Electronic – RF receiver design – estimate received power

This is a harder problem than I think you realize. The basic Loc8tor system works primarily by virtue of a directional antenna in the receiver, and it doesn't really give you the position of the tag directly, it just indicates what direction it is from the location of the receiver, along with a very rough estimate of how far away it might be.

If you want to create a mesh of automatic receivers, each of these receivers will need to do direction finding, either mechanically by physically spinning its antenna in a circle, or by using multiple antennas and electrically "spinning" the reception pattern. Neither method is going to be simple to implement or particularly low-power.

It is only by combining direction information from two or more receivers (plus knowing exactly where these receivers themselves are located) will you be able to derive an absolute position for the tag.

Are loop antennas limited to single frequencies or can they receive specific frequency ranges too? How can i design the best loop antenna to have the broadest bandwith to fulfill the 3-30KHz range? Should i consider anything special?

You can resonant-tune a loop antenna with a capacitor to get high selectivity or you can leave it un-tuned and feed it into a high impedance amplifier. In both cases ensure that the natural resonant frequency of the coil (plus the spurious capacitance between turns) is not resonant anywhere near to your operating frequencies.

I would suggest that you use decent bandpass filters after your 1st stage of amplification/buffering to target any frequency ranges you think are useful.

How can i match the impedance between Antenna, the coax cable and the receiver?

Don't bother with matching - you won't get standing waves at 30kHz or below unless your cable is many kilometres long. A short length of coax will not look like a constant 50 ohms for the loop antenna - your amplifier input impedance will be left largely intact and that will be the "loading" on your loop. Please do consider cable capacitance though - this might be enough to cause a resonant peak and disturb your readings rather like a tight bandpass filter would.

And, how can i match antenna impedance to remove imaginary part?

Your signal won't have an imaginary part unless you are close enough to the meteor for it to dynamically alter the characteristics of the coil!

What is the best way to limit the maximum output of the board to ensure that Sound card of the laptop would not get damaged because of extra voltage/current?

Feed your output via a 1k resistor to the sound card input and clamp it with back-to-back diodes is probably overkill - I'd recommend limiting the final output amplifier to not producing anything more than (say) 2Vp-p either by potential divider or by restricting the final amplifier's power rails.

The design of your loop antenna is just a simple magnetic field detector - you should not bother trying to detect the E-field because your loop would need to be about 1 km in size.