I changed the Subject line since this is a new topic.
If the wattmeter (in this case the P3 sensor) is placed after the tuner
then it will read the SWR of the antenna. To determine the net forward
power, just subtract the reflected power from the forward power.
To calculate reflected power, use the equation:
P(ref) = P(fwd) * [(SWR-1)/(SWR+1)]^2
Since the K3 and K4 already have a built-in wattmeter located before the
tuner, it makes sense to me to put the P3 sensor (or external wattmeter)
after the tuner so you can see what is going on with the antenna.
Alan N1AL
On 4/10/23 09:02, Bob McGraw wrote:
My question would be "where is the sensor for the P3 located in the
feed system?" It should be between the KPA500 output and the KAT500
input. Therefore, when the KAT500 resolves a match, then the line
between the KPA500 output and KAT500 input is nominally 50 ohms.
If the sensor is located after the KAT500, thus in the feedline to the
antenna, the impedance and thus the voltage on the line, will impact
the the indicated value. Most all power reading sensors are voltage
reading devices, referenced to 50 ohms nonreactive and should be
located in a 50 ohm line which has a nonreactive load. Most power
indicating devices are calibrated for 50 ohms. There are some
exceptions.
As example; if the impedance of the antenna is reactive ( most are )
and about 200 ohms Z being fed though 50 ohm coax for the feedline
and the power is 500 watts, the voltage is about 316 volts. If the
impedance were 50 ohms and the power is 500 watts the voltage would be
about 158 volts. Here it is easy to see, using a voltage sensing
circuit, the power indication would be incorrect by some significant
magnitude.
I've considered adding a power/SWR monitor to my P3 but it would be in
the coax to the antenna. I use the internal tuner in my K3S to
resolve a match, thus the power measurement would be after the ATU in
the radio and the actual load Z is really unknown. The only way to
determine if the power indication is correct with the P3 is to use a
50 ohm dummy load or place it in the RF path which is 50 ohms
nonreactive.
73
Bob, K4TAX
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[email protected]
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [email protected]
