4 foot Ku dish question.

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belter-one
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4 foot Ku dish question.

Post by belter-one » Fri Oct 05, 2018 2:37 pm

Hello,

Not too long ago I purchased a 4ft Ku dish, offset I think, that might also get some c band channels as well. A recent video on this dish indicated that in order to receive usable signal from the clarke belt satellites, the dish would not be pointed directly at the clarke belt, like a C band dish, but the elevation would be much lower, and this would allow an LNB to successfully resolve Ku channels, and also C band channels. Is this correct? If so it is going to have to be mounted on the roof, as that is the only place on the immediate property that could possibly have access to open sky required. It seems counter-intuitive to not point this dish directly at the clarke belt. Any input would be greatly appreciated. The dish was the same dish described by this link: https://www.tvrosat.com/forum/phpBB3/vi ... 114&t=4939 .
I attempted to upload a picture of the dish from my computer, but was unable to do this. Given the state of the internet these beautiful days, quite understandable and appreciated. :razz:

Seeking out more signal :razz:

belter-one.

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Re: 4 foot Ku dish question.

Post by skink » Fri Oct 05, 2018 7:19 pm

You are correct about look angle.The Ku dish has the feedhorn located away from the focal point which lessens blockage.It can receive C band with the right electronics attached,but only some channels.

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Re: 4 foot Ku dish question.

Post by RimaNTSS » Sun Oct 07, 2018 9:00 am

skink wrote:
Fri Oct 05, 2018 7:19 pm
Ku dish has the feedhorn located away from the focal point which lessens blockage.It can receive C band with the right electronics attached,but only some channels.
Let me disagree with that statement. Feedhorn should be located exactly in the focal point of the offset dish. But focal point is located away from the direction towards satellite, therefor there is no blockage of signal. Offset dishes also can be used for C-band reception, only the proper feedhorn should be used. So, there will not be big difference in performance between prime-focus and offset same size antennas.

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Re: 4 foot Ku dish question.

Post by skink » Sun Oct 07, 2018 4:48 pm

I stand corrected.I didn't mean the focal point. On the other hand, I would think when you are dealing with a small aperture reflector,a lnb assembly in the center would cause a lowering of a signal in a prime focus reflector?

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Re: 4 foot Ku dish question.

Post by tek2000 » Mon Oct 08, 2018 8:07 am

belter-one,

An offset reflector is really just a section of a prime-focus reflector, where the feed appears to be "offset" and hence the name. When aimed at horizon satellites, it will look like your dish is pointing into the ground (this is normal). The main advantage of such a reflector is that the feed doesn't block any incoming RF. The main disadvantage is the more complex elliptical illumination pattern required from the feed. For small antennas, the advantages outweigh the disadvantages and this is why people use them for ku and ka reception. For large antennas, the blockage advantage is mostly lost and people don't generally use them because it complicates the feed design. A picture is worth a thousand words:
offset_antenna.jpg
offset_antenna.jpg (9.49 KiB) Viewed 1009 times
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Re: 4 foot Ku dish question.

Post by RimaNTSS » Mon Oct 08, 2018 11:00 am

tek2000 wrote:
Mon Oct 08, 2018 8:07 am
The main disadvantage is the more complex elliptical illumination pattern required from the feed.
This can not be disadvantage of an offset dish as looking from feed antenna seems perfectly round and illumination pattern is not elliptical.
But another advantage of the offset antenna design, especially for northern countries is the fact that such antenna does not collect snow on it's surface.

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Re: 4 foot Ku dish question.

Post by tek2000 » Mon Oct 08, 2018 1:30 pm

RimaNTSS wrote:
Mon Oct 08, 2018 11:00 am
tek2000 wrote:
Mon Oct 08, 2018 8:07 am
The main disadvantage is the more complex elliptical illumination pattern required from the feed.
This can not be disadvantage of an offset dish as looking from feed antenna seems perfectly round and illumination pattern is not elliptical.
But another advantage of the offset antenna design, especially for northern countries is the fact that such antenna does not collect snow on it's surface.
I'll give you the snow shedding advantage of an offset dish, and it would sure be nice in Canada!

However, with offset reflectors, you lose the inherent symmetry found in the prime-focus reflector. If you look at a typical offset dish, it has a longer axis in the vertical (up/down) direction. This means an ellipse was cut from the parent prime-focus. Why? If you take a flashlight and point directly at a wall, the illumination is circular. If you tilt the flashlight, you will get an elliptical illumination. So if you tilt the feed, you will be projecting an elliptical illumination onto the reflector, which is why the dish is cut as an ellipse. If on the other hand you cut a circular reflector from the parent prime-focus, you have to change the feed to an elliptical shape so the projection goes back to circular. Does this make sense?

If you want to see what an elliptical feed design looks like, take a look at a modern DirecTV dish. It has a longer surface in the horizontal direction (for multi-satellite reception). The scalar rings and the horn part are elliptical in order to 'stretch' the illumination along the horizontal direction to maximize the gain. A lot of R&D went into that horn design and they probably have it patented too.

There is another important advantage to an offset dish that nobody has mentioned. Because the feed generally points at a steeper angle up in the sky, it will pickup less background noise from the earth (especially for horizon satellites) one side of the arc. This in theory means a slightly better S/N ratio.
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