Just Sharing This Calculation of the focal length of an offset satellite dish antenna, Revisited

[Archive-B]

I found that drawing through your inserted link of ftapinamar blog [Antenas Offset (8)]. And you wrote there

My motto would be: don't invest in programs that are not accurate, when there are others that are accurate: Parabola_6.2 by strannik, or offset_v1_0.xls by F6AGR. (And not to forget my own spreadsheet calculator, but that is not published yet.)

I made google search for Parabola_6.2 by strannik (search results lead to russian language websites) and offset_v1_0.xls by F6AGR ( first result of search prompt to .xls file download). Seems ftapinamar blog (spanish), strannik's posts (russian) and french .xls file are informative but not much useful for someone like me whose academic language is only English.

I could not find formulae for d1 and d2 in that french article (.xls) ! Found only numerical values of d1 & d2.

 

[a33]

Ah! So ftapinamar "borrowed" the drawing from the F6AGR spreadsheet, I think.

Thankfully, a computer can make translations automatically (google translate, deepl.com, ...)! FTAPinamar even offers the translation possibility on the website itself, so I use that.
Not all computer translations make sense, though, as some translations from russian are hard to interpret for me, I found. It helps when you understand some basics from the original language, as I thankfully do with some other languages (but not russian).

For parabola_6.0 and 6.2 there are (short) explanations how to use it, in some english and german (and maybe dutch?) spoken forums, if I recall correctly.

I could not find formulae for d1 and d2 in that french article (.xls) ! Found only numerical values of d1 & d2.

For the cel where d1 is given (D38), when you click (or doubleclick) on it, you'll find this equation:
=(((B93+(D21/2))^2)/(4*D32))+D32
When you follow the cells that are referred to in the formula, you can 'distill' the original equation out of it. (when you've doubleclicked, follow the colored cells! B93 is in row 93, and has a formula of its own...)

Greetz,
A33

 

[Archive-B]

You are right . Double clicking shows cell formula (equation).
When I first open it, I made the mistake of reading it like a pdf/word file by scrolling (top to bottom)

 

[a33]

Attached here is also the article, in which I describe the steps of the derivation: "Calculation of the Focal Length of an Offset Satellite Dish Antenna, Revisited". The base of the derivation is a special property of paraboloid dishes, that I discovered not long ago ~ but it might be common knowledge to mathematicians, I don't know.

I wouldn't expect that everyone would be able to follow the math in the derivation, but for all those who are interested, all the steps are there.

To add to the first post of this topic:

I forgot to add a plus-or-minus sign in the offset angle equation for a non-flat paraboloid offset dish.
So here is the original file again, with an extra page, in which the equation is corrected; to make it 'complete' again.

Also I added another form for the focal length equation, using depth at the center (for a flat, paraboloid dish).
That one is very much like the John Legon equation; with an extra correcting factor. So it might be easier to remember.

Here is the new file, with the addendum:
A33 Calculation of the focal length of an offset satellite dish antenna Revisited Version 1_1 Including Addendum.pdf

Greetz,
A33

 

[scottgs]

Please excuse me for dredging up an old thread, but I'm a new member here and I'm desperately trying to find the offset angle and focal distance of a new-to-me high-quality dish, and this thread seems to be right on target.

This is a non-flat offset feed dish. It came without the feed assembly, arm, etc., and so I have no idea what the offset and focal length are.

I see that there are photos of an excel calculator, but I cannot seem to find an actual excel file in this thread. So I'm wondering, am I just missing it, or perhaps if I put my data here, someone on the forum might be able to plug it in and tell me the results? I realize that I can work from the excellent .pdf explanation and formulas presented, but I would have more confidence from a tried and proven spreadsheet, I believe.

Here is the relevant data, if someone is willing to help me solve this mystery:

a. distance DE, height of the dish, from rim to rim (called 'Hh' further on):
1260 mm
b. distance CA, depth of the dish at the center, relative to the height measure ('dh' further on):
84 mm
c. distance FG, width of the dish, from rim to rim, measured above the center ('Ww' further on):
1268 mm
d. distance CB, depth of the dish at the center, relative to the width measure ('dw' further on):
95 mm

Additionally, there is a marker near the dish center, which I assume might be the aim point for the LNB, but I'm not sure. it is 584 mm from the bottom edge of the dish, along the line Hh, and the depth at this point is 85mm

Any help would be much appreciated.

Garret W8BUG

 

[Channel Hopper]

Please excuse me for dredging up an old thread, but I'm a new member here and I'm desperately trying to find the offset angle and focal distance of a new-to-me high-quality dish, and this thread seems to be right on target.

This is a non-flat offset feed dish. It came without the feed assembly, arm, etc., and so I have no idea what the offset and focal length are.

I see that there are photos of an excel calculator, but I cannot seem to find an actual excel file in this thread. So I'm wondering, am I just missing it, or perhaps if I put my data here, someone on the forum might be able to plug it in and tell me the results? I realize that I can work from the excellent .pdf explanation and formulas presented, but I would have more confidence from a tried and proven spreadsheet, I believe.

Here is the relevant data, if someone is willing to help me solve this mystery:

a. distance DE, height of the dish, from rim to rim (called 'Hh' further on):
1260 mm
b. distance CA, depth of the dish at the center, relative to the height measure ('dh' further on):
84 mm
c. distance FG, width of the dish, from rim to rim, measured above the center ('Ww' further on):
1268 mm
d. distance CB, depth of the dish at the center, relative to the width measure ('dw' further on):
95 mm

Additionally, there is a marker near the dish center, which I assume might be the aim point for the LNB, but I'm not sure. it is 584 mm from the bottom edge of the dish, along the line Hh, and the depth at this point is 85mm

Any help would be much appreciated.

Garret W8BUG

If you can post a couple of pictures of the reflector, somebody should be able to identify it, along with the correct feed parts required.

 

[scottgs]

Channel Hopper, thanks for the response. I'll post some photos later today.

In the mean time, I went ahead and created a new spreadsheet to calculate the offset and focal length for a non-flat paraboloidal-section dish reflector.

Here is the result with my 1.2m dish dimensions:

18.8 degrees offset
1190 mm focal length

At first glance this seems plausible. But, it's not verified. I'm going to run some more tests on the excel calculator to try and verify that it's working correctly, and maybe try out the dish with a temporary LNB, and if it seems to be the correct result, I'll post the excel calculator here in case someone else needs it.

Again, if anyone can help verify, that would be most helpful.

Garret
W8BUG

 

[RimaNTSS]

a non-flat paraboloidal-section dish reflector.

Here is the result with my 1.2m dish dimensions:

What do you mean by not-flat 1.2m dish? Why it is not-flat, is it warped? For sure, without knowing the brand and some pictures, it is not easy to understand.

 

[a33]

I see that there are photos of an excel calculator, but I cannot seem to find an actual excel file in this thread. So I'm wondering, am I just missing it, or perhaps if I put my data here, someone on the forum might be able to plug it in and tell me the results? I realize that I can work from the excellent .pdf explanation and formulas presented, but I would have more confidence from a tried and proven spreadsheet, I believe.

Here is the relevant data, if someone is willing to help me solve this mystery:

Thanks for the compliment for the .pdf.
My spreadsheet isn't made public (yet), I'm afraid. The goal is to add several more calculation methods (also for multifeed dishes, e.g.), sometime.

However, here are the results of your inputs. In line to what you calculated in the meantime, yourself. (But: see my edit addition below this post!)

When you mean to use it as a receive-only satellite dish, I would normally aim the LNB at the deepest point!
The opening angle of the dish seems a bit smaller, than with normal Ku-dishes, though. That might mean that aiming a bit lower might be worth testing, when you would be using a normal Ku-LNBf for offset dishes!



I hope you can work with this?

If it must be tested if the dish is not warped, quite a few more measurements must be collected. I have derived the needed equations somewhere in my notes, but I haven't put those equations in my spreadsheet calculator(s) yet.


Greetz,
A33.


Edit: I just noticed your 1190, noted as "focal length"! I came to 1192 as "dish width", fitting to a flat-faced dish with the height of 1260...! ???

 

[scottgs]

What do you mean by not-flat 1.2m dish? Why it is not-flat, is it warped? For sure, without knowing the brand and some pictures, it is not easy to understand.

Thank you for the reply. No, the dish is not warped, and well, I probably would have already thrown it out if it were. This is a high-quality deployable dish with a fiberglass honeycomb reinforced aluminium reflector.

In the context of this thread, "non-flat" means that it is not a plain circular section of a paraboloid, and thus if the dish were be set face down on a plane, the rim of the dish will be higher across one dimension (for example top to bottom), than it is the other dimension (for example side to side). This is unlike a typical (and much more common) offset dish which would lie completely flat across it's entire circumference. Dishes are sometimes designed this way to provide for advantageous physical and practical geometry, including unique optimizations of feed-point and beam-width. Common examples would be Aviation and Defence radars, such as airport surveillance radars. However, in the context of this thread, this is not to included wide-azimuth view dishes intended for multi-feed applications, (as is common with direct TV broadcast services), but rather, the thread appears intended to address single-focus, single-feed applications only. Please look at post #2 and number #5 in this thread for some insight into the geometry of non-flat paraboloidal reflectors.

I'm just trying to flow with the original intent of this thread, with a practical application which would also be very helpful to me, as well as perhaps helping to validate the original poster/author's goals (member A33), while also contributing and perhaps helping others in the future.

I'll post some photos soon.

Garret
W8BUG

 

[scottgs]

Here are some photos of this dish. It was part of a deployable VSAT system. I'm not sure how helpful they will be. I also need to correct a previous comment. I believe this is a completely aluminium dish, no fiberglass. I got mixed up with another one VSAT dish which I have.

I have tried to contact the maker, and hopefully I'll be able to obtain some information, but so far, I have not received a reply.

Thanks again for all of the help!

Garret
W8BUG

 

[scottgs]

Thanks for the compliment for the .pdf.
My spreadsheet isn't made public (yet), I'm afraid. The goal is to add several more calculation methods (also for multifeed dishes, e.g.), sometime.

However, here are the results of your inputs. In line to what you calculated in the meantime, yourself. (But: see my edit addition below this post!)
...


Greetz,
A33.


Edit: I just noticed your 1190, noted as "focal length"! I came to 1192 as "dish width", fitting to a flat-faced dish with the height of 1260...! ???

A33,
Thank you for much for the information! Exactly what I was hoping for. I will have to take some time to absorb it all, and I also noticed the discrepancy. To be clear, this is not a flat-faced dish.

I would also like to be clear and give you credit for the math. Although I made my own Excel spreadsheet, I did it based on the formulas which you posted here. I did test my spreadsheet with two other offset-feed dishes' geometry, both of which are complete with feed assembly, and it appeared to work perfectly.

After I work through these spreadsheets and calcs again, I believe that my next step will be to fabricate a feed assembly and just seem what happens. I'll report back what I find, but it may be a few days.

Regardless, thanks again, and big thanks to this forum also. I love seeing this kind of detail, and especially the papers which you presented here. So much to learn!

Garret
W8BUG

 

[Channel Hopper]

It's a four piece Celero, though you say there is no fibreglass.

https://www.sat-litetech.com/wp-content/uploads/1233-Celero-850-0022-E.pdf

 

[scottgs]

It's a four piece Celero, though you say there is no fibreglass.

https://www.sat-litetech.com/wp-content/uploads/1233-Celero-850-0022-E.pdf

Thank you!

I appears to be aluminium, front and back, but it's about 3 cm thick, and I don't really know what's inside that. It could be fiberglass, but it's not apparent. It's definitely not like other fiberglass dishes which I've had, which have either a thinner fiberglass, maybe 3-6mm, or a honeycomb like structure in back. On this dish, I can easily see the shiny aluminium showing through the paint, where it's been scraped, front and back.

I did see that datasheet on the Celero when I was googling, but unfortunately it does not have the offset angle or the focal length - or at least I'm not seeing on the sheet. Am I missing it?

Hopefully the manufacturer will respond, although I'm not holding my breath. Thus the reasons I'm here working on plan B.

 

[Channel Hopper]

The sheet is a brochure, not usual to have things other than maximum dimensions and wind loading.

 

[RimaNTSS]

Here are some photos of this dish

Nice dish! Are you sure it is not-flat one? I see you did the string test and it seems pretty flat.

 

[a33]

Nice dish! Are you sure it is not-flat one? I see you did the string test and it seems pretty flat.

b. distance CA, depth of the dish at the center, relative to the height measure ('dh' further on):
84 mm
...
d. distance CB, depth of the dish at the center, relative to the width measure ('dw' further on):
95 mm

11mm difference, it seems.

I'll look forward to what info the manufacturer will give...

Greetz,
A33

 

[Channel Hopper]

Nice dish! Are you sure it is not-flat one? I see you did the string test and it seems pretty flat.

If you check the image of the rear , each panel is individually numbered with notification of where it goes.

 

[RimaNTSS]

each panel is individually numbered with notification of where it goes.

But that does not necessary mean that the face of the dish is not-flat.

 

[scottgs]

But that does not necessary mean that the face of the dish is not-flat.

It's definitely not flat. 11mm difference between top-bottom and side-side. The dish does not appear to be structurally damaged, warped, etc. It goes together perfectly, no sign of any misalignment, and all pieces are matched serial numbers.

Now I've got to figure out what LNB and Feed is useable with it. I'm thinking a standard inexpensive LNBF may have too wide of angle.