I've got a minicircuits ZDC-20-3 20dB directional coupler that I picked up cheap at a rally. I'm interested to see how it matches up to the spec.

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The datasheet claims 0.3dB insertion loss, -19.3dB coupling (forward), and 50dB directivity at 1MHz. Right now, I'm interested in results at 5MHz, but that's the nearest point from the spec.

I've no hope of measuring insertion loss to 0.3dB, so lets look at directivity and coupling instead. Tools available are an FT817, and my W7ZOI powermeter.

To get a low power signal source I've set the rig to low power, and I'm using the tune tone from wsjt-x and adjusting the audio drive to control power.

To measure coupling:

  1. Terminate the cpl port in 50R
  2. Connect the rig to the in port
  3. Connect the power meter to the out port
  4. Transmit and adjust the transmit power (and audio drive level) to show 0dBm forward power on the meter
  5. Holding the transmit power the same, terminate out in 50R and connect the power meter to cpl and record the reading, that's coupling.

To measure directivity:

  1. Terminate the out port in 50R
  2. Connect the rig to the in port
  3. Connect the power meter to the cpl port
  4. Transmit and adjust the transmit power (and audio drive level) to show 0dBm forward power on the meter
  5. Reverse the in and out ports and measure power, that's directivity.
Frequency [MHz] Coupling [dB] Directivity [dB]
1.80 -18 -44
5.36 -18 -40
10.11 -17 -38
50.00 -17 -25
145.00 -18 -14
435.00 -22 -8

The directivity doesn't match the datasheet, even at low frequencies, look at what I'm using for the 50R termination (below left) no surprise that it doesn't perform well at anything about a meg or two.

Not a very good termination Rather better.

Repeating the measurement with a better 50R terminator shows much improved directivity (shock horror!).

Directivity [dB]
Frequency [MHz] Poor termination [dB] Good termination [dB]
1.80 -44 -54
5.36 -40 -52
10.11 -38 -50
50.00 -25 -40
145.00 -14 -33
435.00 -8 -24