On 7/22/2011 11:51 AM, Jeff Liebermann wrote:
I'm helping a friend build a very simple portable VHF antenna for
hiking. Ignoring all the ladder line J-pole antennas on the web, I
decided a simple coaxial antenna would suffice. Remove 19.2" of outer
jacket, peel back the braid over the outer jacket, tape, and instant
75 ohm coaxial dipole antenna. However, I'm not sure if the driven
element cut length is the free space 1/4 wavelenth, as it would be
with a metal rod coax antenna, or if I should include the 0.67
velocity factor of the coax dielectric covering the center conductor.
Is it 19.2" or 12.9" for the driven element? My sense of smell says
19.2" but I'm not sure and can't find any specific references that
deal with the problem.

Note: I would normally test this on the bench with a reflection
coefficient bridge and sweep generator but I have another project
going on the bench and don't want to move or touch anything right now.


Go with your sense of smell. The velocity factor of coax only applies
when the shield is positioned where it is designed to be. Let us know
how it fares.