Circular Offset Dish?

[unk266]

Hello There people, recently I got a new offset sat dish which is circular exactly 127 by 127cm. I'd like to know HOW can I calculate the offset angle, feedhorn beamwidth etc for this non-standard dish.I'm trying to use it for weather satellite reception so thats why i need to know these specs. Thanks in Advance.
Here's a photo.

Advance

 

[a33]

Welcome to the forum, unc266!

An answer how to calculate the offset angle is here: Just Sharing This - Calculation of the focal length of an offset satellite dish antenna, Revisited
There is also another way (multiple depth measures, along vertical center line of the dish), but that is somewhat more difficult/elaborate (and I haven't made a spreadsheet for that yet).

From offset angle, focal length and height of the dish, all other dish specs can be calculated.
(When you give exact input data, I could let my calculator give you the results..)

Greetz,
A33

 

[unk266]

Welcome to the forum, unc266!

An answer how to calculate the offset angle is here: Just Sharing This - Calculation of the focal length of an offset satellite dish antenna, Revisited
There is also another way (multiple depth measures, along vertical center line of the dish), but that is somewhat more difficult/elaborate (and I haven't made a spreadsheet for that yet).

From offset angle, focal length and height of the dish, all other dish specs can be calculated.
(When you give exact input data, I could let my calculator give you the results..)

Greetz,
A33

hi. thanks for your answer. what actually interests me the most is this dishes beamwidth. I need to match it as closely as possible with the DIY Helical L band feed. The offset angle is not actually quite important to me. If this was a standard dish, I would be able to calculate the beamwidth through this calculator: Offset Reflector Calculation
Since my offset dish is not flat, would that mean that this calculator is outputting incorrect values for the beamwidth?

 

[a33]

The given Jens Satre calculator assumes you know the focal length, already; and also that the Vertex of the parabola is exactly at the bottom of the dish (which often is not exactly the case).

For calculating the exact feedhorn beamwidth (or feedhorn illumination angle or dish opening angle, all meaning the same angle), you need the focal length, the offset angle, and the height of the dish; without them you cannot calculate feedhorn beamwidth.

[ In your case you would also need the width measure of the dish, as the feedhorn beamwidth horizontally and vertically are different. So you could use an intermediate value. ]

===========

When you know the exact focal point of the dish already (because you know the LNB feedhorn position, relative to the dish, and you trust that that IS, in fact, exactly at the focal point), then you can use more simple measurements to calculate from: Height of the dish, Topstring length, and Bottomstring length. (TB, TF and BF in the bottom picture of this posting: Just Sharing This - Calculation of the focal length of an offset satellite dish antenna, Revisited

Applying simple cosine-rule, will give you the (vertical) angle.

Calculating the horizontal illumination angle can be done from there (somewhat less simple).

(If you wish, I could do the calculations for you, when you give the above 3 measures, and the width measure.)


BTW Are you really 89 years old? And doing this little project? Nice!!!

Greetz,
A33

Edit: corrected typo, and some small additions.

 

[unk266]

The given Jens Satre calculator assumes you know the focal length, already; and also that the Vertex of the parabola is exactly at the bottom of the dish (which often is not exactly the case).

For calculating the exact feedhorn beamwidth (or feedhorn illumination angle or dish opening angle, all meaning the same angle), you need the focal length, the offset angle, and the height of the dish; without them you cannot calculate feedhorn beamwidth.

[ In your case you would also need the width measure of the dish, as the feedhorn beamwidth horizontally and vertically are different. So you could use an intermediate value. ]

===========

When you know the exact focal point of the dish already (because you know the LNB feedhorn position, relative to the dish, and you trust that that IS, in fact, exactly at the focal point), then you can use more simple measurements to calculate from: Height of the dish, Topstring length, and Bottomstring length. (TB, TF and BF in the bottom picture of this posting: Just Sharing This - Calculation of the focal length of an offset satellite dish antenna, Revisited
Applying simple cosine-rule, will give you the (vertical) angle.


Calculating the horizontal illumination angle can be done from there (somewhat less simple).

(If you wish, I could do the calculation for you, when you give the above 3 measures, and the width measure.)


BTW Are you really 89 years old? And doing this little project? Nice!!!

Greetz,
A33

Edit: corrected typo, and some small additions.

Thank you so much for your answers. Well the height of the dish is 127cm,Topstring length is 140cm and bottomstring length 60cm.

 

[a33]

Thank you so much for your answers. Well the height of the dish is 127cm,Topstring length is 140cm and bottomstring length 60cm.

The 127 is the effective parabolic height (so without the rim)?
The 140 and 60 are really such nice rounded figures?
(Measurement inaccuracies can have much effect on the outcome!)

Then here are the results (feedhorn beamwidth would be 65 degrees, cel O-25):




NB Offset angle of many offset dishes is between 22 and 26 degrees. So 39 degrees is really high.
Edit: Also the vertical distance between vertex and bottom of dish (y-null) is 37 cm, so far more than 0. That is huge!

So I hope your measurements were accurate, and the focal point of the dish really is where you think it is.
(That is why I prefer to calculate the dish specs from the DISH measurements: These measures are always real (unless the dish is warped), whereas the calculation done from the feedhorn position depends on the correctness of that position....)


Greetz,
A33

Edited

 

[unk266]

The 127 is the effective parabolic height (so without the rim)?
The 140 and 60 are really such nice rounded figures?
(Measurement inaccuracies can have much effect on the outcome!)

Then here are the results (feedhorn beamwidth would be 65 degrees, cel O-25):

View attachment 144472


NB Offset angle of many offset dishes is between 22 and 26 degrees. So 34 degrees is really high.
So I hope your measurements were accurate, and the focal point of the dish really is where you think it is.
(That is why I prefer to calculate the dish specs from the DISH measurements: These measures are always real (unless the dish is warped), whereas the calculation done from the feedhorn position depends on the correctness of that position....)


Greetz,
A33

Thank you very much, well i believe the focal point is correct because thats where the signal is the strongest when positioning the feed by hand, again thank you a lot for the help!.

 

[PaulR]

That looks like an Orbitron multi-focus dish for multiple satellites. I have one similar.

The theory is that vertically it's a proper parabola but horizontally it's a succession of circular cross sections. Basically it's a better way of using multi-LNBs across an arc compared to using a standard off-set dish. Yes it's a compromise, but a compromise across all its beamwidth. This compared with a standard dish which is designed to be perfect using an LNB in the normal position but quickly becomes heavily compromised for gain the further out you place and extra LNBs.

There was another manufacturer of these type of dishes but I can't remember their name.

 

[unk266]

That looks like an Orbitron multi-focus dish for multiple satellites. I have one similar.

The theory is that vertically it's a proper parabola but horizontally it's a succession of circular cross sections. Basically it's a better way of using multi-LNBs across an arc compared to using a standard off-set dish. Yes it's a compromise, but a compromise across all its beamwidth. This compared with a standard dish which is designed to be perfect using an LNB in the normal position but quickly becomes heavily compromised for gain the further out you place and extra LNBs.

There was another manufacturer of these type of dishes but I can't remember their name.

Sharp RF Components might be the manufacturer, atleast its written on the dish.

 

[a33]

Thank you very much, well i believe the focal point is correct because thats where the signal is the strongest when positioning the feed by hand, again thank you a lot for the help!.

NB I edited my previous post slightly; don't know if you saw that already?

BTW: Using strongest signal on a mounted dish assumes that the dish's boresight is outlined perfectly to the satellite (so it assumes that the proper offset angle is used, while outlining the dish).
Even when using the sun as the signal source for the parabola, and then finding the dish position (including elevation angle) where the focussed signal is a sharp point, is difficult enough. Finding the focal point with strongest satellite signal, is even more difficult....

Greetz,
A33

 

[PaulR]

There was another manufacturer of these type of dishes but I can't remember their name.

It might have been Raven.

 

[a33]

That looks like an Orbitron multi-focus dish for multiple satellites. I have one similar.

If it is indeed a multifocus dish, then the linked equation I gave above is not valid for this dish.
In that case there is the method with multiple depth measures, along the vertical center line of the dish, as I wrote.

Or indeed use the present LNB feedhorn location, as we did above (though it has rather unlikely outcomes, in my humble opinion).
Or check with someone else who has the exact same dish, at what location that has the LNB feedhorn.

 

[Channel Hopper]

A three bolt reflector is odd, the only Sharp branded product of a similar size is based on the Concentric reflector that I believe was acquired by Raven at some point.

 

[Channel Hopper]

This one ?

Sharp brand satellite dish | TV Accessories | Gumtree Australia Moreland Area - Pascoe Vale | 1299382409

Free Pickup please, we had it removed from the roof as we dont have it connected to anything, 1299382409

www.gumtree.com.au

 

[Terryl]

Hello There people, recently I got a new offset sat dish which is circular exactly 127 by 127cm. I'd like to know HOW can I calculate the offset angle, feedhorn beamwidth etc for this non-standard dish.I'm trying to use it for weather satellite reception so thats why i need to know these specs. Thanks in Advance.
Here's a photo.

Advance

What satellite are you going to be looking at??

 

[unk266]

What satellite are you going to be looking at??

Well, every upper L-Band Weather Satellite I can reach, Elektro-L2, Elektro-L3, Fengyun-2H, EWS-G1, and polar orbiting sats like NOAA15 ,18,19 ; Meteor-M2 and M2-2 and Metop-B and C.

 

[unk266]

If it is indeed a multifocus dish, then the linked equation I gave above is not valid for this dish.
In that case there is the method with multiple depth measures, along the vertical center line of the dish, as I wrote.

Or indeed use the present LNB feedhorn location, as we did above (though it has rather unlikely outcomes, in my humble opinion).
Or check with someone else who has the exact same dish, at what location that has the LNB feedhorn.

So far, I have only been able to slightly outperform the old 80cm offset. Is this type of dish supposed to perform worse than a standard 127cm dish?

 

[a33]

Well, the multifeed dishes that I know, are wider than high, so not circular, as yours.
So maybe @PaulR can give more specifics of his dish, if it is similar?
I'm not convinced yet, that yours is a multifeed dish.

When you don't get signal equivalent to an about 110 cm dish, something is wrong, I'd say.
(height 127cm, estimated offset=26 degree, would give corresponding width = 114cm.)

With what method/procedure did you find your focal spot? Did you also vary the dish's elevation angle, during your hand-positioning test?
And does the mounting bracket have an elevation scale?

When it still is just a non-flat paraboloid dish, a measuring procedure for my calculation method I wrote here:

Measuring and calculating zone-1 and zone-2 dish

Yesterday I managed to adjust my excel satellite dish specs calculator, to calculate non-flat paraboloid dishes (such as the zone-1 and -2 dishes in the UK) as well. Of course you could measure them with the water method (is that from John Legon?), but that measurement procedure is a bit of a...

www.satellites.co.uk

Greetz,
A33

 

[PaulR]

Well, the multifeed dishes that I know, are wider than high, so not circular, as yours.
So maybe @PaulR can give more specifics of his dish, if it is similar?
I'm not convinced yet, that yours is a multifeed dish.

Some photos of the dish in 2011 when I was using it in France. Primary position was aimed at 19E with 13E just to the right as you look at it. The cabled up Quattro LNB is pointed at 28E. Note how the lip of the rim has a variable depth as it goes round, same the OP's. As the signal from 28E became weaker on the UK beam I moved the Quattro to the primary position and then had to replace the dish with a larger one.

I may have brought the dish back to the UK and if you're not convinced that it is a multi-focal dish I'll have a look to see if I can find it!

 

[a33]

... and if you're not convinced that it is a multi-focal dish ...

Well, it is clear that your dish has a higher rim at top and bottom of the dish, and the rim might well be exactly circular.
But what signs do you have that the reflector shape is in fact multi-focus? Is it in the specs, or in the manual?
When in fact placing the 28E LNB at the center of the arm gave a real improvement, then the multi-focus property of the dish would be questionable.

BTW I recently measured up my Cahors/Visiosat Big BiSat multifeed dish, both along the vertical center line, and along the horizontal center line, as I want to know if the circular horizontal design really gives clear non-fitting values as compared to the values you'd expect from a parabolic horizontal shape. Building a suitable spreadsheet for that, that is also usable for other dishes' inputs, will take me some time though; so it will be a while before I come to a conclusion...

Greetz,
A33