Antenna Arrays Could Replace Satellite TV Dishes 183
Zothecula writes "There was a time not so very long ago when people who wanted satellite TV or radio required dishes several feet across. Those have since been replaced by today's compact dishes, but now it looks like even those might be on the road to obsolescence. A recent PhD graduate from The Netherlands' University of Twente has designed a microchip that allows for a grid array of almost-flat antennae to receive satellite signals."
Phased Array antennas (Score:5, Informative)
Phased-array [wikipedia.org] antennas really do work but they are not new. The nice thing about them is that they have electronic steering, so they can steer really fast while a conventional antenna of equivalent size would take much more time to move.
The problem with articles like this (and their Slashdot introductions) is that they always come off as student makes big scientific break-through rather than student applies well-known science.
Re:Why? (Score:5, Informative)
The big advantage I see to this is two-fold: 1. Mounts flat so it is much less of an eyesore. Also you could conceivably hide it behind something that is radio transparent. 2. Can be pointed via software, so that the physical installation only needs to be pointed in the rough direction of the satellite.
Re:Relevant info? (Score:3, Informative)
Antenna elements in an array are usually about 1/2 wavelength apart. L-band, 1ghz to 2ghz, has a wavelength of 20 to 30cm. So... half that, assuming 1/2 wave separation.
Re:No (Score:3, Informative)
Forget about snow. A friend used to lose his TV signal about 45 minutes before it rained.
Re:No (Score:4, Informative)
GP is correct - the dish size has all to do with the gain of the antenna, not the resonant frequency. The actual antenna is at the focal point of the dish and it's length IS frequency-critical. The surface area of the dish directly corresponds to its gain.
The reason we no longer use giant 6' dishes is twofold - because they are using 24 GHz instead of 5 GHz means the antenna at the focal point is much smaller, and the area of the dish is relatively the same size - with relationship to the wavelength - which is also much smaller.
The other reason is the peak power of, say the DirecTV sats, is as high as 150W for some transponders, whereas the older C-Band stuff was about 10W peak.
Dishes typically are designed to produce somewhere around 30dB of gain, which is 1000x magnification of the signal over a straight dipole with no reflector.
Re:What about LOS requirements? (Score:1, Informative)
You will NEVER eliminate LOS requirements for high bandwidth sat links. The reason your Sirius Sat radio works without line of sight is Sirius has ground based repeaters in major metro areas for the tiny chunk of bandwidth that they use for audio
If you were do switch sat tv to this model, 1) why even use the sats anymore? and 2) if you realized the swath of bandwidth that directv's signal required, you would know there's no hole in the band plan that could accommodate it. directv broadcasts off 5 birds right now, each bird is capable of about 500mhz of bandwidth. Tell me where the 2.5ghz wide hole is in the FCC's band plan? Most metro cable plants dont even send a bandwidth this wide over their coax infrastructure most toping out at around 900mhz to 1.2ghz of bandwidth. There's a reason directv has so many more channels than cable.
I forget exactly where i read the fact, but i remember reading somewhere that if you could capture ALL of the data hitting a directv dish at any one moment, you would need a way to capture about 150-200 megaBYTES of data per second.
Yes - RTFA! (Score:3, Informative)
Raw size does matter here. A larger receptor is better.
If you RTFA (yes I know it is Slashdot but hope springs eternal) you'll see that the system uses a GRID of flat antenna which it combines to simulate a larger antenna. By altering how the signals are combined i.e. the delays between them you can "point" the antenna at different sources. Hence you not only have a large detector from combining several smaller one but you can also point the thing without having to mechanically move it. It's brilliant idea and one that radio astronomers have been using for quite a while.
Re:No (Score:2, Informative)
Yes, and no.
The old satellites did use a longer wavelength, it is true. In both cases, however, the parabolic reflector in use is several orders of magnitude larger than any dimensions dictated by the wavelength. The only tuned element (which is where wavelength comes into play) is found at the narrow end of the feedhorn, up in the LNB.
The new ones are smaller because digital signalling has replaced analogue, in turn making error correction possible and sufficient for 99% of the time, and by the fact that the newer satellites put out a more powerful signal.
Re:No (Score:3, Informative)
Have the wavelengths of the sat broadcasts changed?This about using a new type of antenna to handle an existing broadcast, not trying to receive a different signal, no? I suppose the old sats might use a different wavelength than say DirecTV sats I guess.
Yes, the old C-band has a much greater wavelength than the present Ku-band.
Re:No (Score:5, Informative)
GP and YOU are mildly incorrect. WaveLength of the signal is impotant to the gain of the dish. a 5ghz signal has less gain on a 5 foot dish than a 20ghz signal does.
a 3 meter dish at 5 ghz has 21.704db of gain.
the same dish at 20 ghz has 54.415db of gain....
massively more gain on the same size of dish simply because of the frequency of the signal.
you also ignore that the LNA's used today are 20 times better than the ones from only 5 year ago.
Recent graduate with PhD != student (Score:3, Informative)
Re:Phased Array antennas (Score:3, Informative)
Lots of ways. Re-point (electronically) for every channel, even multiple simultaneously. Phase-cancel an interfering signal based on its direction. A more complicated system with dynamically-changing phase delays could probably reduce the impact of dynamic multipath.
Re:No (Score:3, Informative)
"The actual antenna is at the focal point of the dish and it's length IS frequency-critical."
The dish is a simple parabola, it's focal point is not frequency dependent. Its gain IS frequency dependent however. Achieving the same gain at half the frequency requires double the dish size (or, double the frequency and you can halve the dish size for the same gain).
As long as the dish is smooth enough its focal point is not frequency dependent. At the extreme low end things fall apart (i.e. the dish becomes on the order of a wavelength in size).
Re:No (Score:2, Informative)