Hi Ed,
Strong words!
In a message dated 98-07-02 00:00:36 EDT, you write:
<<
W8JI@aol.com wrote:
> 4.) When you measure power, be sure to subtract the reflected power
> from forward power to get true forward power. Use a low power slug to >
measure reflected power, keeping the meter near full scale for both
> readings. Otherwise, you have to add in the error in reflected power, >
since it directly affects the true forward power and SWR readings
Ed wrote:
>> The above is absolute nonsense. Reference "Reflections", Chapter 4,
page 4-10<<
My statement is absolutely correct. The Bird meter, as do ALL directional
coupler meters, reads combined forward and reflected power when in the forward
position.
If you want to know the true forward power applied to the system, you must
subtract reflected power from forward power. Not doing when measuring power is
a common mistake, but none the less absolutely wrong.
> 5.) The best way to measure power is to build a very low loss tuner low
> operating Q tuner using high quality components (efficiency above 98%) >
and use that matching network to insure the load Zo is always 50 ohms. > This
eliminates reflected power meter errors and the accumulated
> errors of reflected power and forwrad power subtraction.
Ed again wrote:
>>Use the same reference. A tuner maintains a constant Zo ONLY between the
signal generater and the tuner. It has absolutely nothing to do with
VSWR on the transmission line, unless of course, the tuner connects
directly to a antenna. And if such were the case, VSWR does not, by
definition exist.<<
I have no idea what you are saying Ed.
I recommended adding a low loss tuner between the meter and the load, so the
meter was terminated in its design and calibration impedance of 50 ohms. Doing
this will remove errors sourced from less-than-perfect directivity in the
meter, and also eliminate the error adding requirement of subtracting
reflected power from forward power. The only additional error is the
attenuation of the network, but that can easily be made less than the
directivity plus absolute reading errors in the meter when the slug is
reversed.
My method is absolutely correct as written.
73 Tom
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