[Elecraft] P3 TX Mon Power Measurement Anomaly

[Fred Jensen]

"> Does anyone have a good explanation for this?"

I don't know if this is a "good" explanation, but it is one that is 
sometimes [often?] true:

Many TX Power sensors actually measure voltage, it's a whole lot easier 
than power.  The meter or display is marked in watts which are computed 
by the designer from E^2/Z.  Z is assumed to be constant, usually 50+j0 
ohms.  That's rarely the case in reality.

Moving the sensor up and down the equipment-coax chain will subject it 
to slightly different values of Z, ergo E^2/Z changes.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the Cal QSO Party 1-2 Oct 2016
- www.cqp.org

On 12/28/2015 4:50 PM, Ken Widelitz wrote:
> I am having an anomaly with the TX Mon power measurements on my P3s. The
> power varies significantly depending upon where I put the TX sensor in the
> RF chain (K3 -> BPF -> Alpha 87A -> Coax Stubs -> Tuner -> Antenna Coax.) I
> have a 3" coax jumper on the appropriate end of the P3's TX sensor. I get
> similar results on both of my radios connecting the TX sensor in the same
> place in the RF chain. For instance, on 20 meters, with the K3 set to full
> power and the Alpha 87A in standby, the P3TX Mon displayed power is as
> follows:
>
>
>
> TX Mon Location                Watts
>
> @K3 RF Out                         106     -> 6' coax ->
>
> @ BPF Input                         101     That is a 5 watt measured
> decrease in a 6' piece of RG-213.
>
> @BPF Output                      92       That is a 9 watt measured decrease
> through an Array Solutions W3NQN design BPF. -> 6' coax ->
>
> @Alpha 87A RF Out          86       That is a 6 watt measured decrease in a
> 6' piece of RG-213 and through an Alpha 87A in standby.
>
>
>
> The Alpha 87A RF Output LEDs show just over 100 watts (the 4th LED
> flickers.) That indicates to me the TX Mon power displayed is incorrect.
> When the Alpha 87A shows full output (all green LEDs lit) the TX Mon reads
> 1329 watts. That is a 13+% difference. I don't have any other power meter as
> a reference.
>
>
>
> I have measured both coax jumpers and the BPF individually using an AIM 4170
> and a 50 ohm load. The SWR is flat at virtually 1:1 for each. The SWR
> displayed on the P3 is almost the same when measured at the K3 (1.53:1) and
> on the AIM 4170 at the end of the coax to the antenna (1.48:1.)
>
>
>
> Does anyone have a good explanation for this? I know there is a calibration
> method for the TX Mon, but I that doesn't explain the measured power drop in
> the RF chain with the amp in standby.
>
>
>
> 73, Ken, K6LA / VY2TT