Large dish gain figures

[cheekymonkey]

I know published gain figures for dishes are not true indications of gain but was wondering what is the difference as far as gain which is quoted as a figure for 12Ghz and gain which is quoted as midband.

ie Prodelin 4.8 mtr dish quotes 53.5dB gain at midband and DH Satellites 5 mtr quotes 54.2 dB gain at 12 Ghz.

ie is the gain on a dish higher for higher frequencies or lower and also how much lower?
If lower then in the above scenario the 5 mtr dish possibly gives a gain increase of over 1dB compared to the 4.8mtr.

Been years since did any antenna theory and that was only yagis etc and did not fully understand the black arts then so got no chance now!!

 

[Channel Hopper]

Prodelin have never made a 4.8metre dish.

Anyway the overall gain at any frequency is not determined by dish area alone, the feed design (narrow or wide band) will affect the numbers at different frequencies.

Whilst it is simple to extrapolate the gain figures to the frequency required, sometimes the instaslled system just won't provide those numbers.

 

[cheekymonkey]

thanks for reply. As I suspected it is simply a case of you do not know exactly what you get until you have it. Especially with how critical alignment is on these big dishes. (ooops...was actually looking at General Dynamics 4.8 cassegrain not prodelin.)
I know DH say that if assembled correctly their 5mtr sectional performs the same as their 5 mtr one piece dish (note IF) whereas the Gen.Dynamics is actually 16 sections so I guess more scope for installation error. I think I will tell the guy who asked me for one to get it himself (and put it in himself).

 

[Channel Hopper]

A reputable manufacturer will (normally) have more than one gain figure, most commonly three of them at Ku band, denoting low, middle and high frequencies. These should be repeatable in the field for them to be published, but as you say, installation techniques vary as do the manufaturing control checks, the delivery to site, and the 'engineers' employed to fit them.

 

[Waveguide]

Got too much coffein today and reading antenna books is a tried and
trusted tranquilizer. The medication was [Per-Simon Kildal - Foundations
Of Antennas p. 331] and [C.A. Balanis - Antenna Theory p. 821].
Whatever you do - No overdoses of those please! It's heavy stuff!

Both books gives figures for surface accuracy and both end up with
a maximal deviation from the optimal shape less than approx. 2 mm to
have small (< 0,3 d impact on directivity. Their "directivity" is
your "antenna gain". Rough surfaces makes the central lobe smaller and
wider. Small f/d ratio makes it more sensitive to imperfections.
(diameter 5000 mm and wavelength 30 mm as input to the calculations)
A diagram in [Balanis] gives the same values for directivity as mentioned
earlier, 54 to 55 dB. Surface errors will reduce it from this maximal value.

Manufacturing method, assembly/installation of antenna parts will
determine if the dish lives up to whats in the specification.

The "finetuning" of the dish in Channel Hopper's image must be something
invented after my books were written.

... now I feel another double espresso would be nice.

(if there are smilies instead of "dB", blame the forum software)

 

[Channel Hopper]

(diameter 5000 mm and wavelength 30 mm as input to the calculations)
A diagram in [Balanis] gives the same values for directivity as mentioned
earlier, 54 to 55 dB. Surface errors will reduce it from this maximal value.

Which frequency is that figure calculated at ?

 

[Waveguide]

30 mm wavelength is very close to 10 GHz.
12 GHz would be 25 mm and directivity 1 dB or so improvement.
(Theoretical values without regard for blocking by LNB holder
and diffraction from support rods and other factors.)

 

[woborny]

This is something I have been thinking about / confused about for a while:

A Famaval 130 dish (measuring 1420 x 1300 from their spec sheet but sold in Spain as a 1.4m dish) has a "Gain at 12,75 GHz of 42,8 db"
A Tecatel 135 dish (measuring 1370 x 1220 from their spec sheet) has a "Gain at 12.5 GHz of 43db"

So does this mean the latter can be regarded as being the better performing dish - despite the PR in the newspapers about the famavals being the best dish?

 

[Channel Hopper]

So does this mean the latter can be regarded as being the better performing dish - despite the PR in the newspapers about the famavals being the best dish?

Does the website have something within it to the likes of 'E and OE' ? If so then the advertising hype is covered under poetic licence and means nothing in the real world

http://en.wikipedia.org/wiki/E&OE