Why are big satellite dishes more critical to align?

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Basil
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Why are big satellite dishes more critical to align?

Post by Basil » Tue Oct 12, 2021 7:27 am

I was unable to find anything on Google to suggest why bigger dishes are more critical to set up, can anyone briefly explain please? Thanks.

g0mjw
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Re: Why are big satellite dishes more critical to align?

Post by g0mjw » Tue Oct 12, 2021 7:45 am

Because the beamwidth is narrower. Therefore they need to be pointed more accurately.

Mike

Basil
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Re: Why are big satellite dishes more critical to align?

Post by Basil » Tue Oct 12, 2021 7:53 am

OK, thanks, but why would a bigger dish have a narrower beam width, it appears the converse of what I would have thought? Something like Jodrell Bank must be ultra finicky! Thanks Mike.

g4eml
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Re: Why are big satellite dishes more critical to align?

Post by g4eml » Tue Oct 12, 2021 10:48 am

Beamwidth is a measure of how much the beam spreads out with distance. The beam is not parallel it is more like a cone, spreading out with distance. Simplifying things quite a lot, imagine it as a cone of light producing a circle on a distant wall. The further away the wall is the larger the circle gets. A larger dish produces a narrower cone so it spreads out less with distance, producing a smaller, brighter circle at the same distance. The circle is brighter so you have achieved some gain, but it is smaller so you have to align it more accurately.

Basil
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Re: Why are big satellite dishes more critical to align?

Post by Basil » Tue Oct 12, 2021 12:36 pm

OK, got it now, that was a nice simple explanation that even I could grasp :) Thank you. I am fabricating a mount for a bigger dish I have acquired, so I had better make sure it's darned rigid with no stiction in its adjustment!

g0mjw
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Re: Why are big satellite dishes more critical to align?

Post by g0mjw » Tue Oct 12, 2021 12:47 pm

Thanks Colin,

I thought this was obvious to everyone but perhaps it is not. https://en.wikipedia.org/wiki/Parabolic_antenna

The half power beamwidth is a measure of how accurately you need to point the dish, if you are just outside the half power beamwidth you are going to lose half the power, or 3dB. The value in degrees is around 70 x wavelength / diameter. For a 1.2m dish it is 1.6 degrees, so you need to point better than that, ideally much better if you are not going to lose anything significant. A 2.4m dish is twice as hard to point, a 0.6m twice as easy.

I would also add that larger dishes are big and heavy which makes it even more difficult to set them up and they need significantly stronger foundations as the force applied by strong winds can be very high. You can expect 12 pounds per square foot from a 70 miles an hour gust, roughly 60kg/sqm. For a 1.2m offset dish the area is about 1.5sqm so around 100kg, not too bad. That loading will probably not be symmetrical with the mount so a few paving slabs or a bag of sand isn't really sufficient, but 70MPH winds at ground level are not that common in UK gardens.

Mike

Basil
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Re: Why are big satellite dishes more critical to align?

Post by Basil » Wed Oct 13, 2021 8:22 am

Thanks again Mike. I must admit initially to me that is counter intuitive, if I'd had to guess I would have said a bigger dish would be les critical, but I see now.

That brings me to another dish question, does dish diameter correlate with ideal aperture diameter in say the copper tube used for feeding the LNB with a POTY antenna?

Thanks , so much to learn here...

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PA3CRX
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Re: Why are big satellite dishes more critical to align?

Post by PA3CRX » Wed Oct 13, 2021 8:54 am

The diameter of the dish does not influence the feed. The feed should illuminate the dish properly and that is related to the f/D, the focus/diameter of the dish. If you have a large and a small dish with the same f/D (for example 0.6), the feed will be the same.

You may look forward the coming issue of CQ-TV to read the article I wrote about this ;)

Basil
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Re: Why are big satellite dishes more critical to align?

Post by Basil » Wed Oct 13, 2021 1:23 pm

Thanks, I will indeed look forward to reading your article in due course, all the best!

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