Just Sharing This Coming to a forum near you soon - mobile dish test rig

[John]

... Interesting results after mounting original CM 1.8 stock arms/feed assembly alongside ref: Jig.

Assuming iv'e not dropped any major clangers (balls ups) with the comparison task it emphasises that for maximum possible performance of a dish / feed, imo, any dish above 1.2 mtr and upwards needs to be double checked and accessed in greater detail than just putting it all together assuming it to be 100% correct. I did the same on my 1.2 Gibby Ka dish set-up.

I'll process some pics asap

 

[s-band]

I agree. I am struggling to get my 1.5 IRTE to exceed the Prodelin Ka 1.2 (on Ku). The Prodelin is on an az-el and has tweakable feed position and skew to get the optimum set-up. The original, fixed position, IRTE feed is about a dB down on an Invacom C120 feed which has variable beamwidth and on which I can move the focal point. (Octagon LNBs, modded to C120, on both)

 

[John]

... Interesting results after mounting original CM 1.8 stock arms/feed assembly alongside ref: Jig.

Assuming iv'e not dropped any major clangers (balls ups) with the comparison task it emphasises that for maximum possible performance of a dish / feed, imo, any dish above 1.2 mtr and upwards needs to be double checked and accessed in greater detail than just putting it all together assuming it to be 100% correct. I did the same on my 1.2 Gibby Ka dish set-up.

I'll process some pics asap

.. Some comparison pics, which helps to illustrate the dodgy illumination mapping on stock set-up.
Next to do is modify the feed position & angles to emulate the ref:
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[Channel Hopper]

So, you suffer from droop.

 

[Channel Hopper]

I agree. I am struggling to get my 1.5 IRTE to exceed the Prodelin Ka 1.2 (on Ku). The Prodelin is on an az-el and has tweakable feed position and skew to get the optimum set-up. The original, fixed position, IRTE feed is about a dB down on an Invacom C120 feed which has variable beamwidth and on which I can move the focal point. (Octagon LNBs, modded to C120, on both)

Without hijacking the thread, which feed assembly are you using on the IRTE ?

I installed a couple of 1.5 reflectors that were substandard in performance out of the box, they gave a second focus point at approximately 2 degrees off prime, which could be improved but not eliminated by standing off the backplate by different sized grommets.

 

[John]

So, you suffer from droop.

... It comes to us all - - eventually

 

[RimaNTSS]

Rima, can you please clarify your ref: dimension B to F on your file - my file pic has that measurement slightly obstructed.

For 1.8m CM it is 1080mm

 

[s-band]

Without hijacking the thread, which feed assembly are you using on the IRTE ?

I installed a couple of 1.5 reflectors that were substandard in performance out of the box, they gave a second focus point at approximately 2 degrees off prime, which could be improved but not eliminated by standing off the backplate by different sized grommets.

This one : https://www.satellites.co.uk/forums/attachments/c-ku_1-5m_dsc1003s-jpg.91098/
It came with the earlier feed which has a PTFE dome but was really only for the FSS band, I think.

Interesting. The dish always had an asymmetric pattern. I've got a new back plate for it so I'll keep that in mind when i finally sort it out.

 

[Channel Hopper]

Hmm

Take the reflector off the backplate, place on a decent flat surface (large plate glass window) and check for edge distortion, refit with grommets, boresight the Ku feed on its own and try again.

Yes, the ice cream cone feed wasn't particularly responsive, though they did make a third type with larger scalar and dielectric plate of hard perspex you could remove.

 

[John]

.. Started the feed re- alignment, didn't really want to butcher the existing feed arms but i can't see me parting with this dish so carried on.
1st job was to put a kick upwards on the bottom lnb arm to cancel out the feed angle error. First attempt pretty close but needs a tad more kick and then shorten the the three arms to bring the focal point to the desired area.
I will also mod the stock CM 1.8 scalar feed so as to allow some forward / backward movement when used with a Ku test lnb.
Any Ka or C-band testing will have a custom feed holder made to suit using the Ref: jig for positional location.
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[RimaNTSS]

@John I have already went through process you are going now. And my final solution is: new, length-adjustable feedarms + universal (C/Ku/Ka) LNB-holder. Holder can hold LNB in any position you wish and pont it to any dot on the surface of the dish. All INOX made. In case you brake original CM feedarms and need replacement I can send a set to you.

 

[John]

@John I have already went through process you are going now. And my final solution is: new, length-adjustable feedarms + universal (C/Ku/Ka) LNB-holder. Holder can hold LNB in any position you wish and pont it to any dot on the surface of the dish. All INOX made. In case you brake original CM feedarms and need replacement I can send a set to you.

Thanks Rima, i'll keep that in mind

 

[RimaNTSS]

But in case when original CM feedarms are used it is better to change plastic LNB-holder. This also can hold C/Ku/Ka LNB and change it's position and direction, but has some limits though.

 

[John]

But in case when original CM feedarms are used it is better to change plastic LNB-holder. This also can hold C/Ku/Ka LNB and change it's position and direction, but has some limits though.

I do like the C-band Inox holder Rima, have you got any already made up that i could purchase ?.

 

[a33]

The aiming point wasn't set by me Rima, the CM is using the stock arms and bottom arm bracket at the bottom of the dish and has the original lnb holder. I accurately 'sighted' through the original CM feed to see where it landed on the dish face and then marked it on the dish face. It would not be possible to bore sight it through to land on the lower spot using the original equipment. I think i mentioned a few posts back that i needed to look at your reference diagram showing all the relevant measurements for the 1.8 because it looked a bit different to your diagrams, but not got around to it yet

Well. I guess there's two things going on here.
One is the Phase Center of the feedhorn, not positioned at the focal point. That is crucial to correct, I would say.

The other is the aiming point for the LNB. I've read about tests (some French tests, I believe) that the best results were gotten somewhere between the G-spot (at the end of the bisector line between AF and BF) and the deepest point of the dish (which lies, of course, between G-spot and middle of the dish).

Of course when aimed at the G-spot, spill-over will be least.
However signal density (per degree) at the LNB will get higher, when aiming higher at the offset dish (due to the fact that the dish surface is further away from the LNB ).
So I guess some compromise between these effects would be best.
(I don't know the size of the effect of the worse X-Pol, when thus using a 'higher offset angle'.)

Therefore I am not holding the position that the G-spot should always be the aiming point, I believe a bit higher is also plausible.
I never tested this myself, I must confess.
But above the deepest point is indeed a no-go, I would say.

Greetz,
A33

 

[nelson_b]

I do like the C-band Inox holder Rima, have you got any already made up that i could purchase ?.

Following this thread with interest .... currently on holiday with very intermittent internet here ... can recommend Rima,s adjustable l.n.b holder ... have used one extensively this past 18 months ..
Cheers

 

[a33]

... here's my measurements of component parts compared to Rimants ref: picture
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Ref: - A to G = 1186. - - - My A to G = 1061

BTW The 1061 seems to be exactly the distance from Top to Deepest point.
See the calculation, line 39, of the attach.



Greetz,
A33

 

[John]

Following this thread with interest .... currently on holiday with very intermittent internet here ... can recommend Rima,s adjustable l.n.b holder ... have used one extensively this past 18 months ..
Cheers
[/QUOTE


... there's one on the way Nelson only just got internet back on here - has been off since 8am

 

[RimaNTSS]

I've found my several year old drawing about 1.2m CM antenna issue. I am pretty sure that 1.8m CM has more or less same problem (perhaps it is too strong word for this )
So, what have I found out years ago: firstly, original LNB-holder does not hold LNB in the way that it's phase center (PC) co-located with antenna's focal point (FP). And it can not be corrected by moving LNB closer/further to the dish, there will always be distance between PC and FP. Secondly, feed pointing is too high and we get noise-catching and under-illuminated areas as @John rightly pointed earlier.
I think that feed should divide opening angle of the antenna (AFB angle) exactly in halve. So, point G is proper point where to aim LNB. I also made some tests, and as far as I remember, pointing feed to G gave me best results.
Another vital parameter of any offset antenna is it's offset angle. All CM antennas have 22.6* offset angle (if we go more precise it is 22.62*). I trully believe, producer really made antennas with 22.6* offset angles. So, if we have antenna pointed to satellite (maximum signal level) with elevation angle 30*, than antenna should be leaning backwards 7.4*. But....if original LNB-holder is used, antenna is pointed to satellite with 30* elevation after measuring leaning of antenna you will not get 22.6*. Moreover, when original LNB-holder is used and you move LNB closer/further from the dish, you will need to adjust antenna's elevation each time.

 

[PrvtN00b]

Further to Rimants observations, it got me curious re: the accuracy of the CM 1.8 stock install as straight out of the box.
This dish never got used in stock mode - the feed support was a custom built thing to take a rotating lnb changer.
The previous pictures showing the illumination landing spot is with a correctly installed set of arms and CM feed that came with the dish.
Tonight i rushed up a temp laser marker fixed onto the end of a 40mm short tube that fitted the CM feed holder at the focal point - most people would take the view that if you did a careful feed install of a highly rated dish, that you can't really get it wrong due to everything being pre-drilled etc etc and that it would be spot on - - well here's the results video of a quick illumination check

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in other words, we are not even using the full potential of these dishes at all??

thanks.