Hi Don:
It's my understanding that all satellite dishes have a parabolic curve which
focuses the signal on the feed.
The C-band dish has a round outline and the feed is located along the dish
center line.
Most commercial Ku-band antennas have a parabolic curve, but have a elliptical or orange peal outline. These are off
center fed so that the feed does not shadow the antenna like it did on the C-band dishes. This is the same problem that
the vast majority of reflecting astronomical telescopes have, i.e. the secondary mirror area needs to be subtracted from
the primary mirror area to get the effective primary mirror area.
A very practical result of that difference is that a C-band dish has it's main beam along the dish center line, but a
Ku-band dish does not.
http://www.prc68.com/I/Images/SB_angw.jpg - showing the beam realtive to the
dish and beam hitting gutter.
Better when dish mounted on roof:
http://www.prc68.com/I/SBvsat.shtml
But the construction of the older dish was better than the newer/cheaper dish.
The Free To Air (FTA) Ku-band dishes also have a parabolic curve & round
outline, but they are offset fed, see:
http://www.prc68.com/I/FTA.shtml
Have Fun,
Brooke Clarke, N6GCE
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
http://www.prc68.com/I/DietNutrition.html
Don Murray via time-nuts wrote:
Hello all...
Not all satellite TV antennas are parabolic. A typical C-Band antenna is
parabolic and aligned for one satellite. But, that could change if the
feed was modified to receive multi-satellites, while the shape of the
reflector remained parabolic. Or the antenna could be an off-center
fed elliptical version.
Satellite antennas for Dish and DirecTV are not parabolic, but they are
off-center fed and either circular or elliptical. The elliptical version
usually supports a feed that will cover multiple satellites.
C-Band satellites in the U.S. Domestic arc are normally spaced
two degrees apart, with some at 4 degrees spacing.
DBS (Direct Broadcast Service) i.e. Dish and DirecTV, satellites
are spaced 9 degrees apart. Clusters of satellites can be parked
at one location to supply additional capacity for spot beam coverage.
DBS service is located in the Ku-Band.
More info at:
https://docs.google.com/presentation/d/1eNMYmcNIxRFpK1PY0GqbvOfvNfzRra4fHxs8
A4hSy7o/preview#slide=id.p18
73
Don
W4WJ
In a message dated 10/9/2014 4:17:20 P.M. Central Daylight Time,
and...@cleverdomain.org writes:
You pick up satellite TV with a parabolic dish that points at one spot
in the sky where the geostationary satellite lives. A sun outage
happens when the sun wanders into the focus and overloads the receiver
with noise that drowns out the satellite signal (at least, it raises
the noise floor enough that you can't receive the high bitrates needed
for a TV picture).
You pick up GPS with a whole-sky antenna that receives signals from
the constantly-moving swarm of GPS satellites. It undoubtedly receives
some noise from the sun, but the only factor in how much of that you
get is the sun's elevation above the horizon. It's not really relevant
whether the sun is "aligned with a satellite" or not. Even if it was,
the satellite would be somewhere else a minute later. :)
Andrew
On Thu, Oct 9, 2014 at 1:40 PM, Bob Stewart <b...@evoria.net> wrote:
Two days this week, there was a 3 or 4 minute outage on DirecTV as the
sun aligned with the satellite and my dish. So I was wondering what kind of
effect this has on the GPS system and especially timing receivers.
Bob - AE6RV
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