On Mon, 10 Dec 2007 16:16:15 -0800, Jim Lux
wrote:

Richard Clark wrote:
On Mon, 10 Dec 2007 21:04:06 GMT, Owen Duffy wrote:


I can't quickly think of a sampling technique that truly takes a
point sample.


Slotted line.

Doesn't take current sample

Hi Jim,

Never needed to with a slotted line.

Probe is of finite size (albeit small fraction of lambda in most cases)

And the time to measure anything is finite too.

The difficulty of making accurate RF current measurements at a point is
why things like slotted lines were invented. You can measure voltage at
a series of points and calculate what the current *must* be.

At worst, only three points, and generally for SWRs folks here would
agree were astronomical (and incalculable, but I could measure them
anyway).

With four fixed points (nominally eighth wave, but a range of
frequencies can be accommodated), I could determine the complex
impedance of any load. [re. "Microwave Measurements," Ginzton, Sec.
5.12] There are a myriad of other slotted line techniques, but I will
leave them for a suitable occasion.

I have never had to consider what the current *must* be - an
unnecessary elaboration.

73's
Richard Clark, KB7QHC