On 12/24/2011 10:07 AM, john@kk9a.com wrote:
> The measured impedance will change depending on the length of the coax. It
> should match the antenna feedpoint impedance at 1/2 wavelength multiples.
> The SWR should not change with coax length, except for the change due to
> loss.
"except for the change due to loss can be a VERY important qualifier."
I recently used N6BV's TLW software (free with the ARRL Antenna Book) to
do the math for 4.5 wavelengths at 21 MHz for some 75 ohm line I was
using. 50 ohms at the antenna looks like 55 ohms at the transmitter end
of the line, due to the line loss. Is this a big deal? Not for this
specific application, but it certainly COULD be for longer lines and/or
higher frequencies -- like 300 Ft of that same kind of line on 6M or 2M.
It's ALWAYS important to remember the simplifying assumptions that make
our problems easier to analyze and think about, because Mother Nature
always solves the more complex problem. This is yet another example.
73, Jim K9YC
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