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
--

Owen Duffy wrote:

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?



A 1/2 wavelength transmission line repeats the load impedance at the
antenna with minimum loss, assuming you cannot get an analyzer at the
antenna itself.

IMO, measuring directly at the antenna and solving the matching/load
problem there is the easiest and best method. You are free to disagree.

On Mon, 20 Mar 2006 17:54:19 -0500, Dave wrote:

Owen Duffy wrote:

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?



A 1/2 wavelength transmission line repeats the load impedance at the
antenna with minimum loss, assuming you cannot get an analyzer at the
antenna itself.

IMO, measuring directly at the antenna and solving the matching/load
problem there is the easiest and best method. You are free to disagree.

Ok, but again you can measure the impedance anywhere and knowing line
parameters, calculate Z at any other point.

Owen
--

Owen Duffy wrote:

SNIPPED

Ok, but again you can measure the impedance anywhere and knowing line
parameters, calculate Z at any other point.

Owen
--

Owen, I don't disagree with you. But, IMO, it is beyond Bob Miller's
expertise/knowledge, at this point in his ham experience, to perform
those calculations.

I didn't perform those calculations until my 3rd year in engineering school.

KISS, and fix it at the source.

On Mon, 20 Mar 2006 19:00:16 -0500, Dave wrote:

Owen Duffy wrote:

SNIPPED

Ok, but again you can measure the impedance anywhere and knowing line
parameters, calculate Z at any other point.

Owen
--

Owen, I don't disagree with you. But, IMO, it is beyond Bob Miller's
expertise/knowledge, at this point in his ham experience, to perform
those calculations.

I didn't perform those calculations until my 3rd year in engineering school.

KISS, and fix it at the source.

Dave, there is no doubt that the closer you are to the antenna, the
less error introduced by tolerance on line parameters. On the
otherhand, the instrumentation / user can be disruptive or real
inconvenient.

As for the calcs, one can be lazy and use
http://www.vk1od.net/tl/tllce.php (if one's cable type is supported).

My antenna analyser can't do the calcs internally, I don't know if
some can, but it would be a pretty handy feature.

Owen
--