Quote:
Originally posted by Glimmie:
It takes a 6 foot or larger dish to get a signal from a C band satellite because it's power output is only 5 watts. DBS satellites have several hundred watts output and thus can be received with only an 18" dish. If preamps were a solution, why aren't C band dishes 18" too with just a bigger preamp? |
While ultra-low wattage used to be the norm, most
C band satellites today have power output in the 10-30 watt range. Still, not nearly the output of most DTH Ku birds. However, part of that large output is required to minimize environmental effects (moisture, etc.) to which the shorter wavelength of the Ku band is significantly more susceptible than is C band. It also affords the use of smaller dishes for most areas which are cheaper and cosmetically more appealing.
The three largest contributors to C band dish size are localized EIRP, satellite adjacency, and dish sidelobes. Effective Isotropic Radiated Power (EIRP) is a measure of power that a satellite directs toward its boresite on the Earth. One's proximity with respect to the boresite will impact the original signal strength with which the on-dish electronics can work (similar to proximity to an OTA transmitter). This holds true for DTH birds as well.
However, because C band satellites are frequently located adjacent to one another (2* spacing) and use the same frequencies, their signals can conflict because of antenna sidelobes and pointing errors. This, despite alternating polarity to minimize such interference. The smaller the dish, the harder it becomes to reject adjacent satellite interference. I have heard of 4' dishes successfully used to receive the NASA channel on GE-2/9. However, GE-2 is as strong as one of its neighbors and the other neighbor is vacant on that frequency. When you have a weak bird sandwiched between two strong birds it becomes difficult to defeat adjacent interference with a small, or poorly designed, antenna. This is exacerbated when the EIRP is moderate to poor.
I have used line amplifiers to mitigate line loss (in one case I run over 200'). As Glimmie points out, if I wanted to boost the signal, I would have to collect more signal from the source (either a larger antenna or a complex computer-controlled multi-dish configuration - not to mention expensive and divorce prone).
------------------
The button is labeled "Play", not "Pay". STOP the MPAA!
Our
Silent Angels