Power measurement of 1mW Tx with DC5ZM UHF power meter



UHF SWR meter


March 2020 Reinhard Weber DC5ZM published a nice design for a UHF SWR meter based on surplus directional couplers [1].
The unit shows forward and reflected power and calculates the return loss and SWR.
The surplus directional couplers range between 800 - 2500 MHz and should be purchased in pairs.
The unit uses two very sensitive AD8313 modules so low RF power levels can be measured.

I happen to have pairs of -10, -20, -30 and -40dB directional couplers and so I tried each set in the DC5ZM meter to get a range of measurements of the power from my 1mW transmitter.
(Note: When a new set of couplers is fitted the Arduino program can be set up (using the LCD shield switches) for the particular coupling loss values).

The results are tabulated below and show that the 1mW transmitter was probably producing less than 1mW
(give or take losses in the system) giving an average power more like 1/2 mW.

Coupler value (dB) recorded value (dBm)* recorded value mW
-10 -4 0.40 mW
-20 -5 0.32 mW
-30 -6 0.25 mW
-40 -1 0.79 mW


* power = 10(dBm/10) in mW
Note: negative values of dBm
e.g. -1dBm = 10(-1/10) = 0.79 mW

[1] Measuring power, return loss and SWR at GHz frequencies, Reinhardt Weber (DC5ZM),
Radio Communication (RadCom), RSGB, March 2020. p30-32





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