Prime focus vs offset big dish

7mdish

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Hello, I'm in the market for a new, professional-grade, 1-piece, 1.8m or 2m dish. I was pretty sure to buy a prime focus dish, but someone suggested to evaluate offset dishes, too. I never used big offset dishes (only less than 1m), so I'm a little bit confused. Offset dish have typical elevation angle shifted of about 20 degrees compared to prime focus, so what about extreme East/West satellites on Clarke belt? It means, when a prime focus dish is at, say, 15 degrees in elevation, an offset dish would be below horizont line! How can it receive the signal? Could I have a shorter "view" using such dish? And what about possible interferences from near satellites? Focus point in PFA antennas is narrow, very little. But it is larger in offset dishes, so what about strong signals from satellites positioned at 2 or 3 degrees at both dish sides? Anyone could clarify to me this points please? Thank you.
 

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Inverting the offset dish is how one gets by with low elevation satellites.

Whilst there is some adjacent overlap with an offset design, with a large enough reflector this will be less than the 2 degrees or so that might impact on choosing an offset design.

What is the largest reflector you (your wife) will allow in the garden ?
 

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I'm planning to install one 1.8m dish, but it seems that all professional-grade dishes of such diameter are offset designed. I have nothing against this type of dish, my doubt was how performing correct setup of a motorised system. Reading your reply, it seems that there would be no limitations to ger all satellites in every orbital locations. A dealer told me that such dishes are provided with its own mechanical and motor mounting. I do not know.... I must evaluate carefully my new setup prior to begin.
 

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I'm planning to install one 1.8m dish, but it seems that all professional-grade dishes of such diameter are offset designed. I have nothing against this type of dish, my doubt was how performing correct setup of a motorised system. Reading your reply, it seems that there would be no limitations to ger all satellites in every orbital locations. A dealer told me that such dishes are provided with its own mechanical and motor mounting. I do not know.... I must evaluate carefully my new setup prior to begin.
Professional grade limitations ?

The only thing that prevents you from getting a professional dish in prime focus or offset in a particular size is who you listen to.
 

7mdish

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I do not know.... today a dealer told me that it is not so easy to find prime focus antennas of around 1.8m in the market. He added that only larger dishes are prime focus yet, I do not know why. But he assured me that offset dishes like Raven/Skyware or Prodelin/DH or similar are supplied with every mechanical parts and motors (and controller, if you need) to complete all assemblies. I want a 2-axis motorised system, so I believe I will choose RCI controller, probably RC2000a. I will take my decision in a few days.
 

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If you can't find a suitable positioner, dreambox1959 may be able to build you one:
 

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Home made positioner?? Ohh, very interesting! Very useful suggestion from you, thanks!
 

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I do not know.... today a dealer told me that it is not so easy to find prime focus antennas of around 1.8m in the market. He added that only larger dishes are prime focus yet, I do not know why. But he assured me that offset dishes like Raven/Skyware or Prodelin/DH or similar are supplied with every mechanical parts and motors (and controller, if you need) to complete all assemblies. I want a 2-axis motorised system, so I believe I will choose RCI controller, probably RC2000a. I will take my decision in a few days.
I have supplied both offset and prime focus reflectors of 1.8m in single panel format for years. Once you go beyond 2-2.2m the issue with prime focus is the manufacture, moving away from spinning to forming. Offset reflectors are almost always mounded once above 1.8m which means strength in their design and the ability to survive the transportation to site.

Perhaps the 'flatness' (think volumetric/three dimensional ) profile of an offset reflector makes them a better product to ship as a single item, but I would suggest the rigidity of a composite / fibreglass / smc product against a steel/aluminium product helps keep the losses / replacement factor down.
 

7mdish

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I have supplied both offset and prime focus reflectors of 1.8m in single panel format for years. Once you go beyond 2-2.2m the issue with prime focus is the manufacture, moving away from spinning to forming. Offset reflectors are almost always mounded once above 1.8m which means strength in their design and the ability to survive the transportation to site.

Perhaps the 'flatness' (think volumetric/three dimensional ) profile of an offset reflector makes them a better product to ship as a single item, but I would suggest the rigidity of a composite / fibreglass / smc product against a steel/aluminium product helps keep the losses / replacement factor down.

Very interesting. I always used aluminium dishes in my 30 years satellite activity, I was not a fan of SMC dishes because I saw some fiberglass dishes thrashed in few years in the past, but it happened many years ago when, maybe, production processes was not advanced like today. Do you think a good 1.8m, single panel, SMC dish is more solid and durable compared to an aluminium one with the same specs? If so, what about brands? DH, Skyware, or what? Thanks.
 

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...Once you go beyond 2-2.2m the issue with prime focus is the manufacture, moving away from spinning to forming. Offset reflectors are almost always mounded once above 1.8m which means strength in their design and the ability to survive the transportation to site...


The BBC, S4C, SIS and Virgin Media were possibly unaware of this point when they purchased Precision/Elite large reflectors, may have been an oversight from their procurement people?
Perhaps it is a good job I selected multi-panel reflectors for a couple of 2.4m and 3.0m setups we recently had installed in Australia, pics below,

.

AUS Project - p01 - Concrete Pads.jpg AUS Project - p02 - Concrete Pads.jpg AUS Project - p03 - Concrete Pads.jpg AUS Project - p04 - Prodelin & Suncom Unloaded.jpg AUS Project - p05 - Prodelin Going Up.jpg AUS Project - p06 - Prodelin & Suncom.jpg AUS Project - p07 - Prodelin & Suncom.jpg AUS Project - p08 - All Installed.jpg
 
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A professional dish of either material will have similar survival rates, some offset manufacturers would also provide an additional stability mount on their fixed dish systems, locking the base of the reflector to the king post.

Some systems have additional parts that are not contained in the inventory (anti-ice covers, actuators, dehydrators and front end switching) that will reduce the survival rates, either wind resistance, weight or moving components.

Skyware is the current name for Channel Master/Raven for many of the reflector sizes
 

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