On Tue, 21 Jul 2009 02:28:18 +0100, "Arfa Daily"
wrote:
http://www.maplin.co.uk/DiscontinuedModule.aspx?ModuleNo=47234
http://www.pulsat.com/satellite/site/details.php?product_id=330
Planning laws and building covenants can be very odd here. Many old
buildings in towns and villages are protected by a National Trust 'listing'.
Depending on the level of the listing, all sorts of provisions are made
about protecting the exterior appearance, and with high level listings, even
the internal features, down to decor in some cases. On buildings like this,
and indeed on some new buildings, the likes of satellite dishes are
expressly forbidden, particularly if they impact on the 'public' face of the
building.
We have something similar with "historic buildings".
http://www.nps.gov/nr/
The owner gets a tax break to maintain the building in good condition.
Dishes aren't totally proscribed, but severely limited. For example:
www.concessions.nps.gov/...pp 26 GRCA Housing Policy.pdf
on Page 6 defines what dishes are acceptable.
As far as I know, transparent dishes were developed to try to get
around this, and I believe some planners and listing inspectors, were
sympathetic to their use. I don't know how successful DBS has been over
there, but here, probably 60% or more of houses in any given street, have a
dish bolted on them, so disguise is quite a big issue in some areas. To that
end, I believe that there were some 'microwave friendly' paints developed as
well, to allow the dish to 'disappear' chameleon-fashion.
About half the UK density.
Satellite 30 million (about 31% of US homes passed)
CATV 60 million (about 62% of US homes passed)
The coating on these transparent dishes is just a very thin vapour deposited
metalic coat as far as I recall reading somewhere long ago. I guess that
it's just the same sort of thing as the 'transparent' electrodes on LCD and
plasma screens.
Yeah, that sounds reasonable. They could probably also have used a
thick transparent plastic pipe for the mount, but didn't.
As to the rotation of the LNB for linearly polarised skew correction, all of
the DBS dishes here are offset eliptical types, and have a fixed horizontal
attitude. Adjustment for azimuth and elevation is carried out by the X - Y
mounting bracket, but skew is carried out by rotating the LNB itself in its
clamp, which is a split circle around the waveguide throat, with a screw to
pinch it up.
Rotating the LNB works just fine for a single LNB. However, the dish
tilt (skew) is intended for multiple LNB arrangements, where the LNB's
need to be line to to be parallel to the satellite belt (ecliptic).
Also, if you look at the photos of the multiple LNB dishes, you'll
probably note that the LNB has a large overhang and are jammed
together. Therefore they cannot be rotated.
A right hand twist (looking from the front) of around 10 deg
is about right for the UK, but I guess with the USA being rather wider than
us, your range of required skew correction could be a lot more than that,
and even in both directions if your birds are located centrally.
Well, let's see. From my house on the left coast, I get 105.5 degrees
skew for the 3 DirecTV birds (101, 110, 119). Moving to the east
coast, I get 53.3 degrees for the same birds. Yeah it varies.
http://www.dishpointer.com
However, for a single location anywhere, the variations in skew are
minor over a 121-101 = 20 degree window. However, if the dish were
trying to see birds from horizon to horizon, the variations in skew
would need to be considered.
In all of the LNBs that I have looked in, the probes are just pcb tracks at
90 deg to one another. There is then a 'steering' transistor, controlled by
the level of the supply voltage, in each probe channel, ahead of the RF amp,
to determine which polarisation is being listened to.
Yep. That's the way it works for linear polarization. I was
wondering how it works for circular polarization switching.
Arfa
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558