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Offset, why?

PoloMint

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#1
Why and where would an offset dish me more appropriate than a standard one?
 

dishdoctor

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#2
On a balcony,or in a narrow gap between houses.........Brian
 

rolfw

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#3
Plus preferable for wall mounts, doesn't need to stand off so far.

Rolf
 

Channel Hopper

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#4
When instaling near the equator, as the dish can be fitted on the pole inverted.

This also lowers the aspect of the system if you are fitting on a roof where there are high winds
 

PoloMint

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#5
Oh so they are all aesthetic type reasons, what’s the technical difference between the two? ie is a 1.4 offset poorer reception wise than a standard 1.4m?
 

jonny bravo

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#6
Hi PoloMint,
I'm not sure about the reception, but as far as I know 1.2M dish is as big as they go with offset, after that i.e. 1.4M you would have to go for prime focus (could be wrong on this)
Hope this helps.
 

dishdoctor

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#7
Measure for measure a prime focus dish has a larger surface area than a offset,so must be better,for instance,a 1.2 offset is sometimes the height measure,the width can be 1 metre........Brian
 

Channel Hopper

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#8
On most commercial offset dishes the minimum diameter is the one stated, ie a 1.8 m will have the longer diameter of 2m which improves gain

Strictly speaking the feed of a prime focus dish will have a known f/d ratio and is matched with a manufacturers feed to maximise the gain and noise characteristics (and this is the same with the larger offset units - we supply up to 3.8m in offset form). Domestic offset dishes leave the end user to use a standard universal LNB with an unknown efficiency and perfoemance curve.

The other difference (ignoring the fact the feed is in front of the dish on a primefocus - this should not affect to a great degree) is the illumiation of the feed by outside effects. The prime focus dish is likley to be affected by some noise generated from the ground behind the dish if this is excessive, the offset having the feed looking up into the air and so seeing effects from the sky.
 

digdog

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#10
Not necessarily - depends on what you want to pull in with it - as a general rule the bigger the dish the better the gain.
An offset dish sits more or less vertical (like the old analogue sky dishes) - a prime focus leans back (like a mini Jodrell Bank).
Can you say what you have in mind - maybe the techies will then be able to advise more accurately... cheers!
 

Channel Hopper

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#11
The offset dish (in most cases) is easier and cheaper to manufacture than a prime focus unit

When it comes to the process, the best prime focus antennas are spun on a large revolving former and pressure is applied until it takes the same shape. This is expensive in both time and labour

Offset dishes are usually pressed out in the same way as sink units, waste bins and other household objects, by the million, so a domestic dish can be made for peanuts. The slightly lower gain can be put down to a number of factors, poor manufacture, poor packaging, Parcelfarce, bad installation, etc etc, and usually the final user may never notice unless he sees a picture from a decent one.

You will see many posts with people wondering why their signal drops out when it rains, or when the wind is blowing, the forums are full of them and it is not always apparent from the post as to what is happening, but I bet that nine times out of ten, something has not been put up right outside, rather than a fault of the manufacturer.
 

PoloMint

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#12
What determines what way the dish should be offset, ie taller than wider or vice versa.

The link to pic below is of A 1.2m x 1.4m that is wider than higher, the same as a minidish, however most/all the dishes I have seen elsewhere are taller than wider.

http://www.satalogue.com/section1/images/mega.jpg

What is the result of the difference and why would one be more appropriate than the other?
 

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#13
[updated:LAST EDITED ON 04-Jul-02 AT 00:16 AM (GMT)]My call I believe

The larger a dish, the narrower the beamwidth of the receiving (or transmitting) signal.

If the dish is larger in one dimension, the received signal is also narrower in this plane

With satellites as close together as .2 degree or so (Arabsat and Astra 2 for example) it makes sense to have a dish that is able to distinguish the slight differences in orbital slot in the best way it can, which is why most smaller dishes now are built fatter than taller

Its a bitch however to design a decent feedhorn for one though