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Originally Posted by mgandalf
Ok, here's something really whacky but sounds like an interesting experiment. Radio astronomers use a little hack of using two dishes together to create a virtual dish the size of the distance between them. Could that work for satellite transmissions? I mean, setup something simular to the following:
1. Two 90cm+ dishes at least 12 foot apart on rotors.
2. Move both dishes to the same angular position using two receivers/DVB cards.
3. Using a combiner, tune one of the receivers/cards to a C-band transponder using both dish's LNBFs.
I know, sounds crazy.. but is it just crazy enough to work?
- Mark. |
Are you sure the astronomers do that with only 2 dishes? I've heard of matrixes of many dishes, but never heard of using just 2 dishes. I would think that would be very complicated with next to no benefit, at least little benefit with respect to the amount of signal collected.
But relative to your TVRO idea, there are 2 different reasons for needing a big dish for C-band, and there are a couple problems with your idea, but it's interesting, and I had thought of doing that myself at one time.
Re why you need a big dish, I said 2 reasons, but it's really one reason, and that is gain. Basically, you need to collect a lot of signal to receive a sat signal, and you need a high gain antenna. The gain of an antenna is usually expressed as a function of the dishes diameter, but it is really more related to the surface area, ie collecting signal over a bigger area. A 12' dish is about 16 times more surface area than a 90 cm dish. Two 90 cm dishes are only 2 times the surface area, so there is little additional gain.
The second reason you need a big dish is resolution, because you have to be able to separate sats that are only 2 degrees apart. A 12 foot dish has about 1.4 degrees resolution at C-band freqs, whereas a 90 cm dish is nearly 6 degrees resolution. I am not positive how to estimate the resolution of TWO 90 cm dishes, but according to the equations, it is related to the GAIN, not to the separation of the dishes, so I think the improvement of resolution wouldn't be significant. Intuitively though, it seems to me, that this is a case where separation of 2 small dishes might help, but apparently not.
However there are a couple problems. One big problem, is that when you combine signals from 2 different antennas, you have to be very accurate with respect to the length of feed line between the antennas and where you join the signals. If you imagine the signal as being a wave, where the signals from the two dishes join, the signals need to be in phase, ie the peaks of the waves have to get there at the same time, or else you'll get destructive interferrence. The wave length at c band freqs is about 3 inches, so you'd have to do a lot of experimenting with changing the length of your feed line to get the signals to add together.
Also, almost as important as the total signal strength is the signal to noise ratio. Since a lot of your noise is coming from the lnbfs and feed lines, you are going to be adding almost as much additional noise as you are adding signal. This is something like trying to amplify a weak signal, ie you end up amplifying the noise more than the signal.
Another problem, is that the stock Ku lnbfs that you get on your 90 cm dishes only receive Ku band, not C_band, so you'd have to figure out a way to attach a BIG C-band feedhorn or lnbf to the little dish, and this bigger lnbf will most likely block some of the signal, reducing the gain of the dish. Plus, you would probably not be able to use a
motorized system, because the motors that drive these little dishes are weight limited, and the C-band lnbfs are going to add a lot of extra weight to the system.
Anyway, the short answer is that you would get only a small amount of benefit from doing this, and it may end up being worse, and to get the equivalent amount of gain, it would end up costing MUCH more than just getting a bigger dish. Interesting idea though, and if you lived in a community that didn't allow dishes bigger than 1 meter, it's an interesting way to get the benefits of a big dish while staying within the legal limits by having 16 1 meter dishes.