Sunlight Rejection And Interference On Lnb

S

some888

Member
My Satellite Setup
relook mutant 200s
My Location
england
hi lads!
sounds a bit scientific but anyway let's talk

When talking about fringe reception counts every db and my question is how mush sunlight interference is degrading performance of LNB?

1 - factor is quite obvious when Dish focuses sunlight on Lnb it heats.... and increases LNB Noise Figure ..... the higher the worse... and obviously the bigger satellite dish -more Radiant heat is focused.
2 - second factor and this is where I'm looking for the advice "the whole sunlight spectrum (except of radiant heat) how is it interfering?

the reason I'm asking I want to find (or modify) existing dish to be more efficient (even in theory)

_http://upload.wikimedia.org/wikipedia/commons/4/4c/Solar_Spectrum.png


I believe the best sunlight Absorber is black Color and the best reflector is white

So I case if we want to eliminate sunlingt on LNB, I think the best colour for the satellite Dish is Black Colour (some Gibertini dishes are) and for the LNBF cap (and LNBF itself) is white colour


what do you think about it ?
Did anybody came up with degrading Performance of LNB because of sunlight?
 
Sorry m8 ...not something I can in all honesty comment on..
Sadly something we Mancunians don't have to worry about..
Now Rust ...thats another thing altogether..
..bit of a speciality round here..
rgds
VS
 
Seriously though ...
If light reflection to the focus ...thus heat as well is a problem for you ..then I suspect a black dish is a better option.
rgds
 
I guess the LNB temperature on a sunny day does not depend a lot on the colour of the dish a meter or so away. It will be heated up much more by the direct sunlight on its body.

But the LNB antenna "looks" in the direction of the dish, and if the dish is hot the LNB sees more noise. I think that is why white is the best colour for the dish!

When the sun is behind the satellite is a different story, the microwave noise from the sun will be added to the wanted signals (independent of colour of dish), and degrade performance. And if the dish is too polished, the heat radiation will also concentrate in the LNB and may melt the plastic!
 
hvdh said:
I guess the LNB temperature on a sunny day does not depend a lot on the colour of the dish a meter or so away. It will be heated up much more by the direct sunlight on its body.

But the LNB antenna "looks" in the direction of the dish, and if the dish is hot the LNB sees more noise. I think that is why white is the best colour for the dish!

When the sun is behind the satellite is a different story, the microwave noise from the sun will be added to the wanted signals (independent of colour of dish), and degrade performance. And if the dish is too polished, the heat radiation will also concentrate in the LNB and may melt the plastic!
Click to expand...

I disagree m8!
first at all I think your post contradicts a bit..... because first you said colour does not matter and at last up to melting of LNB depending of dish Surface....... So colour does matter..... bacause White colour is the best visible sun spectrum reflector (correct me if I'm wrong)
mat and or silk surface - mat is simply uneven surface and it reflects the waive (sunlight) unevenly (out of focus) but it still gets to LNB...... besides uneven paint surface brings out of focus ku band signal as well....let it be even not significant


besides LNB will receive only the wave reflected from the dish surface into LNB feedhorn focal point (sunlight is a wave as well) and if she dish surface absorbs unwanted wave and reflects needed one.... the less interference

in my opinion ideal dish surface should be black (to absorbe sunlight) and silk to focus everything else (unless something else unwanted )

thanks
 
A dish, be it painted white or black, will reflect the microwave radiation just as well.
But a black dish will heat up more in the sun - not by microwave but by the sun's infrared radiation.
Any warm body, in this case the dish, will however radiate noise over a huge spectrum bandwidth: including the microwave frequencies we want to receive. That's also why the best reception is during cold winter nights!

You seem concerned that the (diffuse) infrared radiation from a white dish heats up the LNB. I think (but can't prove)! a few degrees higher temperature of the LNB itself has less influence than, say, 10 degrees more of the area the LNB is looking at (the dish mostly).

Like I said the LNB heating up when the sun is behind the satellite is something else not related to the paint colour, but the smoothness of the surface.

Maybe a native English speaker can formulate things more understandable ? :unsure: Anyway it's an interesting discussion.
 
sure it is interesting discussion :).... but I do not know yet how practical..... I'm not a physic :)
first at all I'd like to find out what is biggest interference for Ku Band caused by solar light, what part of the spectrum is interfering most

from my knowledge solar light consist from: (please do not start talking about Gamma Rays (before they get to us they absorb in the space and atmosphere:) and do not talk about x-rays ) :):

ultraviolet
visible light
infrared
radio waves ( microwaves)

sunlight's composition at ground level, per square meter, with the sun at the zenith, is about 527 watts of infrared radiation, 445 watts of visible light, and 32 watts of ultraviolet radiation.
Click to expand...
- from wikipedia

I believe we can leave out ultraviolet it is too far in spectrum to affect reception
I could not find any data about solar microwaves and I believe it depends on solar activity, but I think it is insignificant as well. If not then any tele / data broadcasting would be really difficult
And I think microwaves generated by domestic,industrial devices other sats or other polarisation but same freq from the same sat ARE MANY,MANY BIGGER AMMOUNT then generated by Sun

I could be wrong, but I think only visible light and infrared may have any impact
in spectrum they next to each other and I hope we agree we do not want them in focal point

for now the question is how to avoid them in focal point :)

because they close in spectrum I think black colour would be the best absorber and it's better for them to be absorbed by the dish instead of to be focused when using white colour....

silk or mat? - we have to find out how much painted (provided there is no metals in the paint) dish surface reflects our Ku Band out of focus with the sunlight together
 
* object heated let's say up to 100C can not generate microwaves (and I simply can not imagine my dish to go even so high I'm not in Egypt as Vipersan said :) otherways every time I do barbeque in my qargen all the neigbours would loose satellite signal:)

* when the Sun is behing behind the Satellite....it is the same line with sat to be in focal point of lnb......
Colour - by absorbsion or reflection and surface type - by reflecting elswhere or into the focus - both affecting it
 
P.P.S
just to proove my point
Working Temperature for LNB is about -30~+60 °C (I believe for the most)
Conversion Gain is measured at about 20 - 25C
 
The best solution is a black dish (with a satin, or matt finish to diffuse a direct reflection) with the addition of a thermal coupled radiator on the back, similar to the heatsinks fitted above computer microprocessors. Use of a fan is optional. For the LNB a clear cap has usuallly been regarded as the optimum, and there are various plastics that are virtually transparent to microwaves but opaque to infrared.
 
heat sink makes sense in hot weather....... but can not imagine dish with fan ))) but anyway quite interesting construction to build

Question about lnb - did anybody had a chance to measure LNB performance in different temperatures?
because if there is any significant difference it is possible to cool down LNB using peltier elements :)
 
So where are you in England some888? Have you located a place that is devoid of cloud and soaked in saharan sun?!! Here in Leicestershire ,i'm lucky if i can get enough sun to warm the slap on my head!!!
 
You could investigate Peltier cooling of the lnb - I think someone tried it a while back here on the forum.

Or spray the lnb with freeze spray - the stuff used to do thermal shock tests on motherboard components.
 
Piltdownpaul said:
So where are you in England some888? Have you located a place that is devoid of cloud and soaked in saharan sun?!! Here in Leicestershire ,i'm lucky if i can get enough sun to warm the slap on my head!!!
Click to expand...

I'm in London m8...
and in summer weather may get really hot in a City.....

but not now :(
with any experiments about LNB cooling I need to wait till next summer :)
 
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