On Sat, 31 Dec 2005 18:14:04 -0800, dansawyeror
wrote:

This is the next chapter in the antenna measuring saga. Today's adventure is
trying to measure SWR with an HP 8405A Vector Voltmeter.

As I see it, your instrument with accessories (directional coupler,
attenuator, etc) can be used to measure the complex reflection
coefficient (Gamma) at the measurement plane.

Reading the mail, it seems have already worked out how to do that, and
to calculate Z given gamma.

If you want to determine SWR, you don't need the argument of Gamma,
you just need its magnitude rho. SWR=(1+rho)/(1-rho) where
rho=|Gamma|. (You are of course throwing away part of the measurement
data, the part that is the real power of the VVM. You could measure
rho with a directional coupler, calibrated attenuator and xtal
detector or a bolometer power meter... but they won't give you the
phase data that the VVM does.)

The return loss of VSWR=2 is 9.5dB, not 4.5 as you suggest. You need
to review some of these simple relationships. You also seem not to be
properly including the directional coupler in the calibration loop,
why the tee as described, doesn't the 8405A have a tee probe?

Is using a VVM an overkill for determining SWR? Probably, especially
if the complexity results in errors.

Owen
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