How much radio power enters a tunnel ?

Assuming a 900MHz transmit antenna with 39dBm ERIP is placed 100m away from a train tunnel entrance measuring 20sqm (cross sectional. Also assume a free space propagation from the antenna right to the tunnel opening.

What is the received signal strength inside the tunnel, at x meter away from the tunnel entrance ? assuming 0dBi receive antenna.

 

If the tunnel was lined with a highly conductive surface, then the incoming waves would reflect from the inner surfaces of the tunnel (at shallow angles) and then the signal strength at X meters would be a very complicated (even chaotic) function depending upon how the waves add up at any particular point in 3D space (x,y,z). There would be nulls and points of focus. In practice, it would be a mess and the nulls would ruin your day.

If the tunnel was lined with absorptive material, then there would be no reflections and the signal strength at X meters would simply be the free space loss pretty much ignoring the tunnel (assumming that the tunnel is straight).

The above two simple cases can put bounds on the answer. It seems that the dominant factor is free space path loss. Any reflections will probably hurt more often than they will help.

J. Harvey
Ottawa

 

[proff]

Ignoring reflections and assuming a parallell tunnel and also assuming a direct line of site between transmitter and receiver, then the strength calculations are exactly the same as in free space. Just simply calculate the received strength as a function of 100 + x metres. If you include reflections, then assume all reflections emanate from the edges at the entrance to the tunnel and nulls and peaks are located at 333mm intervals from there. The reflection strength is highly dependant on the shape of the tunnel entrance.