two degree spacing & KU

[turbosat]

With all these problems reported with G18 and AMC21 possibly interfering with each other, I wonder : What is the minimum size ku dish that would avoid this, with the now 2degree spacing between sats? I remember reading somewhere long ago, this was by somebody who knew the math, and frequencies involved in C-band, that the minimum size for a C-band to avoid such troubles was 8.5 feet. Most of us with 10' dishes properly aligned never had problems with interference back in the old days, and I haven't noticed any since the skies are full now either. Anybody know the answer?

 

[qwert1515]

-3 dB Loss @ 11.7 GHz

70 cm (27 in) sees 2.54° to each side.
80 cm (31 in) sees 2.22° to each side.
90 cm (36 in) sees 1.97° to each side.
100 cm (39 in) sees 1.78° to each side.
110 cm (43 in) sees 1.61° to each side.
120 cm (47 in) sees 1.48° to each side.
150 cm (59 in) sees 1.18° to each side.
200 cm (78 in) sees 0.89° to each side.

Here is a good "Antenna beamwidth calculator".

The following images are from: The Battle of the Bands

 

[turbosat]

qwert, I'm sorry but most of that is over my head, as I don't know how much 'db loss' is equal to one sat messing with another. Which amount is acceptable? Is your " -mark supposed to mean degrees? I never got into it deep enough to study beamwidths and stuff, and I flunked out of the NRI Satellite course because I was like Romy and Michelle in math.

 

[qwert1515]

That should show up as a degree symbol.

-3 dB from the main lobe is what is usually considered for interference.

If all dishes are pointing at 122.85° (G18's exact location) West they will pick up interference from:

70 cm dish [120.31° - 125.39° West]
80 cm dish [120.63° - 125.07° West]
90 cm dish [120.88° - 124.82° West]
100 cm dish [121.07° - 124.63° West]
110 cm dish [121.24° - 124.46° West]
120 cm dish [121.37° - 124.33° West]
150 cm dish [121.67° - 124.03° West]
200 cm dish [121.96° - 123.74° West]

121.0°W is really at 120.77°W and 125.0°W is really at 124.85°W.

So it looks like a 90 cm dish (by 0.03° to the 125.0° West side) is the bare minimum dish size for interference free reception with a 2° Ku band spacing if you have no errors.

Using my pansat 3500 and disecq 1.2 one click is 0.3°
If you look at the dish you can't see a difference, if I take that into account (Alignment error of +/- 0.3°) you really need to have a 110 cm dish.

 

[turbosat]

Thanks!!!!! And your degree sign is right, Firefox is displaying it properly. My eyes weren't quite tiny enough tonite to see it lol.

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[qwert1515]

Changes to the previous post are done

Now I can see why people are complaining about issues with G18 after AMC21 was launched.
(I have not seen any problems with my 1.2 Meter dish and the math above shows that I am ok until they decide on using a 1.5 degree spacing )

 

[Mr Tony]

I must be in the sweet spot because I have no issues with either sat on the 30" dish

the fixed dish at G10 is a 40x30 primestar

 

[turbosat]

1m is ok here

Not having any problems with my 1m primestar , so far with 125. Maybe things will tighten up some when they are done testing, aligning, etc. I got a call about a 10' dish I helped put up back in 95 or 96, I think it's a Sami. The owner doesn't need it any more, said I could come get it. Thinking I might just put it up besides my others and make it a totally dedicated 10' KU signal grabber. Hope the motor still works.

 

[qwert1515]

Have you notice any signal quality changes after AMC21 was launched?

 

[tvropro]

According to the specs on my 84cm dish it's 2 degrees at 3db down. So 123 being 1.8 degrees away from 125 makes sense with me bumping the dish a tad. One thing that wasn't brought up here is the efficiency of the dish. A dish with better surface accuracy has better efficiency and can be smaller, sloppy reflectors don't play well at 2 degrees.

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[tvropro]

I must be in the sweet spot because I have no issues with either sat on the 30" dish

the fixed dish at G10 is a 40x30 primestar

Must be all your other dishes shielding the interference.

 

[Mr Tony]

its gotta be that 6 footer and that 75e Starchoice dish blocking the interference

 

[Cadsulfide]

Explains why my modified superdish is losing signal on 123, time to swap it with the primestar. Thanks for the great link tvrporo