This is our naming scheme of the ports. Others may vary.

PORT 1 is the INPUT
PORT 2 is the OUTPUT
PORT 3 is the INPUT COUPLING
PORT 4 is the OUTPUT COUPLING

Therefore S21 ist the insertion loss from PORT 1 to PORT 2.
S11 is the input port matching. S22 is the output port matching. S33 is the input coupling port matching. S44 is the output coupling port matching. S31 is the input coupling insertion loss. S42 is the output coupling insertion loss. Unused ports are terminated with a 50 Ω, usually ANNE-50+ (DC...18 GHz, SMA Termination from MiniCircuits). And yes, our photo - model has Port 4 internally terminated with 50 Ω.

A low error or a high accuracy is connected directly to a high Directivity. Directivity is a measure of how well the coupler can separate signals traveling in opposite directions, specifically between the forward and reverse signals at the coupled port. It indicates the coupler's ability to isolate the desired signal (usually the forward signal) from unwanted reflections or reverse signals.

Rule of thumb: when the directivity is 20 dB lower than the level of the DUT, the Error is smaller than 1 dB.

Now choose wisely !

Some random models, available in the year 2025 ค.ศ. (2568) are listed in the table below :

TYPE, FREQUENCY	DIRECTIVITY [dB]	APPLICATION
ZFDC-10-6+, 0.005 - 20 MHz	40 dB	SUPERB THINGS
ZFDC-10-5+, 1 to 2000 MHz	30 dB (typ)	LAB GRADE
PE2236-35, 1 GHz to 11 GHz	20 dB (min)	HOME OFFICE

A directivity much larger than 40 dB over a wide bandwidth should make you restrained.

Test-Setup with a Synthesiser and Spectrum Analyser. Of course a network Analyser can be used as well :-)

1) Connect your Signal Source to the Output (Port 2) of the Coupler.
2) Connect a 50 Ω Termination to Port 4 of the Coupler.
3) Connect a Spectrum Analyser to Port 3 of the Coupler.
4) Connect a Short or an Open to the Input of the Coupler.

5) Set the Source to sweep the desired Frequency Range.
6) Measure the Response at Port 3.
7) Now Null the displayed / measured Response.

You are now ready to measure the Directivity ...

Now, as you have calibrated the Setup, add a high quality 50 Ω Termination to the Input.

The displayed Response of Port 3 is now the Directivity.

We used an hp 8753 ES and a ADCB-20-82+ from MCL and got the measurement below :

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