Some care has to be taken when interpreting the LP-100A's power readings.
On my version it indicates Pfwd when the "F+R" mode is selected, and
Pfwd-Pref when the "Net" mode is selected. I got caught out on several
occasions assuming the "F+R" mode meant Pfwd+Pref - it doesn't!
Note also that the derivation of Pfwd and Pref from the Voltage and
Current sensors assumes a 50 Ohm Zo; I don't know any way to alter the
characteristic impedance that the LP-100A assumes, unless it's buried
deep in the menus and I haven't found it?
Steve G3TXQ
On 22/07/2014 13:03, Carl Moreschi wrote:
The LP-100A has two connections to the sensor. One is for current and
one is for voltage. Power is simply the product of voltage times
current. So for any load impedance, power will always be calculated
correctly.
Most less expensive meters only measure voltage. They calculate power
as Voltage squared divided by 50. They always assume the load is a
pure 50 ohms. But with a 1.5 swr, you could have a 75 ohm load or a
33 ohm load or a complex load with both real and imaginary
components. If you have a 33 ohm load, your power will read high. If
you have a 75 ohm load, your power will read low. The point is unless
the swr is exactly 50 ohms, your power measurement will be wrong by a
lot.
Carl Moreschi N4PY
58 Hogwood Rd
Louisburg, NC 27549
www.n4py.com
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