How to test LNB and area it can see ?

[Keano]

Hi guys,

As some of you may know I have a bigger dish ( 2.4mtr CM ).

The thing is I don't think the LNB can see all the dish, a "normal" LNBF gave me nothing on H beam on 7.0w.
The LNBF was invacom 0.3.

If I use a c120 with a feedhorn I have here then I do get 20%, but thats it no matter how little I move dish it will not increase.

The dish is spot, no dents, no marks nothing and looks fine.

The thing is I thought I would try something, what I did was I took a mag and placed in the center of the dish this cause no signal then I moved the mag outward to wards the edge when I got about a foot or so from the edge it went back to 20%.
This is the same all over the dish, no matter where I put the mag on the outers of the dish signal does not decrease.
This making me think feedhorn is only seeing about 200cm of the dish and not the full 240cm.

What you guys think ?

Keano

[proteus]

Logic is making me say move the lnb further back from the dish............Regards N.

[Keano]

Cant m8 the feedhorn is channel master feedhorn i have and it sits in the holder perfect.
There is a little rim in the holder and on the feedhorn arm there another rim and the two go together.

Here is a pic of the feedhorn I am using but with invacom c120.

Forgot to say, its not the one which came with the dish, its one i've had here for ages.

[ouagadougou]

Hi mate,

if you have the lnb installed using the CM feedhorn and feedhorn mount then you are at the focal point of the dish so you wont get much improvement.

If you are using another non CM feedhorn then the feedhorn has to have the correct illumination taper angle for the focal length/diameter ratio for the CM dish. Also you have to get it at the phase centre for the feedhorn which you do by the normal moving the feed closer or further away from the reflector for best result.

When you place objects on the dish surface they are in the near field zone of the dish which can be a weird area at times with strange results. The most likely result though is that you block a percentage of the reflector surface area which gives less signal reflected signal to the lnb. In this near field zone you can completely destroy the far field pattern of the dish which probably means very little or no gain at all.

You will only ever see the full surface area from the focal point so if you have the feedhorn at th correct focal length then you cant gain anymore unless you have a feedhorn that shapes the energy more efficiently over the required surface area.

I will see if I can find some stats for that CM240 dish, only got the 1.8m here for now.

Cheers.

[ouagadougou]

Forgot to mention that a typical feedhorn has a gain taper from the centre to the edge of the reflector. The difference is typically around 10dB so if the dish centre is 0dB then edges are typically -10dB down. This will also contribute somewhat to you seeing less/no signal when you block the reflector centre compared to blocking the edge.

[Keano]

Cheers pal, wished you were still living up the road m8:D.

Being doing some searching on net, and have found another feedhorn that is suppose to be a CM feedhorn that looks nothing like the one I have here.

One thing for sure something is not right, because 11747V on 7.0w is giving about 51% well I use to get about 47% with the gibertini 1.5mtr.

[ouagadougou]

Yours is the receive only version looking at the pics in the other thread so you should have 47.6dBi gain at ku according to the Andrew specs.

Gib 1.5m should have around 43.5dBi so you should be around 4dB better off which is huge all other things being equal of course.

We have an Andrew CM 1.8m on the office roof for VSAT with a Transmit/Receive feed and dual optics so there are quite a few different feed types for the same basic reflector. Yours looks ok in the pics so maybe its just the lnb.

Biggest problem with these dishes is you cant fine adjust the focal length of the Andrew CM feed especially on the receive only version like yours.

I spent ages with my NERA 2m dish playing around with focal length on weak signals and each feedhorn was different with a slightly different peak spot.

[ouagadougou]

Dual optics type

_http://www.andrew.com/search/BN_PA-100556.1-EN.aspx

Receive only type

_http://www.andrew.com/search/BN_PA-100555-EN.aspx

there are a few variants of these as well depending on age etc.

If you look at the Nilesat website or Lyngsat and the footprints then it seems Nilesat has a shaped reflector with higher gain on the vertical plane relative to the horizontal hence the horizontal pattern is much broader.

This could be one of the things that contributes to the big difference between vertical and horizontal signal levels when you are so far off the intended coverage like the north of England. It would seem a bit strange if all horizontal transponders are running significantly less power than the vertical ones so I doubt EIRP differences are a major cause of the horizontal reception problems.

_http://www.nilesat.com.eg/satfootprint.htm

_http://www.lyngsat-maps.com/maps/nile102.html

[Keano]

This is the other LNB i have seen.

[ouagadougou]

Thats for VSAT transmit and receive but the actual feedhorn is probably the same as yours upto the C120 flange. Its also somtimes used with dual optic type with a small subreflector for better cross polarisation discrimination.

[Keano]

OK m8,

Thanks I'm going to get another feedhorn, just to try it and see what happens.

Here is a picture of what I am using now.


By the way if you do ever come back home ( Wigan is lovely this time of year..honest ) then let me know.

Cheers m8.

[ouagadougou]

I might pass through Englands pleasant NW around xmas so will give you a shout.

I just got Nilesat here in Lagos and have just about the same signal levels on the Vertical as in NW England but no Horizontals yet and thats with a 2.4m fibreglass dish.

We just got 2 new houses here in Lagos so I am busy trying to figure out what size dish we can fit in the plot. Saw a lovely 7.6m Andrew for sale surplus the other day here for silly money :-D

[ouagadougou]

Just spotted this from Andrew with some of the feed type numbers on it.

Best check which one of the Dual Mode feeds you have ?

_http://www.andrew.com/search/BN_PA-100533-EN.aspx

[Keano]

Thanks mate.

I am going to ordered another feedhorns/lnb because tonight I stood in front of the dish at the edge and it never effected the signal:confused:.

Surely some big daft 6ft 2 idiot should block some signal ?

The link you posted I am currently using the feedhorn on the far right upto the flange then with c120 invacom.

[ouagadougou]

Its best to check the part number to be sure which Dual Mode Feed it is you have at the monent because they make a few for different applications and it could possibly affect the performance.

Notice that the Dual Mode Feeds are specified from 11.7 to 12.2GHz although performance wont cut off suddenly below 11.7Ghz.

[ManikM]

blimey - what a caper!!