Jim Lux wrote in news:f9i1i3$8v5$1
@nntp1.jpl.nasa.gov:


BTW, directional wattmeters for the ham market are often not capable
of
reasonable accuracy on loads other than the nominal 50 ohm load. There
are a range of tests that such an instrument should satisfy, but for
hams, it is usually considered sufficient if the "reflected" reading
is
approximately zero on a 50 ohm load.

I should think, though, that one could calibrate such a
reflectometer/directional wattmeter. That is, you could test it with a
suitable variety of source and load impedances and develop a fairly
simple arithmetic correction that would be accurate.

Yes Jim, some of the deficiencies of the instrument fall to things like
an equal response from the separate forward and reverse couplers. Scale
shape is an issue (especially where the sensitivity is continuously
adjustable using a pot). Phase and amplitude response of the coupler over
the frequency range is another issue not so readily calibrated out. A
coupler that is long will underestimate rho, and some couplers insert
more mismatch than they pretend to measure.

In my experience, many of the instruments that are claimed to work up to
144MHz band might well indicate close to 1:1 on a dummy load, but they do
not indicate rho=1 on a s/c or o/c. Whilst they may serve their purpose
as a null indicator on a 50 ohm load, they are not suited to the loss
measurement such as Jimmie is performing.


The interesting question might be whether you could unambiguously take
a
particular fwd and rev reading and turn that into a true fwd and true
rev, essentially solving for the mismatch.

I don't think you can compensate for lack of f/b ratio in the coupler,
for example because the coupled lines are too long.


Down in the lab here at work we have a whole rack of precision
misterminations (1.1:1, 1.2:1, 1.5:1, etc.) that some talented engineer
built and calibrated some decades ago. They're built on the Maury
bluedot N terminations.

I have always though that a budget priced set of mismatches would be real
handy, and have wondered why MFJ (or someone else for that matter) don't
offer a set for checking / calibration of the MFJ259B etc.

Owen