Just created my dish an offset
- Thread starter batzen1981
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Interesting, now I wonder if offset works better than focus point in terms of signal gain quality?
Maybe not much I guess?
Maybe not much I guess?
I think it is a useless creation. I see no point to go this way.
why? less chance of signal loss due to a big obstruction (feed assy.). especially for a small reflector. si?
.....what happens when you stick a finger right in front of your pupil a foot or so out there versus below your eye lid?
Instead of just dissing the OP how about giving him an explanation as to WHY you think what you do?
Very simple...such a setup will diminish performance of the antenna at least 3-4 times. So, a simple 1.2m offset antenna will perform better than this one. That is why it is a useless creation, IMHO.
Most certainly.
As this prime focus dish itself is already no more than 1.22 meter (4 feet)
, I guess:
But indeed, I don't see much reason to put an LNB in such an offset position from the focal point (20 degrees or so?), unless it is in a multifeed setup where you have no alternative.
batzen1981 , what was your idea behind this setup?
Greetz,
A33
Thank you for explaining it to the OP. I know from your posts that you have done extensive work with dishes and are very knowledgeable. Even as a relative newbie to this hobby (2013) I also knew from years of studying that this might work to some degree but would not be an optimal setup. My point, when I saw your post, was that perhaps the OP did not and it is always best to give reasons why rather than just knock someone's work. That is why Satellite Guys was created - to be a friendly site where we can all learn and share our knowledge.
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When you take a shallow round dish like that and change it from prime to offset don't you risk degrading side-lobe rejection? In other words don't you risk increased interference from adjacent satellites? I also think the proper way would have been to run signal tests before and after in order to make comparisons, at least as far as gain goes.
Despite having owned large C-band dishes years ago, I claim no dish expertise. That said, does anyone have some insight as to why the DBS services, Dish, DTV, etc, chose to use offset LNBF's with their small reflectors if it diminishes performance?
You mean that they have the LNBf in an offset position towards the dish? (I don't know anything about DBS services, Dish, DTV, etc.)
My guess: maybe because they are offset reflectors?
Greetz,
A33
Performance is not diminished because they are designed as offset dishes.The purpose of the offset is so the feed does not block the signal from getting to the reflector when dealing with the tiny ku/ka wavelenghts.
Ok, but the reflectors seem to be concentric, just as they would be for a prime focus LNB set. The smaller 10-12" dishes used in the Winegard and King auto aiming dishes use a double reflecting prime focus LNB. The design decisions must be interesting.
With an offset feed, the signal comes in from a higher point. Therefore the dish does not need to be tipped up as far to get the same signal. I think this could be a design decision. This can be a big advantage for reliability, especially up north. Not having the dish fill up with heavy snow, wet leaves, etc. near as often. The customer base for small dish isn't tinkerers, they just need it to work without messing with it or even thinking about it. With an offset feed on a concentric dish, if the signal is less than optimal, it's still better than asking the customer to keep cleaning it out. The newer small dishes are probably optimized for the offset.
In the wintertime, especially when there's a storm coming, I typically park my big dish to a lower satellite when I'm not using it, so I don't have to go clean it out as often.
Ok, yeah, the snow consideration does make sense as a design decision factor. The small 10"-12" dishes used in the portable auto aiming setups are enclosed in typically round housings, so it's not a factor with them.
Back in my C-band days, our dish was located on a side hill that made cleaning off snow difficult, so I programmed a bogus satellite into the receiver that drove the dish to an almost complete flip that made reaching it with a standard broom easy from the uphill side. Sometimes just flipping it over was all it took to clear it.
With a "concentric dish" you mean a perfectly round/circular dish?
That could still be an offset dish, but these then have a non-flat dish face.
Those round non-flat offset dishes exist (usually the dish rim on top and bottom then is higher than the rim at left and right),
though 'normal' offset dishes are elliptical (higher than wide) and have a flat dish face (measured on the working (paraboloid) area of the dish).
By the way. An offset dish can also be mounted upside down, or rotated 90 degrees (or at any angle you like). That is sometimes practical, when the dish is on a balcony or so...
Greetz,
A33
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