Adding Chris's BAT41 and 0.1uF cap to read the peak voltage with a DVM is a good idea, will work great for a QMX.
Han's 20W dummy load does the same, but uses a 1n4004 diode, good for 400 Volts, assembly instructions have the schematic.
The BAT41 is a shottky diode so will be more accurate, but 20W would be close to blowing through it's 100 Volt max spec.
The DVM will see the peak RF voltage from ground, so multiply by 0.707 to get Vrms (the rms voltage of your transmitted signal).
The power out in Watts is then equal to Vrms*Vrms/50, and that should be fairly accurate assuming the RF is a sine wave.
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Chris has two 100 ohm resistors in parallel to form the 50 ohm on the source end of the coax that goes to
some measuring device such as a tinySA, which we assume has an internal 50 ohm termination resistor.
So his circuit is the same as what I was suggesting for the 50 dB attenuator
His 2.5k resistor is very close to the ideal value of 2475 ohms that I had calculated.
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At 10 Watts out into 50 ohms, the voltage across the 50 ohms is sqrt(10*50) = 22.4 Vrms.
And the voltage across Chris's two 100 ohm parallel resistors is? ?22.4v/(2500+50) * 50 = 0.44 Vrms.
Power dissipated by that 50 ohms is? 0.44*0.44/50 = 0.004 Watts.
Even ridiculously small 0402 (imperial size) chip resistors are rated for 0.062 Watts.
Two 100 ohm resistors in parallel present half the inductance so will be more accurate at 50 MHz,
but if using a surface mount chip resistor that shouldn't matter much.
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Jerry, KE7ER
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On Tue, Apr 22, 2025 at 04:38 AM, Chris, G5CTH wrote:
My dummy load uses 4 x 51R 2W SMD resistors connected in series parallel, then 2k5 to 2 x 100R to give a dummy load come 40dB attenuator.? It also has a BAT41 Schottky diode and 0.1 uF capacitor to give the peak reading.? AIUI SMD resistors tend to be lower inductance than through hole ones.
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A dummy load is about third on my list of essential test gear, first is a multimeter, then a controllable voltage and current power supply with voltage and current display, then the dummy load.
