Extension Petals

DishDick

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A qiuck question. It seems logical, but I am sure there is a complicated answer.

As I understand it an offset dish is in essence a section of a prime focus, so taking a 2.4m dish, the offset of that would be an oval shape above the LNB, giving approx a 1.2m offset dish. Now if the 2.4 had extension petals, to 3.1m, then the offset dish equivalent would be 1.55m approx. The prime focus original part has the same shape as before, so using this as an example, why is it not possible to get extention petals for offset dishes.

I ask as a little increase in my dish size would bring in the PSB channels perfectly.

Thanks
 

Captain Jack

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I think the geometry is more complicated for creating extension petals for offset dishes, in the fact that it's an oval. PF dishes are uniform in their structure, so are easier to predict what shape the petals should be and where they would go.

@Vipersan created his own petals for his PF dish and so have others with a decent degree of success.
 

PaulR

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Assuming you could get the shape right it would bring in more signal but the LNB wouldn't see the extra. The f/D ratio would change and you would need an LNB with adjustable rings in order capture it all.
 

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Well, with the right shape petals, the FD wouldn't (and shouldn't) change. After all, the signals should converse at exactly the same point from all areas of the dish. The petals would have to be "deeper" than the rest of the dish, though..
 

PaulR

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Well, with the right shape petals, the FD wouldn't (and shouldn't) change. After all, the signals should converse at exactly the same point from all areas of the dish. The petals would have to be "deeper" than the rest of the dish, though..
If f is the focal point and D is the diameter of the dish then adding petals will, by definition, increase the diameter overall. The focal point remains the same, as you're merely extending the parabola, so the f/D ratio therefore changes.
 

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Hmm, you're right. A larger dish would, in theory, mean moving the LNB further away from the dish to "see" it all. But then the beam from the original reflector shape would still converse at the original point, so moving the LNB further away means it would be out of focus.

To overcome this, the petals would have to be "deeper" than the original reflector in order to direct the beam towards the original point. But then that formula comes into play...
 

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Thanks for the input.

I realise the F/D is important, but as I said, I understand an offset is effectively a portion of a prime focus, and if so, I thought that if you can increase a Famaval Prime focus by adding petals and the F/D stays the same (arms are not changed in the process and the feedhorn hasn´t got much play), then I thought the same must be possible with an offset. Obviously the angle of any extra petals is critical.

I am just applying my own logic here, though it may be flawed.

I have asked the manufacturer of my dish if they are planning to make any petals. I will let you know their response.
 

PaulR

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Hmm, you're right. A larger dish would, in theory, mean moving the LNB further away from the dish to "see" it all. But then the beam from the original reflector shape would still converse at the original point, so moving the LNB further away means it would be out of focus.
To overcome this, the petals would have to be "deeper" than the original reflector in order to direct the beam towards the original point. But then that formula comes into play...
Exactly! Some people might find an improvement in signal by just putting petals on but what shows is that the LNB was already "looking" at a larger area than the original dish and probably picking up interference as a result.
 

PaulR

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I understand an offset is effectively a portion of a prime focus, ...
Yes. you can imagine it as an angled slice.

... I thought that if you can increase a Famaval Prime focus by adding petals and the F/D stays the same (arms are not changed in the process and the feedhorn hasn´t got much play), then I thought the same must be possible with an offset. ...
But the f/D does does alter. The size of the dish increases making the diameter increase.
 

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These discussions bring back many memories ..some good some bad .
The FD does change I found ..but the focus remaining much the same ..
My experiments were full of compromise ..but then I was working in C band which is much more forgiving of errors.
Plus I didn't get too ambitious and try enlarging the dish diameter too much ...
The subject is not an easy one ..and not one I'd contemplate attempting on an offset slice..

That said ..Pride or possibly Aceb if I remember had a solution involving hinged aluminium plate sections attached to the dish outer edge ..thus he could adjust these by trial and error to get improvements in signal.
Sorry if it wasn't either ..my memory aint what it was.
rgds
VS
 
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..the additonal extension petals would just catch more wind ;)

http://www.satellites.co.uk/forums/threads/lnbs-aiming-point.160762/#post-874943

full-ring-jpg.61977
 

Vipersan

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Well Christina certainly caught more wind recently ...95 >100mph to be exact ..and no damage to the dish or petals ...
the actuator jacks were the only things to move ..
I'd say that was certainly a test ..and proof of good construction.
;)
 

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Yes. you can imagine it as an angled slice.


But the f/D does does alter. The size of the dish increases making the diameter increase.
Yes sorry PaulR, I meant the focal point.
 

DishDick

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From the horses mouth

"As to extension petals in case of offset it is no economic sense. We'll have to prepare a lot of devices and moulds because in offset every petal is different in shape."

So possible, but not practical.
 

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Hi VS yes it was me who extended an offset dish with ali petals as you say moving each petal on a hinge to get extra gain then super gluing the hinge so it does not move after I was satisfied with the end result using ali strips I connected all the petals on the outside so that in strong wind it would have more resistance not to move.cheers
 

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DishDick

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At the moment I am getting very marginal signal on 2E. If I was to increase my dish from its current 2.71m x 3.01m, to one say 3.015m x 3.347m, on a flat elliptical that would equate to an increase in surface area of 12%, so being bent the surface area increase would be even more. What increase in reception would that likely equate to, assuming the efficiency of the dish was proportional to the old one?

Cheers.
 

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Provided it's linear the increase would be:
10 x log(1,12)=0.5dB
 

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Yes sorry PaulR, I meant the focal point.
Actually, the focal point remains the same if you follow the parabolic curve. On a PF dish position of the LNB is unchanged.
 
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