[Antennas] RG178 B/U / Choke/Balun?

[Niel - W0VLZ]

I have some RG178 B/U I'd like to try as a choke/balun for a light 
weight QRP C-pole (see 
http://w0vlz.blogspot.com/2010/05/c-pole-antenna-for-qrpxpeditions.html 
for my heavy weight model).

I have a 2" diameter coil form. How do I calculate/test for the number 
of turns required to be an effective choke at 14MHz? Right now 15 turns 
of RG8X single layer wound on a 4" plastic coffee can works.

Also, anyone know the attenuation factor of RG178 B/U at HF frequencies?

73,
Niel