Does "warm Earth" radiation really worsen S/N ratio ?

[polgyver]

Popular view is, that reception through offset dishes has some advantage over prime focus dishes, because LNBF used in offset dish "sees" cold sky around the dish. LNBF in prime focus dish "sees" warm Earth, and it causes receiving more thermal noise, thus worsening Signal-to-Noise ratio, and Quality.
I tried to verify this popular view, by introducing source of heat between the dish and LNBF.
Assumed, that the higher the temperature of heat source, the more thermal noise would be, and Q would drop significantly.
As a heat source, BBQ heater was used, its power around 600 W, and temperature reached above 500 degree Celsius (many times more than "warm" Earth... ).
Introducing any small metal object between dish and LNBF will cause drop of Quality, in the range of 1 to 3 points.
However, heating up the coil to almost cherry red, did not cause the Q to drop more than these 1-3 points.
Seems, that LNBF is not sensitive to infrared and red radiation (its wavelength in the range of fractions of millimetres), whereas it is most sensitive to waves 20-30 mm long (Ku frequencies).
Therefore, "warm" Earth must emit electromagnetic radiation also in Ku range.
What do you, more experienced members, think?

 

[Titanium]

Ground noise is often referred to as thermal noise, but it really isn't referencing increased RF level caused by a higher soil temperature or elevated ambient air temperatures.

Thermal noise is molecular movement which occurs in all matter when temperatures are above absolute zero (-460F). The less reflection of these undesirable signals radiated from any mass into the feedhorn, the better the signal to noise ratio.

 

[lost_mesa]

I've also heard that offset dishes have an advantage because the feed would see the cold sky around the edge of dish and am a bit skeptical about that. I see plenty of offset dishes that have tall trees or buildings behind them, so the feed isn't seeing any cold sky. I haven't heard of these dishes taking a performance hit. One advantage of offset dishes is the feed isn't shading the reflector surface. Probably important for a small dish with 2 or 3 feeds.

Ground noise is from the ground radiating RF. Same with your hand or anything else. As an experiment I measured the RF power out of an LNBF pointed at the sky, then the ground. The power increased about 5.5 dB when pointed at the ground. That is because the sky has a noise temperature of about 10K to 15K (depending on weather) at 12 GHz and the ground perhaps about 290K.

I think the reason you don't see much noise from the BBQ heater is because temperature as we perceive it with our skin, isn't the same thing as the noise temperature at 12 GHz. Things that are lossy at RF (the ground, a tree) tend to have much higher noise temperatures then things that are not lossy, such as metal, even if the metal is at a higher physical temperature.

The ground and the metal reflector of my dish are at the same physical temperature, but the power out of the LNBF shows very different noise temperatures.

 

[Cham]

Also remember lower latitudes the offset dish LNB will be pointing lower and eventually at the earth (behind the dish).

Some dishes will be installed with a ring or wall around the edge to help keep RF (transmitting) from spilling out the side creating interference to other services nearby; also minimizing interference (receiving) from other services/noise. This is often the case for terrestrial microwave antennae where they are usually pointing at the horizon.

Depends too how much of the dish the LNB/Feedhorn illuminates; if there is a significant gap around the edge that is not utilized, there would be less interference no matter where the dish is pointing.

Then there is noise created by the dish reflector itself and the electronics inside the LNB/LNBF...

All boils down to the basic signal/noise equation...

The Moon is a nice target, about the size of the beam from a 30-40" dish on Ku. Might be interesting to see what the moon's radiation reads when full compared to <1st quarter.... Might turn some of you into radio astronomers!

 

[stanleyjohn]

One thing i do know is that during the summer months my 6 ftr has a harder time keeping a lock on some of the weaker signals out there.