[time-nuts] Sun Outage
W4wj at aol.com
W4wj at aol.com
Fri Oct 10 01:44:51 UTC 2014
Hi Brooke...
You are correct. My semantics were confusing!
The offset feed certainly has an advantage because of no shadowing,
but a lot of commercial Ku-Band antennas are complete parabolic
reflectors with a sub-reflector and cassegrain feed.
There obviously is some loss because of the sub reflector, but these are
larger antennas and the loss is acceptable.
TNX for you feedback Brooke!
73
Don
W4WJ
In a message dated 10/9/2014 7:26:55 P.M. Central Daylight Time,
brooke at pacific.net writes:
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,
> andrew at 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 <bob at 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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