[Elecraft] Where to put the wattmeter

[Alan Bloom]

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