On Mon,11/24/2014 7:56 AM, Jim Thomson wrote:
Has anybody actually measured the loss on their coax from back of amp to top of
tower, for various freqs if a multiband yagi used..... or what ever band your
monobander is? I mean using accurate watt meters at both ends?
I've measured loss in various lengths of various coax types on the
ground by several methods. The first time around, I used an HP generator
as a source and an HP Spectrum Analyzer as a load and voltmeter. In
later years, I've used the DG8SAQ VNWA Vector Network Analyzer.
As to your data comparisons between coax types -- remember that skin
effect dominates resistance above mid audio frequencies, so resistance
of both shield and center conductor are largely due to diameters of
both. Also, remember that RG-numbers don't mean much with respect to
cable specs. Rather, we must look at the PART NUMBERS for the coax.
Belden makes several dozen RG6s, many of which are quite different from
each other. Likewise a dozen or more RG11s, and dozens of RG8s and
RG213s. Differences include center conductor composition (copper, copper
clad steel), center diameter, dielectric, shield composition (copper,
aluminum, braid, foil), and diameters of both conductors.
The equations and graphs on the Times data sheets are based on those
fundamental properties of the coax and the wave equation. Dan Maguire,
AC6LA has developed Excel spreadsheets that will plot graphs of Zo, loss
and velocity factor as a function of frequency when you import sweeps
from a vector antenna analyzer. http://www.ac6la.com/
73, Jim K9YC
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