Advice Needed Gain on a dish

RichardinSpain

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Hi All
What does 'gain' on a dish actually mean?
If i have seen a 150cm offset dish advertised as KU-Band Gain 12.5GHZ44.26dB, is this any good?
Is a dish of the same size with lower gain or higher gain better?
Many thanks Richard
 

Lazarus

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Higher Gain is better.

The figure refers to Gain relative to a half-wave dipole, which is deemed to have a Gain of 0 dB

Another way of quoting Gain uses an Isotropic Antenna as the reference (ie An Antenna that is wholly omni-directional {or radiates equally in a spherical pattern}). As a dipole has a theoretical gain of its own in relation to an Isotropic Antenna - of 2.4 dB - then that can be used to mislead if the reader is not wholly aware of the reference being used. Strictly, the figure relative to Isotropic is quoted in dBi.

Either way, more is always better for Satellite Dishes!
 

woborny

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So can you help me with this one then...

One dish has a "Gain at 12,75 GHz of 42,8 db"
Another dish of similar size has a "Gain at 12.5 GHz of 43db"

Which is the better dish gainwise, given they have different db at different frequencies...
 

Lazarus

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Not possible to say without access to the full Frequency vs. Gain plot over the range of interest.

Same, same for LNB Noise Figures: For Marketing purposes only a Headline figure is quoted.

So, if the inevitable variations across Ku Band are likely to be an issue, further technical enquiries must be made.
 

Channel Hopper

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So can you help me with this one then...

One dish has a "Gain at 12,75 GHz of 42,8 db"
Another dish of similar size has a "Gain at 12.5 GHz of 43db"

Which is the better dish gainwise, given they have different db at different frequencies...


Does either dish have a dedicated feedhorn ? If they both do then the second one listed would be better (unless the feed chokes the signal above 12.5GHz). If neither do then there is really no way of telling how those figures were reached, nor which feed or LNBF will perform the best.
 

Huevos

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Forget gain figures, just measure the dish. Anything else is just BS.
 

terryashbyash

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The main question is what are you trying to receive, I would like gain figures from 10.7 GHz to 11.1 GHz for freesat reception. However the higher the gain the better generally. It all depends what frequency is important. I go for the middle and tune for maximum smoke.
 
A

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Forget gain figures, just measure the dish. Anything else is just BS.
Well, size isn't everything, is it?
I agree that with most dishes, the size is the key determinant for selection when buying a new dish.

But in theory, the gain also gives an indication of the effeciency of the dish design and manufacture.
I.e. the SMC dishes tend to perform slightly compared to pressed steel dishes of same size (I say, bracing myself for the oncoming storm).
And indeed, when talking about cheap petalised stuff (Jonsas), it's a different kettle of fish as they are both badly made, and difficult to assemble really well. The gain figure given by the mfg may have nothing to do with the end result...

In practicality, the gain of two similar sized dishes (180cm) is roughly the same, so that figure alone is not enough for a choice.
And I agree, therefore irrelevant to most users.
 
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On that note; is there a difference between polarisation dispersion properties of various dishes?
I seem to remembet that the parabolic mirror screws up some of the polarisation depending on what bit of the reflector the magnetic wave hits. (And a second mirror fixes that on e.g. a gregorian.)
Is this the same irrespective of the reflector being solid steel or, say, a mesh dish?
 

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As there are so many variables involved with polarization and signal rotation I think it is better to look at the uniformity of the dish surface and the overall quality of the dish construction.
 

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gain also gives an indication of the effeciency of the dish design and manufacture.
Yes, if you measure it. But this is just BS from a spec sheet. For a start it is a perfect dish being measured under lab conditions. And that is if you believe the figures.
 
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