23mm LNB vs 40mm

delorehal

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Has the LNB's diameter any incidence on the LNB efficiency.
A seller told me that 40mm LNB are better.
At 10.7GHz (lowest Ku frequency), the wavelength should be equal to 2.8cm, so 23mm should be insufficient ?
 

Captain Jack

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23mm LNBs are only really useful in multi-feed setups where you need two LNBs close together... They also tend to suffer quite badly from rain fade.
 

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A slightly smaller diameter throat will pass lower frequency wavelengths adequately within a certain bandwidth. It is when you wish to pass higher frequencies through a larger diameter that the problems are apparent.
 

RimaNTSS

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I am not completely sure, but perhaps 23mm and 40mm those are only diameter of the plastic body of the LNB. The waveguide in 40mm LNB has same diameter as waveguide of 23mm LNB. So, should not be big difference in performance.
 

s-band

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C120 flange has a 19mm internal diameter and using
Circular Waveguide frequency calculator | converters and calculators
gives a cut-off of 9.3GHz that's the low frequency limit and it's best to keep clear of it.

On the high side things go wrong if the guide is over-moded which is a bit more complex. Practical effects can be confusing.
Properties of Circular Waveguide Modes (formulas) - RF Cafe
Rectangular & Circular Waveguide: Equations & Fields Formulas & Calculator - RF Cafe

Useful table for standard circular guide sizes: http://www.cernex.com/PRODUCTS_NEW/Tech_info/Circular Waveguide Sizes.pdf

I have no evidence but I would have thought that the small diameter ones that are not so good are those that use dielectric feeds or under-size horns. If you take the plastic off most modern LNBFs you'll fine the innards are 22mm ish outside diameter. (Octagon PLL single is 21mm o/d)

Edit: RimaNTSS beat me to it
Edit 2: Attached a pic of one I pulled apart some time ago
sky-q_DSC1670.jpg
 
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delorehal

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sonnetpete

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To be honest, if you are using the LNB as the only one on your dish, I'd avoid the narrow ones. I've had to use one or two on my multisat setup and as has been mentioned, they are more susceptible to rain fade.
 

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>Does the 23mm feed reduce signal entrance or only "field of view"

Neither necessarily. "Field of view" is a function of the feed part of the LNBF

I presume that you are using an offset dish. If so, I have found this one very good for the money: LNB OCTAGON SINGLE PLL OSLO It is 21mm diameter under the plastic, not that it has any bearing on the choice

I suggest that you phone 2galli, who have a good reputation. Tell them what you want to do and with what dish and try their suggestion.
 
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