Offset dishes

[PaulR]

I see you are still suffering, Paul!

Yes, back at work and the same old problems with the forum software. I forgot and simply saved my reply. Can't be bothered to sort it out any more.

The illumination of the dish will be kept within the confines of the oval part that sits within the pentangle shape

That does answer one of my questions in a way. A Penta will considerably underperform compared to an ordinary dish if you compare the maximum dimension (which will be a diagonal). I would also imagine that if you take the letter of the law with a local authority regarding maximum dimension in any direction you would be at a disadvantage.

Having a generic feedhorn on the LNBF on a standard shaped dish is going to compromise far more than the Penta dish with its apparent cut-outs

I don't understand this at all.

Cassegrain/Grgorian dishes work better than single reflector systems not as a result of higher gain (larger area) but by a lower signal to noise ratio at the feed.

But we're not discussing double reflector dishes so that's irrelevent.

The Penta shape goes some way to ensure extra rigidity - it's more difficult to bend the reflector out of shape compared to a standard offset design.

I'll have to take your word on that. I've never had one one in my hands as they're ugly as sin. Rigidity can be designed in using a more pleasingly shaped dish - just ask Andrews/Raven/Channelmaster/Prodelin.

The Penta is a good compromise bwtween offset vertical (as per the Gibertini where the maximum area of the dish is 'seen' by the satellite, with an associated loss at the feedhorn) and the offset horizontal (as per Zone 1 and 2 dishes) where the feedhorn is matched to the reflector for improved narrow aperture - beamwidth - and cross-pole isolation, at the expense of satellite illumination

I think that's a matter of debate. If the Penta is just using a standard universal LNB then it's only as good as the area of the dish that it illumintes.

 

[Captain Jack]

Paul, you really hate Pentas, don't you?

 

[PaulR]

Now that you mention it...

 

[Channel Hopper]

But we're not discussing double reflector dishes so that's irrelevent.

The dish size and reflector design difference is relevant however. From memory the Penta was designed with improved sidelobe performance and strength over the standard oval shaped reflectors and what you dislike most was the result of this research.

Oooh lookee !

www.ndsatcom.com/en/products/shop.php?catID=136&artID=63
www.holkirk.com/Quick-Deploy-Ka-Band-Flyaway-Antennas--TP2-120.htm

 

[PaulR]

I'm not sure why improved sidelobe performance should be thought necessary for dishes no smaller than 60cm as they have enough directivity for satellite seperations of 3 degrees or more. It's even debatable for 2.5 degree of separation I would have thought. It's only when you get down to $ly minidish sizes that you need to think about extra sidelobe rejection; hence the squashed shape and special LNB horn to match.

In fact if improved sidelobe performance was supposedly a reason, why is there not an LNB horn designed to match? I suspect that the shape is simply a design statement. And we all know about style over substance.

There's nothing in either link about the shape of the dish either.

 

[Channel Hopper]

I'm not sure why improved sidelobe performance should be thought necessary for dishes no smaller than 60cm as they have enough directivity for satellite seperations of 3 degrees or more. It's even debatable for 2.5 degree of separation I would have thought. It's only when you get down to $ly minidish sizes that you need to think about extra sidelobe rejection; hence the squashed shape and special LNB horn to match.

In fact if improved sidelobe performance was supposedly a reason, why is there not an LNB horn designed to match? I suspect that the shape is simply a design statement. And we all know about style over substance.

There's nothing in either link about the shape of the dish either.

Only that the design is no longer exclusive to Fracarro, others have realised the importance of the shape, and with it the rights to manufacture.

On a receive only dish the manufacturer has to put a great deal of extra investment to come up with a matching feed, a few years ago only those dabbling with VSAT equipment would have offered one (Channel Master, Precision, Prodelin etc).

An olde advert,

 

[archive10]

The dish size and reflector design difference is relevant however. From memory the Penta was designed with improved sidelobe performance and strength over the standard oval shaped reflectors and what you dislike most was the result of this research.

Oooh lookee !

www.ndsatcom.com/en/products/shop.php?catID=136&artID=63

www.holkirk.com/Quick-Deploy-Ka-Band-Flyaway-Antennas--TP2-120.htm
I buy the strength argument, as these are both fly-away systems that requires extra rigidity with as little thickness as possible, making them better for flat-pack than an SMC dish (that has the ribs in the back providing rigidity).
(Wouldn't this make them the ideal IKEA dish? Now there's a prediction for you...)

I suspect the prodelin/CM/Visiosat still use the almost oval shape for the fixed and non-gregorian installations as this still give better signal compared to the amount of surface area you have in the wind.

Or maybe there's a niche market there... dishes in any shape you want (as long as it as an illuminable inner oval). Apple-dishes that cost 15% more than others, bat-dishes, you name it.

(and erhm... aren't we supposed to not post external live links on the site?)

 

[BlindFaith]

Why use a 105cm dish when a Penta 85 works better? *ROTFL*

 

[Channel Hopper]

Or maybe there's a niche market there... dishes in any shape you want (as long as it as an illuminable inner oval). Apple-dishes that cost 15% more than others, bat-dishes, you name it.

(and erhm... aren't we supposed to not post external live links on the site?)

The links aren't live (shortly after posting anyway)

If there wasn't variety then the world would be a boring place.