Basically directional coupler
is a 4 ports device (port1 - input, port2 -transmitted, port3 - coupled, port4 -isolated), which has many uses in HF technik (forward and reflected power measurement, automatic level control, etc.).
Three ports directional coupler (without isolated port) is RF passive device used to couple a part of the traveling power (from port1 to port2) out to the other (coupled) output port (port3). I needed several tens of decibels to attenuate abt. 100W to the low measurable power with my mW-meter. Typically used transformation ratio 100:1 or 1000:1 was selected. It means 20dB or 30dB of attenuation. The 50 Ohms load at the coupled output is transformed as 0.5 or 0.05 Ohms serial resistance in the main direction (from port1 to port2). This means that unused coupled port must be allways terminated with 50 Ohms.
Toroidal cores from 43 material was used for transformers. For 20 dB transformer 3 pieces of 10mm (out dia) due to higher power handling and for 30dB one piece of 43006301 core. The 20 dB transformer has 10 turns of 0.4mm enamel, 30 dB transformer has 31 turns of the same wire diameter (SQRT 100 = 10 and SQRT 1000 = 31.6).
I removed the upper insulation and braid from piece of RG316 coax cable. Inner conductor with insulation will be used for connection between port1 and port2. Both wound toroidal cores were strung on this isolated wire. The winding outlets of both transformers were soldered to the coaxial connectors. I will use coupler as a symetrical device (input and output can be swapped), it means that order of winding outlets was not important.
Directional coupler with two coupled outputs was installed to the Hammond case 1550A