> With an open or shorted piece of coax, there will be forward and
> reflected voltage and current, but relatively little power, since the
> impedance will be almost purely reactive. That's why I emphasized
> measuring voltage and current rather than power. A true RMS power meter
> in such a circuit would show close to zero in spite of large amounts of
> voltage and current.
>
> 73, Bill W7TI
Hi Bill,
RF power meters traditionally measure voltage across the line and
current through the line. Current and voltage are both expressed as
a voltage from a low impedance source.
The current is read via transformer coupling to the line which
converts line current to voltage. Voltage is read by a voltage divider
either with capacitors or a transformer.
These voltages are summed at radio frequencies and the sum fed
to a voltmeter system.
Even a "RMS" meter will show VAR (volt-amperes-reactive) power
until the reflected power is subtracted from the forward power.
Because of the many stages in this process, there are also many
additional errors.
The most accurate way to read power into a load in situations
where there is a lot of power factor (reactance or reflected power) at
radio frequencies is to measure temperature rise of the load.
I can't think of a way you can read RMS voltage and RMS current
without also knowing phase angle between them and read true
power with a reactive load.
73, Tom W8JI
w8ji@contesting.com
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