The Bird Model 43, and other thru-line wattmeters, are based on a “lumped constant” directional couplers. The directional coupler is called an element. Many users also call it a “slug” or a “plug-in”.
Each plug-in element (or coupler) samples the voltage at the point of insertion and samples the current via a loop. Turning the element 180° reverses the loop (and consequently the current pick-up) while the voltage sample remains unchanged. By proper combination of the two samples we obtain an RF voltage proportional to the square root of main line RF power.
The RF sample is then rectified and a signal proportional to the envelope is delivered to the meter. When the Model 43 wattmeter is used on CW – its intended purpose – this signal is DC. It is not, however, when the carrier to be measured is modulated.