What makes you think your coax is bad? Have you tested it or is there a physical problem?
The issue with coaxial cable loss is a function of cable length, cable characteristics, SWR, and operating frequency. With the length required for a C-30 being less than 50', I would worry more about using a quality coax, protecting the weather end from water intrusion, and minimizing SWR if possible.
I've operated the world on ham radio SSB and CW at 5 watts with RG 58 into a 40 meter dipole, so far out of frequency the SWR was through the roof. The line loss was high but it was matched with a transmatch and worked good enough. I'm not suggesting this for VHF operation but I don't think it will be required to use a large diameter low loss coax for a short run at about 150-160 mHz.
For specifics on coax loss look here:
http://www.timesmicrowave.com/cgi-bin/calculate.pl
http://www.ac6v.com/techref.htm#coax