On Fri, 01 Sep 2006 23:47:44 -0500, wrote:

I am getting such wild differences between the meter on my IC736 and that on
the MFJ969, that I feel that I need a 3d meter to tell me which of the others
to believe.

would appreciate suggestions on something for HF 100W and less than $100.


chasm,

There is a fundamental flaw in your approach. You cannot have
confidence that the meter you buy for $100 is significantly better
than the two you already have, and so, it doesn't add useful
information in solving the problem reliably, it just adds complexity
and noise.

Some key tests that you can perform on the MFJ969 a
- does it indicate 1:1 on a good 50 ohm dummy load (and that the
internal load doesn't qualify as a good 50 ohm dummy load)? (Goodness
is 1:1);
- terminate the MFJ with a S/C and measure the forward and reflected
power, are they exactly the same at any power level(reduce your
transmitter power output to do this test)? (Goodness is that they
match exactly at all power levels);
- terminate the MFJ with an O/C and measure the forward and reflected
power, are they exactly the same at any power level(reduce your
transmitter power output to do this test)? (Goodness is that they
match exactly at all power levels);

If it passes all those tests, it may be ok. We have not validated the
scale shape, nor the absolute power calibration (which is not needed
for VSWR measurement). You cannot do either of these easily without a
reliable means of measuring RF power, voltage or current.

What's that, you don't have a dummy load! Oh well, go ahead and use
the internal load... but remember it is not as accurate as you would
want, and so contributes to error. (I shouldn't be surprised however
if the MFJ969 meter is calibrated (ie nulled) on the internal load so
that customers think that since they agree, they must be correct...
remember what MFJ stands for! Seriously though, if you tightened all
the loose screws, shook out the loose solder bits, clipped ends of
wire, extra nuts etc, the sampler and meter in the MFJ969 are good
enough for the purpose at hand.)

Indicative readings from my MFJ949 (which uses the same sampler /
metering), on the internal load with 100W fwd, reflected is ~0.5W
(very hard to read, using the low power scale, non linear) which is a
VSWR of 1.15. On an external Bird load I cannot see meter deflection
(this might not be the factory cal, I may have cal'd it along with
other repairs.)

Owen
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