Ray,
Dependent on the tunnel length, size, and construction methods you may want to look at a product such as "Radiating Cable" or leaky cable as we use to call it. Basically it is a high quality transmission line that has a series of milled holes or apertures along the outer conductor, which allow the RF energy to pass thru. The cable tends to provide a lower level of energy at a given distance from the antenna, but fairly consistent along a great distance, useful if the tunnel is curved. This may or may not be the right solution or even a cost effective solution but one you should look into. Check with Tessco ( www.tessco.com) as they distribute this from several vendors.
Don’t forget to include the potential loss from making the transition from the connector on the AP or Bridge to the cable (typically a “N” series connector) when looking at a power budget.
It’s been years since I used it with some 900MHz radios but it did the trick in specific applications.
