the red line to the center of the parabola, the same distance as (b) the estimated 10-12 m of the offset focus for rays moving vertically downward in the diagram?

No this is incorrect
It would be incorrect IF it was a normal full dish antenna

Go read up on offset fed dish antennas ... there's lots of ray diagrams and info on the net
You really start need to read up on this stuff.

Another question: The photograph in post #22 shows the monopole antenna in the shed rather than inside the horn. Is this correct? Or does the horn continue into the shed, and the monopole is inside the end of the horn?

it's inside the horn, right at the end of the horn as I drew in my diagram (which is inside the shed)

 

[Buzz Bloom]

Go read up on offset fed dish antennas

Hi Dave:

Thank you for mentioning "offset fed dish antennas". I found several articles that I was unable to find without knowing the proper vocabulary.

https://en.wikipedia.org/wiki/Offset_dish_antenna
http://www.qsl.net/n1bwt/chap5.pdf
http://www.john-legon.co.uk/offsetdish.htm​

The third one had a particularly useful picture.

The term "offset paraboloid" in the last post from @sophiecentaur was also helpful, although he was mistaken that the term had been in a previous post. (I could not find it by searching through all the thread pages. I also could not find the reference he referred to as "iirc".)

The reason I asked you about the equality of focal lengths from different angles was because my intuition told me that they should be different, and also that the cross sections of the offset dish would be elliptical rather than circular. I tried to confirm this by doing the math, but it was to difficult for me. I was able to deduce that the focal length F for a circular parabolic dish was

F = 1/(4a),​

where the parabola is defined by

y = a x².​

Unfortunately that was as far as I could take the math.

Thank you again for all your help and for your patience.

ADDED
Here is a better photo of the Holmdel. It is much clearer to see its geometry.

Regards,
Buzz

 

[sophiecentaur]

lthough he was mistaken that the term had been in a previous post.

The reflector is a segment of a parabolic reflector, so the antenna is really a parabolic antenna which is fed off-axis.

It was quoted in a post of your own - quoting from that paper with the photo and other info. (I assume that off-axis and offset are near enough for the meaning to have got across).
Edit: "Off Axis" is not a good term for it as the focus is always on the axis of the parabola. It's just that only part of the parabola is actually there. Off axis distorts the image.

 

[Buzz Bloom]

"Off Axis" is not a good term for it as the focus is always on the axis of the parabola.

Hi Sophie:

As I have come to understand this, the axis of the reflector is perpendicular to that point, P, on the surface of the reflector that has the greatest curvature. Since the cross-section is not a circle but a ellipse, the curvature refers to the maximum curvature in both the major and minor semi-axis directions. If you imagine a flat mirror at this point tangent to the surface, the parallel incoming rays that hit the mirror at an angle relative to the axis, θ, are all reflected in parallel towards the focus, F. The line between F and P is also at an angle θ relative to the axis, on the opposite side of the axis than the incoming ray that hits P.

This explanation is well illustrated in the "Figure 5-1" diagram in post #62.

It was quoted in a post of your own

I confess that I tend to read technical things in a formal way, and that may well cause me to miss the proper use of vocabulary. I failed at the time to interpret

"a parabolic antenna which is fed off-axis"​

as being the proper phrase from which to choose the words to use for an Internet search.

Regards,
Buzz

 

[sophiecentaur]

I confess that I tend to read technical things in a formal way, and that may well cause me to miss the proper use of vocabulary. I failed at the time to interpret
"a parabolic antenna which is fed off-axis"as being the proper phrase from which to choose the words to use for an Internet search.

Yet again you are proving that your approach is not inherently successful. Why not do this thing properly?

 

[Buzz Bloom]

Yet again you are proving that your approach is not inherently successful. Why not do this thing properly?

Hi Sophie:

I do not think it will be fruitful for us to continue to debate "what is the proper way" for me to learn new things as a hobby. Although I have a large number of friends, some of whom have technical backgrounds, none are interested in my particular technical hobbies. I also do not have a good technical library easily available. I have found the PFs very helpful regarding most of the topics about which I have sought help. A lot of the help has been citing specific references, and/or suggesting particular key words to use when searching the Internet.

When I have, for various topics, made an effort to get my local librarian to find a copy of a textbook and arrange for an inter-library loan, it generally has not been particularly helpful. The reason, as I see it, is that the purpose of the textbook is to teach a broad subject in an organized fashion. The main difficulty I have had in this thread, aside from the errors in my calculations, is the use of vocabulary which I found confusing. When I use a textbook about a broad subject, I make an effort to generally read the few chapters that directly relate to the problem that interests me. Frequently there are terms I don't know. Text books generally do not seem to believe that a glossary is useful. I use the index, and after some searching, I find the definition of the term I did not understand, Unfortunately, the definition I found uses more terms I do not know, and the chase continues for more levels than I can manage. I agree, that to understand a non-elementary topic in a textbook, I would need to carefully read and absorb the entire book, at least up to the chapters that contain the specific narrow topic of my interest. I find it takes months to do this, and my skills deteriorate as I age.

If I had found much sooner the diagram and photograph posted in post #63, I would have been able to understand Dave's and your discussions much sooner. However, the 66 post process still took much less time than reading a textbook would have taken, assuming I had the where-with-all to finish it.

Regards,
Buzz

 

[sophiecentaur]

I do not think it will be fruitful for us to continue to debate "what is the proper way" for me to learn new things as a hobby.

Perhaps not. It is clear that your method leaves so many holes in your knowledge that my comments are demonstrably valid.

Unfortunately, the definition I found uses more terms I do not know, and the chase continues for more levels than I can manage.

Which is just my point. There are many aspects of Science and Engineering that are interesting and entertaining and appreciation at a superficial level is a good thing. But you can't dip in, here and there and be surprised when you come to a problem you cannot solve. One doesn't attempt a concerto before being able to do perfect scales and arpeggios.
BTW, I am not a girl. I am a boy - see my profile.

 

[Buzz Bloom]

BTW, I am not a girl. I am a boy - see my profile.

Hi Sophie:

I did look that up and made an effort to use masculine pronouns. I apologize for my failure to correct all of my gender errors.

But you can't dip in, here and there and be surprised when you come to a problem you cannot solve.

I confess there have been some problems I posted questions about that I failed to ever understand. (For example, I have completely given up regarding quantum mechanics.) However, these have been relatively few. Most of the questions I have explored I have learned what I wanted to learn.

Regards,
Buzz

 

[sophiecentaur]

I apologize for my failure to correct all of my gender errors.

No problem at all. I just thought you should know a bit about who you are talking to.