Tests of 120cm antennas

[RimaNTSS]

Great idea! And reference dish should not be 1,2m, it could be also 1,8m.

 

[timo_w2s]

Great idea! And reference dish should not be 1,2m, it could be also 1,8m.

Or even a Gibertini 125.

 

[RimaNTSS]

Or even a Gibertini 125.

Yes, if you have it in stock
Add: And remembering it is only 121cm (maybe even smaller )

 

[RimaNTSS]

So, CM1,2m is clean and some homework could be done with it. First lesson, even without precise measurements, is that original LNB holder holds LNB at the wrong angle! I suspected that, and now got evidence. You see red dot, it is exactly in the middle of the dish, but should be much lower.

Add: And I suspect that other CM1,2m owners have same issue.

 

[samoloko]

have read an opinion from very experienced (ex member here Likvid) that If you move slightly down lnb on channel master 1.2 you will got better snr
never tried

 

[skomedal]

Did you rotate the boresighter on its axis when in the holder to see if the diode was centralized?

 

[RimaNTSS]

Did you rotate the boresighter

Yes, it perfectly stays on one spot.

 

[RimaNTSS]

If you move slightly down lnb on channel master 1.2 you will got better snr

That could be true. But if you just lean LNB little bit down, than you also need to lift it up little bit to make sure LNB's Phase center is located where antenna's focal point is.

 

[archive10]

So, CM1,2m is clean and some homework could be done with it. First lesson, even without precise measurements, is that original LNB holder holds LNB at the wrong angle! I suspected that, and now got evidence. You see red dot, it is exactly in the middle of the dish, but should be much lower.

Add: And I suspect that other CM1,2m owners have same issue.

Anf why is it that it should be much lower?

 

[RimaNTSS]

Anf why is it that it should be much lower?

It is oofset geometry. On the attached picture I draw red line, which represents middle of the dish looking from satellite, but that will be lower then antenna's geometrical middle. That what I was trying to say in this thread.
I just made a string test to my CM120, no problems at all. and calculated LNB's proper aiming point (green circle), it is about 140mm lower than original holder points LNB (red circle).

 

[skomedal]

Offset so called g spot

Still waiting for a explanation

 

[archive10]

I think you need to factor in that the CM120 (100 and 90 too) have the feed arms attached to the front of the dish.
The active area of the dish probably does not include the points where the feed-arms are attach to the dish.
This means that the active area is offset upwards in relation to the dish face.
The G-point of the active area is therefore *higher* than the "offset center" of the reflector (your green dot).
It may be close, if not spot-on the red dot..
(After all, somebody thought long and hard about this when designing the antenna...)

See attached modified drawing for putting this visually.

This does not detract from the CM120 being a 120, IIRC the reflector is in fact 125 x 134 or something like that.
This probably will include an (almost) full 120 cm working area of the reflector.

It could be different on reflectors sich as the Echostar, where the feedarms attach to the sides of the dish.
But we need to measure...

 

[RimaNTSS]

3 pipes through dish do not make big shadow, maybe 2 square sm each. Do not agree with you on this. I think it is simple- producer did not pay attention to this. And maybe this is only mine problem, maybe on another dishes, your for example, lazer would make spot where it should be?

 

[archive10]

Well, that's exactly the point Trust1 was making in your other thread on LNB's aiming point

Rimants , thats what I was thinking to the first time , maybe the feedarm was bended by this old mistreated dish .
So I drove to satshop Hamido (we've been there together) and tested at a new Triax and SAB dish with exactly the same result .
Both brands seem to use the lower section of the dish for the attachment of the feedarm through the dish .
Then I drove to my satfriend PaulvR here in town and tested it at a fiber Hirschmann dish with the feedarm under the dish .
There the spot was following exactly the edge of the dish .

To truthfully find out, you probably need to go beyond that russion focus calculator, and find some way of measuring the topology of the dish, and see where on the paraboloid it maps, and therefore where the maximum focus point is (and what lies outside of the active area).
Or, potentially easier, find the guy who did the design and ask him...

 

[RimaNTSS]

Do not like Russian, use English, such as Parabola Calculator 2.0. If you put there same data you will get more or less same data. 1200 x 1300 mm is antenna's working area according to producer. So, Parabola Calculator gives you distances from top of the dish to focal point and also distance from the bottom of the dish to focal point. So, you get triangle (remember such a figure from school times? ) where 3 lines are 1300mm, 721mm and 1220mm. Draw this triangle and then draw bisector line from antenna's focus. And, o sh*t, you are not getting bisector line to the middle of the dish, but somewhere lower, for example 140mm!
I am not trying to re-convince somebody if I am not sure in what I am saying. Do you?

 

[Riverblue]

I have to say that I find it a bit strange that a specialist company would get the design of one of their products so spectacularly wrong for the focal point to be 140mm out, maybe 5 to 20mm but 140mm? That represents approximately 12% of the dish face vertical measurement! I realise that some things are not made or designed well, but this is a dish that has been made by three different companies in succession for 15 years or so, apparently Skyware Global, the current manufacturers, still use the original Andrews moulds for the SMC dish faces. I find it hard to believe that these dishes have survived being manufactured for this length of time if they have been designed so poorly.
Maybe the maths of the "Parabola" program are not totally accurate?
Having said all this, if it's a nice day tomorrow, I might unscrew the top of the lnb holder on my 120cm Channel Master, hook it up to my meter and see if I can get any improvements by tilting the lnb slightly.

 

[RimaNTSS]

spectacularly wrong for the focal point to be 140mm out, maybe 5 to 20mm but 140mm?

Focal point is no problem, but aiming point is out 140mm. At the same time, remember, we found out another day that IBU's illumination angle is 84*, but opening angle of CM120 is 80*, so even if LNB slightly out of direction we still getting good signal

 

[Riverblue]

Sorry , I meant "Aiming Point"

 

[Riverblue]

Rima, we have already hinted at the possibility of the lnb not being in the right place before in this thread;
Make Gregorian dish | Page 2 | Satellite & Digital TV Support forums my post *27
It will be interesting to see if it is out. ;)

 

[Channel Hopper]

I don't understand this at all. Do you mean to compare an offset with a primary focus dish perhaps? Or maybe a "conventional" offset dish with a $ly type offset dish?


I believe that Gregorian, and the other dual reflector type Cassegrain, benefit in three areas. Firstly having a longer focal length gives extra gain, Secondly altering the optical characteristics on the secondary reflector gives greater selectivity and thirdly, but least, by placing the LNB in a position where it can accessed more easily.

Just saying that comparing one 1.2m against each other, purely on gain, or disallowing another dish, purely on 'diameter', is unlikely to bring a result that identifies which design/manufacturer is 'the best', especially on a hobby forum.

Different strokes and all that