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

a33

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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.

Looking at the horizontal characteristics of the dish, for an offset dish 1340 * 1268 * 95, the opening angle of that dish dimension would be 64 degrees, so 4 degrees more than in the vertical plane.

So it might indeed be an issue. Not my expertise, though. But when using a normal Ku-LNBf, I gave an aiming advise, to test to aim lower than 'normal'.

@RimaNTSS has an Equatorial dish, which I believe has an opening angle ~49 degrees.
It would be interesting to know what kind of feedhorn/LNB he is using with it?
(Same dish is in use with the blog owner of ftapinamar, Calcular la distancia al segundo LNB (6) )

Greetz,
A33
 
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ea3cv

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¡Gracias, @ozumo!

Aquí están las especificaciones calculadas (en la columna V).

El espacio libre de este plato es aproximadamente cero: y-nulo es -0,25. Eso significa que un multifeedholder, al ocupar espacio encima de los LNB, le quitará señal al plato.
(No sé si ese también es el caso con el plato Sky en este tema reciente: Multi sat en un plato Sky. ¡Pero podría ser un problema, como lo muestra este plato!)


View attachment 139469


El ángulo de visión de NB LNB, etc., se calcula para un plato de cara plana con una altura de 55,3 y un ancho correspondiente de 51,43.
La posición del punto focal se puede encontrar por la longitud de la cuerda superior y la longitud de la cuerda inferior.

En la (pequeña) colección de especificaciones de antenas parabólicas offset que tengo ahora, el ángulo de visión del LNB suele estar entre 70 y 80 grados (o, en promedio, entre 73 y 74). (Pero hay algunas excepciones por debajo de 70 o por encima de 80. ¡Y no soy un experto en esto!)
Este plato tiene 77 grados. Supongo que esto significaría que un LNBf Ku normal (a diferencia de un LNB Sky con bocina de alimentación especial) "vería" toda la parte vertical del plato (sin desbordamiento), pero solo una pequeña parte horizontal del plato.

La especificación de los puntos indicados se muestra en esta imagen (¿supongo que debo intentar hacerlo un poco más pequeño, creo...?):

View attachment 139470


Así que ahora tengo curiosidad por saber si el titular del LNB realmente coloca el LNB en la posición correcta.

Saludos,
A33

Editar: algunas adiciones,
 

ea3cv

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Hi,
I found your presentation for the calculation of offset antennas very interesting. I have also seen that you have a calculator in Excel. Would it be possible for you to share it? Great job.
Thanks for sharing.
Kin EA3CV
 

a33

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Hi @ea3cv , and welcome to the forum!
:-welcome1

I'm perfectly willing, when you supply me your input data, to enter them in my spreadsheet, and share the results.

I assume you know, that the measures (height, width, depth(s)) must be taken just for the parabolic working area of the dish, so not including the dimensions of the rim.
And all measures to the millimeter precise, depth even to parts of a mm precise, when possible.

Do you have a special dish, or special project, that you would want the calculations for?



Alas, there doesn't seem to be much interest in the calculations on parabolic/paraboloid dishes nowadays. That is why most of my projects for calculating on dish geometry are on hold.
Instead I recently managed to derive/develop a way to calculate multifeed skew angle precisely (many calculators give quite different values, so are just estimates from often unclear calculations, I would say),
and am presently considering the 'modified motor angles' for satellite motor setups again (many charts give forward axis tilt values that unnecessarily overcompensate, I found).


Greetz,
A33
 

ea3cv

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Hi @ea3cv , and welcome to the forum!
:-welcome1

I'm perfectly willing, when you supply me your input data, to enter them in my spreadsheet, and share the results.

I assume you know, that the measures (height, width, depth(s)) must be taken just for the parabolic working area of the dish, so not including the dimensions of the rim.
And all measures to the millimeter precise, depth even to parts of a mm precise, when possible.

Do you have a special dish, or special project, that you would want the calculations for?



Alas, there doesn't seem to be much interest in the calculations on parabolic/paraboloid dishes nowadays. That is why most of my projects for calculating on dish geometry are on hold.
Instead I recently managed to derive/develop a way to calculate multifeed skew angle precisely (many calculators give quite different values, so are just estimates from often unclear calculations, I would say),
and am presently considering the 'modified motor angles' for satellite motor setups again (many charts give forward axis tilt values that unnecessarily overcompensate, I found).


Greetz,
A33
Hi Greetz,

I am using a Kathrein CAS-90 antenna.
The measured values have been:
Largest diameter: 960 mm
Minor diameter: 930 mm
Max. depth: 84mm

I am currently using it for Band-X, 8.4 GHz for DSN, and soon I hope to be active in 10 GHz. I would like to optimize it as much as possible, especially for receiving interplanetary probes.
Therefore, I am interested in having values on which to work on possible overflow and spillover.

Regards.

Kin
 

a33

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I am currently using it for Band-X, 8.4 GHz for DSN, and soon I hope to be active in 10 GHz. I would like to optimize it as much as possible, especially for receiving interplanetary probes.

Ah! Not my expertise at all! :):)
But nice to have new pioneers using this knowledge!


I had a contact with an Austrian specialist/hobbyist some time ago, using a Kathrein dish of about 90 cm for things like this, so probably a CAS-90 as well. He came with measures height = 945mm, width = 925mm (are you sure you measured without the dish rim?); but that one did not have a flat dishface!
So that part B of my article would be applicable.
In that case, apart from height and width, two depth measurements at the center are needed. And the measurements should be done with some extra care, as a measurement error has more consequences here.

Have you read part B already?
Measurement procedure is also here: Measuring and calculating zone-1 and zone-2 dish
Or a shorter one: OTHER - Question about LNB position
When you want precise results, please invest time and effort in precise measurement! (As I invested time and effort in the precise formulas.)


By the way, I seldom use maximum depth as input; I prefer the calculating methods for depth at the center (easier to determine/measure, I think).


Greetz ( = 'popular' word for "greetings"),

A33.
 
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