Closed circuit Wifi over coax?
If you used RG-58 at about 1dB loss per meter, you could theoretically run a cable of about 80 to 100 meters, depending on the output power of your transmitter. In reality, I would rather keep it to 50-60 meters and place a 5 dB attenuator at each end to make sure I wasn't saturating the receiver. You can take out the hard attenuation when you're done but I like to gradually work up to full power to make sure there aren't any kinks.

Also, if you want to hook up multiple devices, just have your clients linked through a power divider (2-way, 4-way, etc.).

Basically (excuse my first time math syntax)

\begin{aligned}
\ distance = \ (PowerOut - ReceiverMinimum)/ dBLossPerMeter \\
\
\
\end{aligned}