Channel Master 1,2m issue

RimaNTSS · Feb 5, 2015

polgyver said:
I suggest making simple holder for LNBF

Perhaps it is not good idea to use metal strip around very head of the LNB, as it can influence wave flow. BTW, maybe, also metal feedarms (side bars) not making any good for signal.

polgyver · Feb 5, 2015

RimaNTSS said:
Perhaps it is not good idea to use metal strip around very head of the LNB, as it can influence wave flow. BTW, maybe, also metal feedarms (side bars) not making any good for signal.

Perhaps... the best would be to do a small experiment (I know, not during winter...).
There are 3 possibilities:
1. Metal ring around LNBF head will reduce Q, or,
2. Metal ring will increase Q, or
3.Will not affect Q
I bet 50 % for 3., 20% for 2., and 30% for 1.
Mitigating factor for 2. is that the extra ring would be enhancement for already existing scalar rings, hidden inside plastic.
In case of (1.), a holder could be made of plastic. But, as it is ONLY for testing the angle sight of LNBF, probably is not worth bothering.
Fully agree regarding side bars, as they are a weight burden on flabby dishes like my 4' Ariza.
Not sure, though, if such bars constitute any radiation obstacle.
Some members (myself included) made reinforcing straps with turnbuckles to correct distorted by side bars geometry of reflector.
What is the focal length of your dish? I checked the info in link, quoted by FaT Air, but there is no focal length mentioned.
Cheers, polgyver

RimaNTSS · Feb 5, 2015

This is mine CM1,2m dish profile with all dimensions

danristheman · Feb 6, 2015

Rima I have 1 meter dish just like your 1.2 meter dish my lnb for that dish is pretty low but had one great signal. Hispansat 30w 75q on my back porch.

RimaNTSS · Feb 8, 2015

Yesterday I visited @Trust , we placed his rotating laser in proper focal point and rightly pointed. You can see on VIDEO how dish is illuminated form there. Due to several reasons we could not make provable tests of system when LNB is in focus and LNB is out of focus (multifeed), but, I think that focal placement will always win

RimaNTSS · Feb 20, 2015

1 Query to everybody, but actually to those who has digital protractor and know offset angle of his/her dish/dishes:
Can you measure elevation on your dish when exactly tuned to satellite. Of course, dish should be on fixed mount (no polar mount or Diseqc motor). For some time I have some suspicions, that some dishes, particularly CM 1,2m, has LNB placed not in antenna's focus and, therefore when antenna is tuned to satellite with elevation let's say 31,4*, it is leaned backwards 8,1*. But we all know that CM 1,2m has 22,6* offset angle, so it should be leaning 8,8* to that satellite. There is 0,7* mistake. And those are real numbers from one of the CM 1,2m owner.
So, I would like to know is that everybody has same mistake. ATM I do not have possibility to install CM 1,2m, but very eager to find out what is going on.

RimaNTSS · Feb 25, 2015

Installed CM1,2m on balcony. Will do some testings soon (hopefully NLT Ester) about proper LNB placement.

RimaNTSS · Mar 4, 2015

Last weekend when we had sun outage I found out that great part of my dish obstructed by balcony's infrastructure. So, decided to make new stand. This will have possibility to get pole plumb. Still need some work to do (drilling couple of holes, painting), but hope to have it installed and CM120 on it newt weekend.

RimaNTSS · Mar 5, 2015

Finished with stand. After painting it looks better. Installation- soon

RimaNTSS · Mar 6, 2015

Back to the topic: I started this with statement that CM dishes are designed in the way that LNB is not located in the antenna's focal point and not pointed to the right spot. Then I had issue with balcony's rail obstructing receiving of signals, which I overcome by lifting antenna some 40 cm. So I made test- installed LNB in original CM holder , pointed antenna to satellite (in my case that was Eutelsat 7,3*W) and scanned all available frequencies, recording signal levels and putting them in the spreadsheet. Afterwards I installed self-made LNB-holder, which holds LNB in antenna's calculated focal point and points it to the right direction. Tuned again antenna to same satellite, recorded all signal levels and put them in the same spreadsheet for analysis. And I clearly see improvement at about 2% average. Did both measurements within ~20 minutes, and weather conditions did not change in this period of time. Will try to repeat this kind of tests just to make sure I am right.
Data is attached:

polgyver · Mar 6, 2015

Would be interesting to see also comparison of these LNBF two settings with optical test.
Could you, please, temporarily glue (with 2-sided THIN glue tape) a few small (10-15 mm), round mirrors to the dish?
Hopefully, the Sun would be present.
At this time of year (close to Sun outage), the change of elevation would be minimal (for getting the reflections on LNBF "face").
Surely, easier than serious job you just did with dish stand, and, hopefully, more fun ...
Cheers, polgyver

RimaNTSS · Mar 7, 2015

polgyver said:
with 2-sided THIN glue tape

Not easy to find one

and shopping day is over.

polgyver said:
Hopefully, the Sun would be present.

and this is not allays the case.

RimaNTSS · Jun 1, 2015

Hi everybody! I would like to share with you all drawing of CM120 dish. Everything what is in green color is measured (as precise as I could), and blue digits are calculated. I will elaborate little bit more about this issue later and also try to make some conclusions. I do not mind to receive some critics if needed

RimaNTSS · Jun 2, 2015

I will make now my conclusion (strait forward and perhaps provocative): Producer of antenna Channel Master 120 made feedarms and LNB holder in such a way that it is impossible to get maximum this antenna could provide. Perhaps there is a way how to adjust original LNB holder, but I would suggest to make completely new one.

Titanium · Jun 2, 2015

Thinking outside the geometry calculator box.... As provocative as my post may seem in response to your calculations and drawings, the factory engineered design may be intentional for increased sidelobe or terrestrial noise attenuation. Might I suggest real world testing to provide a graph depicting the main lobe vs side lobe/terrestrial noise attenuation with the feedhorn opening inline with the factory placement vs your calculated revised placement?

Something to consider, increased primary lobe gain may not provide increased performance if the side lobe signals (noise) are not attenuated at the same or increased levels.

RimaNTSS · Jun 2, 2015

Last Saturday I repeated following test: Installed original LNB holder to antenna, tuned it, as perfectly as I could (no cheating, I do not have any interest here), and scanned all available transponders from Eutelsat 7 West A @ 7,3*W. Results put in spreadsheet. Within not more than 30 minutes changed LNB holder and with same LNB tuned antenna to the same satellite. Scanned the same 44 transponders and put data in the same spreadsheet. Compared data and got at least +5% increase of performance.

RimaNTSS · Jun 2, 2015

I would like to believe producer of Channel Master 120, but cant! :
- LNB is located not in antenna's focus;
- LNB is not pointed to the right point on the surface of the dish;
- Performance of such system is worse than it could be;
So where is the logic? Should I take it for granted?

RimaNTSS · Aug 14, 2015

Decided to remove original feedarms and, instead of them, install self-made ones. Firstly, central LNB will be fully adjustable in all dimensions. Secondly, sidearms will not be obstacle for signal (original arms are obstructing). Thirdly, dish will be more ready for multifeeds. Yes, some additional holes in the rim of the antenna will be made.
There is the plan: