TV antenna dish: depth, f/D, or focal distance

  • #1
nomadreid
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Summary:
What is the approximate depth, f/D, or focal distance of a standard TV 20 inch (58.8 cm) parabolic (offset) antenna?
I am covering parabolic antennas with a school student in elementary physics, and I was looking for the specs of the (offset) antenna on her family's roof in order to calculate things like focal distance. She sent me a photo (she is on another continent), I found the brand specs, which lists it as a 20 inch antenna.... but I could find nowhere the depth or focal distance, or f/D ratio listed. Not being an engineer, I am probably looking incorrectly.

I am not sure whether it is allowed to list a particular brand here, so out of caution, I won't. But if someone tells me it is allowed and desirable, I will post the precise model.
 

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  • #2
Baluncore
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It would help if we had a link to the particular model.
The focal length is the distance from the focus to the vertex. An offset dish is designed so the shadow of the LNA at the focus, and LNA support do not fall on the dish surface, so the vertex can be hard to find or missing. The approximate vertex is probably the surface of the dish that is closest to the focus support.
 
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  • #3
nomadreid
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Thanks, Baluncore. The model is:
Dish Network Satellite 500 KIT Pro Twin LNB Antenna 110 119 DP LNBF DishPro plus
 
  • #4
Tom.G
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I tried tracking down the information on their website. Pointless, all they have is consumer information there. However I did find a page with a link for support with live chat:
https://my.dish.com/support/contact

Another possible approach is try to get a side-view photo taken at the same distance with the same camera and zoom setting. Knowing the dish diameter, you may be able to do some photogrammetry and calculate the distance from the dish surface to the receive head. Try a Google search for Photogrammetry Software (there are even some free ones).

Please let us know any results, good or bad, so others may benefit from the knowledge.

Thanks,
Tom
 
  • #5
sophiecentaur
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I don't know if this would help but this link suggests a formula for an offset dish
f=D2/16c
where
f is the focal length of the reflector
D is reflector diameter in same units as wavelength
c is depth of the reflector

It looks just like a rule of thumb but it may be near enough for you.
 
  • #6
nomadreid
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Thanks, Tom G. and sophiecentaur.

Tom G.: the link you gave appears to be broken: I tried it on two different browsers, two different computers, two different days. I tried going to the website directly, same result. Timing out, cannot connect to this site, Error message, etc.

sophiecentaur: Thanks, but it is precisely the value c which is missing.
 
  • #7
dlgoff
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but it is precisely the value c which is missing.
I would think you could make a fixture to measure this.
From: https://askinglot.com/how-do-you-measure-the-depth-of-a-parabolic-dish

Place a rigid straight edge across the dish and measure the depth to the center of the dish keeping the measuring instrument square to the straight edge. The measurements should be of the actual parabola. Do not include the rolled edge of the dish in your measurement. When measuring, be as precise as you can be.
 
  • #8
Tom.G
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Tom G.: the link you gave appears to be broken: I tried it on two different browsers, two different computers, two different days. I tried going to the website directly, same result. Timing out, cannot connect to this site, Error message, etc.
Hmm... Still works here in the USA. They could be selecting which countries they will accept. Their list of language packages includes English, several European, Hebrew, some Asian and Asian-Pacific Islands, and a few African.

It would be worth trying to get access from a computer using a different ISP.
Also if you are using an Educational account, those could be blocked.

Here is their contact info as listed by
https://www.networksolutions.com/whois/results.jsp?domain=dish.com
Registrant Name: Dish Network LLC
Registrant Organization: Dish Network LLC
Registrant Street: 9601 S MERIDIAN BLVD
Registrant City: ENGLEWOOD
Registrant State/Province: CO
Registrant Postal Code: 80112-5905
Registrant Country: US
Registrant Phone: +1.3037231000

Cheers,
Tom
 
  • #9
tech99
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Summary:: What is the approximate depth, f/D, or focal distance of a standard TV 20 inch (58.8 cm) parabolic (offset) antenna?

I am covering parabolic antennas with a school student in elementary physics, and I was looking for the specs of the (offset) antenna on her family's roof in order to calculate things like focal distance. She sent me a photo (she is on another continent), I found the brand specs, which lists it as a 20 inch antenna.... but I could find nowhere the depth or focal distance, or f/D ratio listed. Not being an engineer, I am probably looking incorrectly.

I am not sure whether it is allowed to list a particular brand here, so out of caution, I won't. But if someone tells me it is allowed and desirable, I will post the precise model.
For an offset paraboloid, in concept, the reflector is cut from an off axis portion of a very large reflector, so the simple formula relating dish depth and focal length will not work.
 
  • #10
sophiecentaur
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sophiecentaur: Thanks, but it is precisely the value c which is missing.
If the student can't get to measure the depth of the dish then you would have to use a 'typical value' for f/D. But do you really need this information? The length of the feed support is a good indicator of the actual focal length f of the parabola. To estimate that remotely then take a photo from the side and, if the dish is a typical 20" (60cm) diameter then simple geometry and scaling can tell you the spacing of the feed from the centre of the parabola. Most offset dishes for domestic TV have the centre of the parabola on the edge of the reflector, just below the feed.

A similar exercise can give you a good approximation for finding f/D if you feel you really need it. What is the context of the OP? It's a school exercise so it will not be too technically demanding.
 
  • #11
nomadreid
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Thanks very much, sophiecentaur, tech99, TomG and dlgoff. Some comments:

For an offset paraboloid, in concept, the reflector is cut from an off axis portion of a very large reflector, so the simple formula relating dish depth and focal length will not work.
I have found that out, so, if I understand correctly, the "20 inch" is not uniform, but I need the width and the height (by which I presume is meant the maximum and minimum diameters of the border?) , so that a widespread formula would be Focal length=(width^3) /(16*depth*height) (Taken from http://docshare01.docshare.tips/files/23977/239773115.pdf) , although there seems to be several other formulas proposed on other sites.

I would think you could make a fixture to measure this.
From: https://askinglot.com/how-do-you-measure-the-depth-of-a-parabolic-dish
Unfortunately, I do not have direct access to the dish -- it would be about ten thousand kilometers of flight -- and I am not about to ask an eleven-year old pupil to go out on her roof to measure it.

Hmm... Still works here in the USA. They could be selecting which countries they will accept. Their list of language packages includes English, several European, Hebrew, some Asian and Asian-Pacific Islands, and a few African.

It would be worth trying to get access from a computer using a different ISP.
Also if you are using an Educational account, those could be blocked.
I tried from a different ISP without success, and so that is not the problem. I am not sure what the language package has to do with it -- but the language of the country I am writing from is included in that list. I am not using an Educational account. The idea of trying it from the US induced me to turn on my VPN for the first time in a long time, only to find out that there seems to be a glitch in it -- so I wrote "support", but that may be a while to get it straightened out, so I will just try to find an acquaintance in the US to try it.
If the student can't get to measure the depth of the dish then you would have to use a 'typical value' for f/D.
That would be fine, but I don't know what a typical value is. In https://www.tutorialspoint.com/antenna_theory/antenna_theory_parabolic_reflector.htm they say it varies between 0.25 and 0.5, but then a calculation in https://www.qsl.net/n1bwt/chap5.pdf the author uses a ratio of 0.69
What is the context of the OP? It's a school exercise so it will not be too technically demanding.
That is correct, I don't need answers to be down to the micrometer. Most of the calculations will actually be done by me before I present it, in simplified form, to the student.
The length of the feed support is a good indicator of the actual focal length f of the parabola. To estimate that remotely then take a photo from the side and, if the dish is a typical 20" (60cm) diameter then simple geometry and scaling can tell you the spacing of the feed from the centre of the parabola. Most offset dishes for domestic TV have the centre of the parabola on the edge of the reflector, just below the feed.
Excellent suggestion: that is probably my best bet, as the student has shared quite a good photo of it from as close as she dared get to it. I will probably do that ( as it is her photo and I have not asked for her permission to use it publicly, I guess I don't have the right to post it here). I shall give that a try in the absence of the direct specs.
 
  • #12
Baluncore
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Excellent suggestion: that is probably my best bet, as the student has shared quite a good photo of it from as close as she dared get to it.
I still think you should do what I suggested in post #2. Measure or estimate the distance between the focus and the point on the dish surface closest to the focus support. That will be close enough for an educational exercise.
 
  • #13
nomadreid
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Measure or estimate the distance between the focus and the point on the dish surface closest to the focus support.

Just to see if I understand this, I refer to a generic picture (this is not the antenna concerned, but a photo with no brand names on it, so as not to trespass any legal boundaries with which I am not familiar): are you saying that the focal distance is the length of the bottom bar in this picture?
offset antenna.jpg
 
  • #14
Baluncore
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are you saying that the focal distance is the length of the bottom bar in this picture?
Approximately yes, but it will depend on how you mount the LNA at the focus.

Find the point on the dish where a normal to the dish surface passes closest to the focus.
That is the vertex. It will be close to where the bottom bar is attached to the dish surface.
Measure the distance from the vertex to the focus.
 
  • #15
sophiecentaur
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I ask again what is the context of the enquiry. Why would someone need the f/D ratio unless it was for some relatively advanced comms communication. If it's just to study the action of a parabolic reflector then is the Gain value required? There's a lot of chat about that sort of thing but, as you've already found, manufacturers and suppliers don't want you to 'worry your pretty little head' about that sort of thing if you're 'just' a potential customer. Too many details would enable you to have a valid opinion about their stuff. Like HiFi manufacturers, they thrive on customer ignorance.
If you leave aside the offset issue and looked at what's written about conventional reflectors - e.g. many radio telescopes and microwave link terminals - you might find the sort of general knowledge that your guy may actually be needing.
 
  • #16
hutchphd
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I still think you should do what I suggested in post #2. Measure or estimate the distance between the focus and the point on the dish surface closest to the focus support. That will be close enough for an educational exercise.
If I understand it, this would be exactly correct if the dish were an offset cut from a hemispherical collector. As long as the dish is not too deep this is probably adequate. The spherical reflector is a whole lot easier to understand so that may be a pedagogical plus.
 
  • #17
sophiecentaur
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But what does the student actually want to know and why? Are we actually addressing the right question here?

Perhaps the student needs help with formulating a meaningful question. I remember being asked some crazy questions when I was teaching.
 
  • #18
dlgoff
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I am not about to ask an eleven-year old pupil to go out on her roof to measure it.
I would say it depends on the eleven-year old.
 
  • #19
sophiecentaur
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So what are the terms of the study that this 11 year old is doing. Why is f/D relevant?
 
  • #20
berkeman
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It looks like the mounting ring for the LNA points directly at the important point in the dish, no?

1630369768827.png
 
  • #21
Baluncore
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It looks like the mounting ring for the LNA points directly at the important point in the dish, no?
That depends on why a point is "important". In an offset dish, the LNA certainly does not point at the vertex.

The focal element points at the centre of the aperture, which is the area of surface that is present. Think transmitter; The antenna element at the focus illuminates the dish surface. Best efficiency per dish area is when the surface is evenly illuminated by the centre of the primary lobe from the focus.

Offset dishes are not usually round, they are elliptical, but not quite as much as the 45° diagonal mirror at a 90° bend in an optical system.

Obviously, the rolled edge of the dish increases the rigidity of the surface so it remains an accurate paraboloid. But less obviously, it introduces a gradual phase error at the edge of the aperture that has been carefully designed to reduce the side-lobes by blurring the sharp step edge of the aperture at the wavelength of operation.
 
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  • #22
nomadreid
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Thanks, Baluncore, sophiecentaur, hutchphd, dlgoff, and berkeman. Some comments:
However I did find a page with a link for support with live chat:
https://my.dish.com/support/contact

There's a lot of chat about that sort of thing but, as you've already found, manufacturers and suppliers don't want you to 'worry your pretty little head' about that sort of thing if you're 'just' a potential customer. Too many details would enable you to have a valid opinion about their stuff. Like HiFi manufacturers, they thrive on customer ignorance.
Indeed, I had a friend in the US contact the company involved and ask for the depth, focal distance, and/or f/D ratio, and when, in response to the chat request for a 16-digit number on the bill, my friend explained that it was for an example for a science class, nothing more, the company refused to give any information. (The company has just dissuaded my friend from ever buying anything from it.)

I ask again what is the context of the enquiry.

But what does the student actually want to know and why?

Perhaps the student needs help with formulating a meaningful question.
The student herself has not posed a question: it is I who hoped to find some meaningful questions based on a real-life example. In general I try to know more about a subject than I am going to ask the student to know, so that my simplification is not too simple (e.g., billions of books on quantum theory get boiled down to "physics of the tiny"....) and that I will be ready to answer any questions the student may have, even if I need to simplify them. The student is only 11 years old, and is not (yet?) into electrical engineering, so much of the interesting and elsewhere useful information presented here, for which I am very grateful, is not going to end up in the final questions that I will pose the student (or other students), but is nonetheless worthwhile for me.

Finally, having put my cursor incorrectly, one quote got embedded in another, and I don't see how to delete the quote-references:
 
  • #23
Baluncore
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Finally, having put my cursor incorrectly, one quote got embedded in another, and I don't see how to delete the quote-references:
Click on the [ ] in the reply box toolbar to toggle BB code. Then edit and move the text.
 
  • #24
sophiecentaur
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The student is only 11 years old, and is not (yet?) into electrical engineering,
In which case I would say that the level of demand that your question to the student would be far too high and would introduce something very far down that particular line of enquiry. At that age, the idea of ray tracing to show how a parabolic dish basically works (laws of reflection to produce a point image from parallel rays) and an idea of why size matters would be, imo, more than enough. The amount of energy intercepted by a dish of that area could be introduced but the numbers involved are so big and so small that there will be a lot of zeros floating around. This is why we use dB in this context but would an 11 year old cope with dB (a lot of adults are totally flummoxed by them)?
 
  • #25
nomadreid
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Click on the [ ] in the reply box toolbar to toggle BB code. Then edit and move the text.
Thanks. Also thanks that you have removed them from the visible post. (Interestingly enough, they reappear when I press "edit".)
introduce something very far down that particular line of enquiry. At that age, the idea of ray tracing to show how a parabolic dish basically works (laws of reflection to produce a point image from parallel rays) and an idea of why size matters would be, imo, more than enough.
Indeed, I am starting quantitatively on finding the focal distance of a simple parabola, and then only qualitatively discussing the idea of cross-sections of a dish that will give two different parabolas, and why this will alter the focus, and why, roughly, one even wants an offset antenna. (As I work out the information that, for example, Baluncore wrote in #21, may or may not figure into the qualitative explanation, but the information will at least increase my understanding.) Most of the quantitative information I was seeking were for me to work out the details to see if any of them would be worthwhile to present, either in qualitative or quantitative form.
This is why we use dB in this context but would an 11 year old cope with dB (a lot of adults are totally flummoxed by them)?
The exact mathematical relationships are indeed too advanced for an eleven year old; in schools decibels are first introduced with a brief explanation of a decibel being used as "a measure of intensity, for example, of sound" (because that is where they will first encounter it) as an example of a logarithmic scale, and only a few years later, is the definition connected to power quantities; the kind of depth you are referring to is usually left for the university.
 

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