On Thu, 29 Dec 2005 17:50:59 -0800, Roy Lewallen
wrote:

Owen Duffy wrote:
On Thu, 29 Dec 2005 12:13:17 -0700, Wes Stewart
wrote:



I've provided a spreadsheet that facilitates the calculations over a
range of frequencies.

www.qsl.net/n7ws/8405.zip

So a couple of trips to the end of the cable are all that are required
to calibrate the setup.

(I must confess, I haven't tried this program with a line much over a
few inches in length to determine whether my calibration functions can
handle it, but I think so.)


That's a good idea!

Wes, does it collect enough information to be able to correctly
calculate a phase constant on longer feedline. Some work for the next
version perhaps?

A potential problem is cable loss. When the line Z0 is close to the
impedance being measured, loss doesn't have much effect. But if the two
impedances are very different, a surprisingly small amount of loss can
have a significant effect on the observed input impedance.

Wes' procedure calibrates both loss and phase, but my suspicion is
that it does not calculate a correct phase constant for longer lines.
The loss constant is probably simple, assuming a straight line between
the two cal points, but that is probably adequate for the task given
the object being measured.


Of course, the short circuit measurement will give you the cable loss,
which can then be used in the calibration process. It's just that you
wouldn't be able to do the correction by the simple equivalent of a
Smith chart rotation.

Agreed, that was someone else's suggestion, and if the line on the
Smith Chart was a lossless arc rather than a lossy spiral, some more
error creeps in, and the error is larger as VSWR increases.

I knocked up a small spreadsheet solution myself, it uses the gamma
(with a small g) figure returned by my line loss calculator at a
frequency, and the line length to calculate the impedance
transformation. (Gee Excel is ugly with complex numbers.)

A more general solution would be one that calculates the fundamental
RLGC model from k1, k2, vf, and Zo, and can calculate the impedance
transformation as a function of Gamma and freq. I have a Perl library
that I use for such things, but it won't port to Excel very easily.
(If only Microsoft would extend Excel's capabilities instead of
renaming and relocating functions from version to version.)

It highlights the convenience of a direct reading impedance meter!
Still, I can see the advantages of the VVM over an impedance bridge,
and they are both in a different class to the MFJ.

Owen
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