On Mon, 20 Mar 2006 21:31:02 GMT, Bob Miller
wrote:


Second, you should measure the antenna feed point impedance at the
feedpoint, or 1/2 wavelength from the feed point [coax corrected for
velocity factor].


Why the half wavelength?

The VSWR in this situation varies smoothly from the value at the feed
point to the value at the source end of the line as dictated by the
(known) line loss.

In this scenario, knowing matched line loss and the VSWR at a point,
it is possible to estimate the VSWR anywhere else on the line (as I
have done in another post).

I'm confused, wouldn't it be more important to have the right
impedance and swr at the end of the feedline, where the transceiver is
expecting 50 ohms?

There are two objectives:
1. delivering the transmitter its rated load impedance (so that it
safely delivers its rated power);
2. minimising line loss.

1. is achieved by a load approximately 50 ohms at the souce end of the
line.

2. is achieved by a load approximately 50 ohms at the load end of the
line (which will also satisfy 1.).

Owen
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