One of the most difficult tasks of the QRPer is to measure transmitter output power with any form of accuracy. Low power meters can be rather expensive to say the least.
The figure gives the circuit diagram of an SWR/POWER meter that can be built using an existing SWR/"POWER" meter. These instruments have two meters, one for relative power and the other for SWR indications. The problem with these meters is that the readings are very frequency conscious and there exists no point of calibration to set the in-built potentiometer. All this can be overcome.
Between the two meters on the front panel, drill a hole to mount a single-pole two-way miniature toggle switch and wire it in circuit as shown in SWR-PWR.GIF using the right-hand meter. This meter is usually calibrated in SWR. Now you can use the SWR bridge as exactly as normal but using only one meter.
Add the extra components shown in SWR-PWR.GIF and highlighted with an asterix (*). Connect the diode to the existing Left-Hand (power) meter to give a true power indication. The values shown assume the meter is 100uA movement and the full scale reading required is 10 watts. Note that the scale follows a square law and although FSD=10 watts, 10% FSD is equal to 100mW. This reading is accurate for almost any frequency from 1.5 MHz to 200 MHz. For 70cm (430 MHz) a correction capacitor can be placed across the upper RF voltage divider (R1); the value selected for the correct power at 432 MHz. My meter needed a 6.8pf correction capacitor.
If you change the additional resistor values you can have a 1 watt FSD scale which will give reasonably accurate indication down to less than 10mW. You could provide a switch (beside the SET/SWR switch) to switch between the two ranges.
Note that meaniningful power readings can only be obtained when using a true resistive load (50 ohm dummy load etc.).
Have fun, de HARRY, Upplands Vasby, Sweden
I am very grateful to Harry, SM0VPO for giving me permission to use his circuits and ideas. He can be reached by Email at firstname.lastname@example.org
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