Help with Lnb for dish?

joe250

HI

I was asked about getting a Lnb for a dish but cannot figure out which Lnb to get.
Dish is about 2 meters diameter.
I have attached a few pics of dish.

12 element

joe250 wrote: »

HI

I was asked about getting a Lnb for a dish but cannot figure out which Lnb to get.
Dish is about 2 meters diameter.
I have attached a few pics of dish.

What diameter is the collar that's holding in the LNB that's in there it looks like it's held in with tape? Where did you get the dish?

joe250

HI

Not sure I will ask him.Lnb at the moment is held by tape which he has done himself picking up 190channels on 26east.

He got the dish out of a skip in dungarvan about 3 years ago

settopbox

Nice bit of kit there mate.

Well wear.

BTW I have no idea what lnb you would need.

Apogee

You need a Prime focus LNB. You can get an LNBF with the feedhorn integrated from MTI like this:



Or else a C120 LNB (e.g. Invacom or Inverto) with bolt on feedhorn like this:


You can get brackets from the US which will attach to your feedarms and hold the Ku LNB/F in place.



http://www.satellitesuperstore.com/feedhorn.htm
http://www.inverto.tv/
http://www.invacom.net/
http://store.fridgefta.info/wp-content/uploads/2010/08/kutocband.jpg

watty

ALL LNBs and LNBFs are actually used "prime focus" (unless it's a Multifeed boom or a Wave Frontier)

Some dishes are offset. like a cut out of side of a regular parabolic dish.

F/D is the important issue.
Offset dishes have fairly standard F/D (0.8?)
Large symetrical dishes with centre feed can have ANY F/D
You need Flanged LNB and feed horn with matching F/D for make and model of dish.

Nothing to do with "offset", Prime Focus or Center Feed.

Three most common kind of dish

All examples above have LNB at "prime focus". A non prime focus is an LNB offset to receive a different satellite to the one the dish is aimed at. If the LNB arm is bent, then the LNB is no longer at the prime focus, but the dish will still work if aligned, but with a gain reduced depending on the squint. The highest gain is at the prime focus. For f/D, the D is the straight line across the middle of dish at widest point, the diameter. The f is harder to measure. f/D is best taken from the Manufacturer's specifications. Many dish are oval and also an offset dish is only "circular" if the vertical axis is longer! Oval dish ideally need a oval horn or focus rings on the LNBF, which is why while Sky LNB can be used on regular dish and vice versa, best performance is LNB designed for elongated horizontal axis. (Circular or vertically elongated dish gives more gain for same amount of metal, but sky dish shape gives slightly more adjacent satellite rejection).

All parabolic dishes have the parabolic curvature, but some are shallow, while others are much deeper and more like a bowl. A convenient way to describe the depth is f/D ratio, the ratio of the focal length f to diameter D of the dish.

The rings or plastic or horn shape of LNBF or feed with flange has a certain "beamwidth". That has to match the f/D for the dish. Too narrow and you don't get all the signal the dish is focusing and too wide and the the LNB "sees" past the edges and picks up cosmic noise or noise (sun reflections or thermal generated mostly worse in daytime) from walls.

see http://www.qsl.net/n1bwt/chap4.pdf

Apogee

Difficult to see the benefit to the OP by referring to all LNBFs as 'prime focus' when they are sold/marketed as 'offset' and 'prime focus'.

watty

I can't help it if many shops have no clue what they are talking about.

Actually the wikipedia picture for Cassegrain is wrong, if the secondary reflector was offset it would be Gregorian. The Wave frontier I think is a modified toriodalised Gregorian. Cassegrain uses an inverse secondary reflector. Hyperboid or something.

The essential issue is that LNBFs (integrated feed) is for ordinary cheap domestic dishes up to about 1.1m that are usually offset feed point (not offset focus) and centre feed, usually only on larger dishes usually need a feed horn specific to the dish and flanged LNB, almost never an LNBF. Some dishes are offset feed and need horn + flange, usually only 65cm to 1.2m for VSAT (two way).

This is for C-Band. It has adjustable focus for different f/D


I have a painted C-Band LNBF which is slightly unusual as it's not flanged, but the annular ring part (focus rings instead of horn) is adjustable on waveguide over a common f/D range.

Apogee

watty wrote: »

This is for C-Band. It has adjustable focus for different f/D

It isn't. Had you bothered to click the first link I provided, you'd have seen that it is an Inverto C120 Universal Ku LNB and a prime focus Invacom feedhorn.