On Tue, 28 Jun 2005 01:11:31 -0400, Scott in Baltimore
wrote:

So ... I have an older CB - Cobra 21 LTD Classic with weather stns etc. ...
a 102" Shakespeare Antenna - 18' of cable - and this


What makes you think 18 feet of coax is even a half wave?

At 27.185 MHz (ch 19) a half wave is 17.21 feet.

At 66% velocity factor, an electrical half wave is 11.36 feet.

At 77% velocity factor, an electrical half wave is 13.25 feet.

What's so special about a half wavelength of coax?

The Mobile antenna websites practically tell you to keep the coax at 18 feet, or
else. I thought that was true, until numerous people at this group and several
websites said that is nonsense.



It's the point that the SWR at the feedpoint is reflected to the other
end of the coax. At any other point in the coax, the phase angle affects
the apparent SWR seen by a Voltage reading SWR bridge. You can swap in
different lengths of coax to see this in action for your self. If your
antenna feedpoint is 50 ohms NONREACTIVE, coax length does not matter.
If there is a reactive component to your antenna system, the reflection
travels back through the coax and at different points the voltage and
current will affect the reading on the meter. A matchbox does not fix the
mismatch of the antenna, it only fixes what the load sees the antenna as.

The mismatch does not go away just because you've adjusted a few knobs.


Vinnie S.