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What's in glass that blocks a Sat signal?
tr_ramsgate
Posts: 866 Forum Member
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I've been playing around with my Lidl SL65 receiver with an indoor dish and found that I have to have the window open to get a signal from the SKY satellite. But I did manage to get Atlantic Bird through glass, though signal quality knocked back.
As it happens the glass in the window is at right angles to the beam from the satellite for Atlantic Bird, but at an angle of perhaps 40 degrees off the beam for Astra 28.
However, when I put a sheet of perspex in the open window (to cut the draught) the signal strength and quality are barely changed ie 71% becomes 70%.
What's in the glass that is impeding the signal? Why doesn't the same thickness perspex barely affect the signal?
The glass I am talking about may be as old as the house ie early 1800's. Would modern glass be different?
As it happens the glass in the window is at right angles to the beam from the satellite for Atlantic Bird, but at an angle of perhaps 40 degrees off the beam for Astra 28.
However, when I put a sheet of perspex in the open window (to cut the draught) the signal strength and quality are barely changed ie 71% becomes 70%.
What's in the glass that is impeding the signal? Why doesn't the same thickness perspex barely affect the signal?
The glass I am talking about may be as old as the house ie early 1800's. Would modern glass be different?
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It's very convenient using a dish indoors. It's a doddle to change satellites and far cheaper than a motorised system.
By the way, how do owners of motorised systems deal with the fact that the LNB has to be tilted differently for different satellites. The motor only drives the pole and the tilt of the dish, doesn't it?
LNB tilt for Astra 28 is 12 degrees anticlockwise, but for Atlantic Bird is a few degrees in opposite direction.
Exactly - the LNB is fitted to the Arm in the dead upright position for the due South Satellite relevant to your location - as you rightly say, the dish, along with the Arm and LNB "tilts" as it traverses the Arc, thus automatically skewing the LNB to remain in the perfect attitude for each Satellite.
It's all done by the design of the locus described by the Motor Stub.
Clever stuff - yet so effective. Elevation and Skew all taken care of simply by initiating a change of Azimuth.
To change azimuth, I rotate the table. To change elevation, I slide the book towards the dish or towards the LNB, depending on whether I wish to raise or lower the elevation.
If I could make a manually operated support that tracks in an arc like in motorised dishes, then switching satellites would be as simple as lining up on this chimney or that lamp-post.
Any ideas for such a design?
By the way the LNB is a Skyware Universal quad with markings of from -30 to +30 round the barrel and markings of 2,3,4 (very close together) on the collar.
Clearly it is meant to click into positions corrresponding to the 2,3,4 markings - which positions are easily set by slackening the collar.
But what do these positions relate to?
The -30 to +30 is clearly for setting skew with the zero position looking about right for the skew for SKY at 28 degrees.
But when you slacken the collar, the LNB doesn't really want to be at any other skew than where it slots into the 2 3 4 positions (all of which produce pretty much the same skew).
http://ppl.creol.ucf.edu/pub-189.pdf
At 12 GHZ wavelength is 2.5cms, if my rusty maths is right
WH
Even so, that article is looking at light/UV frequencies, but it does refer to hydroxyl ions. Water is a pretty good absorber of microwaves (witness the effect of rain/hail on a satellite signal) so I suppose it's possble that the hydroxyl ions in the glass could absorb microwaves at 11GHz or so.
Providing the glass is flat and parallel refraction would not bend the beam - merely displace it to one side ( refraction nearer to the vertical going in and refraction away from the vertical coming out)
Is the microwave oven frequency close enough to satellite frequencies for the test to be revealing?
Have you ever noticed any glass dishes that don't get warm in a microwave? I haven't got one myself. Still cooking on a 1980 gas cooker. And my electric toaster probably dates from the fifties.
The microwave range includes ultra-high frequency (UHF) (0.3-3 GHz), super high frequency (SHF) (3-30 GHz), and extremely high frequency (EHF) (30-300 GHz) signals.
Satellite and Radio Microwave frequency bands
Designation Frequency range
L band 1 to 2 GHz
S band 2 to 4 GHz
C band 4 to 8 GHz
X band 8 to 12 GHz
Ku band 12 to 18 GHz
K band 18 to 26.5 GHz
Ka band 26.5 to 40 GHz
Q band 30 to 50 GHz
U band 40 to 60 GHz
V band 50 to 75 GHz
E band 60 to 90 GHz
W band 75 to 110 GHz
F band 90 to 140 GHz
D band 110 to 170 GHz
Most satellites are in the C to Ku bands
Typical microwave ovens operate at around 2.4GHz.
Most plastics do not heat up in a microwave to the same extent as glass so would appear to absorb a lot less of the microwave energy.
Whether the absorption is related to power or not may be a different matter. Satellite dishes are receiving in the milliwatt to microwatt range compared to the 600+ Watts of a microwave oven.
http://www.satellitesuperstore.com/dishes-motors.htm