"Path Loss" is what happens to your signal between the time it leaves your antenna and the time it arrives at your partner's. If you have a perfect, clear line of sight, then the path loss is predictable based on the distance and frequency. Mostly, it comes about because of the "inverse square law" rule which applies if the antennas are reasonably far apart: if you double the distance, you get one-quarter the signal. Here are a few equivalent formulas for path loss:
- PL = 20 log (4 * pi * distance / wavelength) PL = 20 log f + 20 log d - 28
- ( f - frequency in MHz, d - distance in meters )
- ( f = 2.4 GHz, d - distance in meters )
For example, if you need to make a point-to-point link over 1000 meters, (about 5/8 mile), log(d) is 3 and the Path Loss is 100 dB. That would mean that, with isotropic antennas, only one ten-billionth of the power leaving the transmitter reaches the receiver. Your 30mW (+15dBm) Orinoco card is going to be down to -85dBm at the other end of the link, not even accounting for fading, cable losses, etc. This is really marginal. (A lot of real-world systems are engineered with 30-40dB extra signal for "fade margin". And if there are any obstructions, your signal will be reduced.)
Thank goodness for directional antennas -- they have AntennaGain ! If you add an 18dBi antenna at each end (36dB total), your theoretical signal level goes up to -49dB. Now you're talking!
Passive Reflectors
The old microwave links used by telephone and power companies occasionally use passive reflectors (big flat metal sheets) to bounce the signal around an obstruction. I have a book which provides this formula for path loss on such a bounce path:
- PL = 20 log ( d1 * d2 / A ) + 22
- ( d1, d2 - distances to reflector from the two stations,
- A - area of reflector )
- ( d1, d2 - distances to reflector from the two stations,
so, if you had a 1-meter-square reflector in the middle of your 1km path, you would get
- PL = 20 log ( 500 * 500 / 1 ) + 22 = 130 dB
This is 30dB worse than a straight line-of-sight -- 99.9% of the power fails to make it around the bend. However, with good directionals, you still might have enough signal.
A reflector at a "glancing angle" is not a great idea -- it needs to be really wide (the "projected area" A is reduced by foreshortening). A double-bounce reflector with two panels might work better if the bend angle is less than 45 degrees.
