On Fri, 7 Sep 2007 22:13:55 -0500, Gary Schafer wrote:
>If it is 1.2:1 at the shack it is also 1.2:1 at the antenna. The impedance
>may be transformed but the swr will not be.
WRONG! The loss in ANY transmission line will cause the SWR to be reduced
as you travel along the line AWAY from a mismatched load. The loss can be
predicted by Smith Chart calculations, by some equations published in the
ARRL Antenna Book, and by N6BV's TLW program (on the CD in the Antenna
Book). The amount of reduction in SWR increases with the loss in the line
-- it will be greater for longer lines and for lossier lines.
As an example, a mismatch of 2:1 at the antenna may be reduced to 1;5:1
when measured at the transmitter when using RG58, but 1.8:1 when using the
same length of RG8. An 8:1 mismatch at the antenna might be only 3.5:1 at
the transmitter!
The best way to undersand why this happens is to realize that both the
transmitter wavefront and the reflected wave are attenuated by the loss in
the line. Thus, at the transmitter end, the direct wave is strongest and
the reflected wave is weakest.
It's quite instructive to do an NEC model, read the values of R +jX at
various points in the band, plug them into TLW to compute the loss for
various types of transmission line, and plot the result as a graph. By
replacing the RG59 feedline for my high 80/40 fan dipoles with RG1l, I
figure that I've picked up about 1.3 dB of transmit power (averaged over
the two bands). That's a 1.35 power ratio. No big deal for casual Q's, but
can make the difference if you're right on the edge of the other guy's
noise.
73,
Jim Brown K9YC
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