[Elecraft] Calibrating the P3 Station Monitor?

[Peter Dougherty]

Hi all,
I recently installed a TX MON to my P3, and the readings are seemingly far off
compared to two other meters. The 1.8-54 Mhz/2kW sensor was placed where my
existing Alpha 4510 meter was connected (after the amp and low-pass filter,
before the tuner), so I was expecting readings that were more-or-less close.
However the TX MON is reading far below both the Alpha meter it's replacing and
the power indicated on my Palstar HF-Auto tuner.

Here are some sample readings, all of which are using the default 500ms
TXMonMenu/SensorCal setting, running into a Cantenna resistive dummy load,
keying down with 27 WPM dits 

 

Frequency           K3s Drive             Alpha                    HF-Auto
TX-MON

14025    CW         100 W                    101-107 W           96 W
80-82 W

                                100 W + amp      1425 W                  1229 W
1140-1150 W

 

7025       CW         100W                     105-107 W           99 W
79 W      

                                100W + amp       1595 W (!)            1475-1500
W       1240-1250 W

 

1825       CW         100W                     86-90 W                94 W
73-77 W

                                100 + amp           1350-1425
1320-1350 W      1000-1130

On SSB, the readings discrepancies are even more noticeable. While reading the
same two sentences at an even cadence and voice level, using 100W of drive to
the amp, tuned to 14210 and transmitting into the dummy load, the following
levels were noted.

14210 SSB                                            101-105 W           88-92 W
33 - 35W

                                                                1530-1560 W
1348-1386 W      675-800 W

I have adjusted the sensor scale settings, and 594 seems to be fine for 20m SSB,
high power, but it's still low at 100W radio-indicated output. When switched to
CW, using 594ms, it's reading high; the HF-Auto's internal meter is indicating
100W even while the TX Mon is indicating 115 W, and 1650-1675 Watts with the amp
on, into the dummy load, with the HF-Auto's meter indicating 1425 W. In this
case, for reasonably-accurate CW on 20m, the SensorScale setting needs to be at
549ms. At that lower setting, indicated power on the TX Mon is once again far
lower (1050W vs 1325 on the HF-Auto). Also, once the TX Mon gets a reading it
locks it there, and doesn't seem to change a whole lot if I continue talking.
Both the Alpha 4510 and the HF-Auto react as I raise or lower my voice, or speak
closer or further from the mic.

I can chalk up minor discrepancies of up to 5-7%, to instrument error-these
aren't lab-grade meters, after all, and I'm fine with that. But the numbers I'm
seeing on the TX-Mon are way out of whack with what I'm after. Has anyone else
run into this situation or did I get a bad sensor, do you think?


----------------------------------------------------

Regards,
Peter Dougherty, W2IRT

DXCC Card Checker/CQ-WAZ Checkpoint

www.facebook.com/W2IRT