AVR Freaks

Hi, Alex,
For example, a coal mine. There is lots of rock to traverse... Which frequency should I choose to traverse the rock. 912Mhz? 433? 2.4 ghz?

The rescue experts in your coal mine should already be equipped with ELF-Radio ("Extreme low frequency"), using frequencies as the radio to submerged submarines.

Ask them. I assume, they are already expecting your questions.

Another example, a chemical plant which has lots of outdoor metalic pipes. Suppose you need to put some wireless sensor nodes to study some physical phenomenum. Which is the best frequency for those sensor nodes (more imune to reflection on the metalic pipes, etc)

If you can't get rid of the reflections - use them:
a) Packet transmission
b) Diversity for the nodes, single antennae at the sensors. Use two or three antennas at the nodes. Let your nodes learn for each sensor, which antenna performs best.

Ciao
Wolfgang Horn

Perhaps the reason people have been having so much difficulty answering your question is because there isn't a simple answer to 'which frequency is best' for any give application. There are just too many variables to consider.

In general, lower frequencies are better at penetrating through and around obstacles than higher frequencies. At the same time, since higher frequencies tend to be line of sight, interference sources for higher frequencies also have to be line of sight, and therefore higher frequencies tend to suffer less interference than lower frequencies. There are also environmental concerns to worry about. 2.4GHz radio signals are very strongly absorbed by water in the air, making them useless for anything beyond short range communications. Some radio frequencies, like 2.4GHz, are also absorbed strongly by the human body, making high power transmitters a health hazard. (think microwave ovens)
Metal building walls will also completely block radio signals, leaving openings in the metal such as windows as the only way in. Radio signals won't go through an opening less than 1/4 wavelength across, so that's another limit that may need to be considered.

You may be able to do a few tricks to help you. For example, in your coal mine, it's hard to get radio signals to penetrate rock, especially if the rock is damp enough to be conductive. It might be possible to choose a frequency such that that mine shaft functions as a wave guide, and carries the signal throughout the mine.

By far the biggest restrictions on finding the frequency to use will be legal. Most radio frequencies are already assigned to specific uses, and you're almost certainly going to be restricted to what radio frequencies are already in use in the area, and what is available for licensing.

And I will add to Jims info.
2.4 GHz is bad near a lot of water (moist).
That is why the freq. is used in a microwave oven.

For SSB radios there a some tables for how good different freq. are at the different time of the day, but they are all based on vessels, in freq 1-20MHz.