Propagation of different frequencies

This question was posted on www.element-14.com:

I am developing a project where we need to track mining equipment in underground mine shafts. The problem is GPS obviously doesn’t work underground so our solution is to use small stationary transmitters embedded in the walls of the shafts and with each trackeble piece of equipment carrying a receiver we record the time and ID of a transmitter.

The transmitters have to be cheap and very low power as we are planning to make them disposable. The receivers are usually mounted on trucks and such and don’t need to be very efficient. We considered RFID but that could become very expensive if every piece of equipment would need a several thousand Euro standalone RFID reader. So we went with low powered active beacons and receivers. Well all is well when testing in the office – we get ample range (upto some 100m in open space) and reliable detection but when we take the equipment outdoors in the rain or snow we get problems. We tried 2.4 GHz and it was very cheap and efficient, but we had trouble when the receiver or transmitter were near the ground or a big mass like a truck. Snow and moisture had very little effect on range.

Next we tried RFM12B modules at 868 MHz that were unbelievably cheap but also performed well below levels advertised in data sheets. We could get the range but power usage was unacceptable because of low transmission speed and high packet loss.

Now we have new modules and are just about to begin testing them but as these are 868 MHz again I have my doubts.

Pinter Electronics Consultants’ answered:

Chris Pinter answers, “You will need to design your system for a multipath environment. As wave propagates through space it will bounce off any obstacle that is between the transmitter and receiver.  I can completely understand that you are getting 100 meters in an open field but not getting that in your tunnel.  By the time the signal has reached your receiver it has changed.

This problem will not necessarily be solved looking at the frequency.  You need to look at the way you are transmitting and how the wave changes as it bounces around.   We here, at Pinter Electronics Consultants have done several tests in this area of radio frequency research and have found a number of key characteristics that have helped.

1. Multipath has been known to change the frequency of the high frequency (>1 GHz) propagating wave.  Frequency hopping spread spectrum is a good way to to deal with this. The receive needs to tune into the frequency so you will need to create some type of real time calibration so the radio can change the correction factor for each location.

2. Multipath also creates minima and maxima power levels so a MIMO antenna system is also very helpful in order to get the most powerful signal.”

If you want to read the full discussion click here.

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