RG-58U (solid center) with foam dielectric is 0.57dB/meter RG-58C/U (stranded center) with foam dielectric is 0.90dB/meter. RG-58A/U (solid center) with polyethylene dielectric is 1.5dB/meter.
I'm too lazy to convert to feet so just assume 1 meter = 3ft. Close enough. Don't forget to add about 0.5dB loss per connector pair if using TNC connectors and about 3 times that for crappy BNC connectors.
For reference, 6dB loss is half your range. 12dB is 1/4th your range.
Try again. CATV is 75 ohms. It's probably RG-59/u (crap) or RG-6/u. If you search Google Groups, you'll see some of my postings on using RG-6/u for wi-fi and for 10base2 ethernet. It works but there are limitations. If it's RG-59/u, forget it.
No logarithms involved. If it's 38.9dB/100ft, then it's 1/100th of that per foot or 0.389dB/ft.
It would be an odd multiple of a 1/4 wave electrical line section with an impedance equal to sqrt(Z1 * Z2) where Z1 and Z2 are the load and line impedances. For 50 ohms to 75 ohms, that's 61 ohms. At 2.4Ghz, building one of these is tricky as the velocity factor of the matching section is not very well defined for random pieces of junk coax.
If it's good quality RG-6/u satellite grade coax, just use it directly with no matching section. Going from an F connector to a TNC is possible with adapters. The 1.5:1 mismatch loss (2 * 0.18dB) will be less than the extra connector (0.5dB) losses.