Tests of 120cm antennas

[lovelyperson]

.............. Please do not suggest any new dishes for me.... I promised to wife that Visiosat is my last dish! ................But, at the same time.... if I see some dish at real bargain somewhere nearby, I will probably not be able to resist That is the dilemma

Contradiction or what ....nature of the hobby..........good man....you will certainly have more dishes .........we all do!!!

 

[RimaNTSS]

Contradiction or what ....nature of the hobby..........good man....you will certainly have more dishes .........we all do!!!

Do not like your post!!!

 

[lovelyperson]

Do not like your post!!!

You really are not going to have another dish......I/we all bet you will!
Sleep well RimaNTSS

 

[RimaNTSS]

Common Lovelyperson, I am serious this time! And, BTW, I promised to my wife! Can not sleep well under such a bad thinking of my fellow forumers! Perhaps some scoutch can help me to get well rested!

 

[RimaNTSS]

As for your dish warehouse, perhaps you could get rid of some of them?

Great idea!!! I like it.

once you're done with testing

You mean - never?!

 

[RimaNTSS]

Pointing LNB to the right spot on the face of the antenna could be done from anywhere. It is also important we have right angle between antenna's face (line A and LNB's direction (line FG). This angle (AB-FG) could be easily measured by using digital protractor with precision up to 0,1*. Measuring this angle could help us to find out whether LNB is located in focus. So, my query would be: Can somebody measure this angle and share results?

 

[Riverblue]

Pointing LNB to the right spot on the face of the antenna could be done from anywhere. It is also important we have right angle between antenna's face (line A and LNB's direction (line FG). This angle (AB-FG) could be easily measured by using digital protractor with precision up to 0,1*. Measuring this angle could help us to find out whether LNB is located in focus. So, my query would be: Can somebody measure this angle and share results?

What's the difference between blue 'F' and red 'F' in your diagram?
Which 'F' do you want to use for the angle measurement?
Is point 'A' the top edge of the dish and point 'B' the bottom?
Is point 'F' the phase centre of the lnb?

If the weather is ok I'll try tomorrow.

 

[RimaNTSS]

It is not a diagram, not even drawing, just a painting. All dimensions and sizes not real and can not be applied to any known dishes, it was just to show principle. Yes, point "A" is very top of antenna's working area. Point "B" is bottom point. Red "F" shows true focal point of antenna. Blue "F" - any point not far away from red one, in such position sometimes LNB is located (for different reasons not in focus). But from both those positions LNB can be pointed to point "G".
So, I would appreciate if you measure angle FG-AB on your antenna/antennas and then we think about number you got.

 

[Riverblue]

Here are my measurements;
Channel Master 120cm dish

 

[RimaNTSS]

And tuned satellite and location ARE? Thanks, I am on the walk now, will examine your angles later

 

[Riverblue]

On my dish I determined where the lnb is aimed by placing a long metal straight edge in the lnb holder (with the lnb removed) and noted where it touched the dish. Maybe it is not as accurate as a laser, but it clearly indicates that my dish (and probably all Channel Master dishes) is aimed at a dead central position. Also I measured the distances from the point of focus to points A & B and found them to be equal, 665mm.

Dish is aimed at 7.2W, my location approximately 50.8º N 0.0º E

 

[Riverblue]

More angles for Rima to peruse!

 

[RimaNTSS]

@Riverblue Just do not go bananas right away but:
- CM1,2m has 22,6* offset angle. Right?
-At your location 7.2*W has elevation 31,4*. Correct?
- Your dish is looking at something with elevation 30,7* (offset angle + AB line angle). Do not you agree?
Therefore, I suspect, LNB is not in antenna's focus! And dish is not performing at it's maximum capacity.

Add:

Also I measured the distances from the point of focus to points A & B and found them to be equal, 665mm.

Sorry, I do not understand this! AF=665 and also BF=665?!

 

[Riverblue]

@Riverblue Just do not go bananas right away but:
- CM1,2m has 22,6* offset angle. Right?
-At your location 7.2*W has elevation 31,4*. Correct?
- Your dish is looking at something with elevation 30,7* (offset angle + AB line angle). Do not you agree?
Therefore, I suspect, LNB is not in antenna's focus! And dish is not performing at it's maximum capacity.

Add: Sorry, I do not understand this! AF=665 and also BF=665?!

Not going to go bananas
CM has 22.6º offset; agree
Dish pointer shows 31.4º elevation; agree
22.6º + 8.1º = 30.7º; agree
Dish elevation and signal peaked on meter after much tweaking and fine adjustment, I could not get anymore out of it as it is put together (i.e. standard set up). If I were to adjust the elevation to 31.4º, then I know that the signal will drop off.
Now whether this confirms your notion that CM dishes are not manufactured with the focus of the lnb set up correctly or not needs to be investigated further. As I stated in post 131 the current set up on my dish focuses the lnb at a dead central position, not where your parabola program suggests. Until I get round to trying the lnb in slightly different positions then I won't be able to say whether I can get any more or not!
It sounds like it may need a bit more than just releasing the lnb from the holder and holding it in slightly different positions to see what difference it makes! Sounds like a prolonged 'fiddle' probably. I'm thinking of taping fine nylon string to the top and bottom of the dish, so that they are touching the surface where the useable edges of the dish start and marking off the lengths from the parabola program on the string so I can see where the lnb 'should' be positioned, take some readings, adjust elevation and repeat etc. I just need a spare day with nice weather!

AF-BF = the distance along the dish face from the top (or bottom) to where I determined the focal point to be, in this case, dead centre!
Make that AG-BG as per my diagram, my bad!;)

 

[RimaNTSS]

AF-BF = the distance along the dish face from the top (or bottom) to where I determined the focal point to be, in this case, dead centre!
Make that AG-BG as per my diagram, my bad!

No, F is antenna's focal point and G- is LNB's aiming point on the surface of the dish, let's stick to those aberrations to not get tricked.

If I were to adjust the elevation to 31.4º, then I know that the signal will drop off.

Yes, right, signal will drop, but then you change position of LNB (put it in antenna's focus) and signal will come back and perhaps at better level.

CM dishes are not manufactured with the focus of the lnb set up correctly

I am more and more sure about that.

I'm thinking of taping fine nylon string to the top and bottom of the dish, so that they are touching the surface where the useable edges of the dish start and marking off the lengths from the parabola program on the string so I can see where the lnb 'should' be positioned

I would suggest to use not flexible material, like metal wire (see attached picture)

I just need a spare day with nice weather

Could happen that one day is not enough

 

[Riverblue]

No, F is antenna's focal point and G- is LNB's aiming point on the surface of the dish, let's stick to those aberrations to not get tricked.

I would suggest to use not flexible material, like metal wire (see attached picture)
Could happen that one day is not enough

G= Aiming point, not focal point, Agree, getting my 'terminology' mixed up!

The nylon cord I have in mind isn't stretchy ;)

Yes, might take more than one day! Definitely if fabricating the lnb holder is needed. ;)

We are of course assuming that there will be some gain in signal, but if so, then how much, will it be worth the hassle involved?

The thing is as well, all this talk of non standard adjustment of a dish interests me and maybe a few others because seeing as we've been touching on this subject for a while I would like an answer! But the majority of people (especially "professional" installers) will "just" assemble their dishes as intended and get what they can out of them, they will not spend the time even considering what we are discussing let alone attempt anything. Obviously for enthusiasts on the reception fringes of a signal, all this could potentially make enough of a difference to enable them to receive a decent signal if they can be bothered to try, especially if any modification of the lnb positioning/holder is required.

 

[RimaNTSS]

You know, tiny bit on better cable, bit on better LNB, chunk of additional signal strengths from proper adjustment of the LNB and we get much better result than usual "installers".

 

[Riverblue]

You know, tiny bit on better cable, bit on better LNB, chunk of additional signal strengths from proper adjustment of the LNB and we get much better result than usual "installers".

Yes, I know, it becomes a "challenge" to see if you can 'just' get a little bit more!

 

[RimaNTSS]

We are of course assuming that there will be some gain in signal, but if so, then how much, will it be worth the hassle involved?

If headlights of your car are not properly aligned and you align them correctly: Will you see better at night? (perhaps not good comparison).
Installers do not care about precise alignment of dishes, they are too busy and do not have time.
Producers do not care as well, they should make money.
WTF
Only hobbyists are left alone.

 

[Riverblue]

Going back to the current elevation of my 120cm CM dish, 30.7º, I remember now, when I set it up I set the elevation as per Dishpointer, 31.4º, with my electronic angle gauge, then found that I had to adjust it to receive more signal.