I have a couple bridges that I need to check the directivity on before we sell them,
This is a bridge with an integrated scalar detector. You can only test it if you can connect it to a scalar network analyzer (Like an 8757). If you do so, then the test is to measure an open and short and take the average: that is your "response" term. Then add a high quality load, that is your "load response" term. The directivity is the difference (in dB) between the two. You can only verify it to the quality of your load. (in fact, there are more complicated ways to verify it, but this is the first order simple way).
Directivity is dependent upon 3 characteristics: insertion loss, coupling factor, and isolation. In order to find the directivity of the directional bridge we use the following equation:
Directivity = Isolation/(Coupling*Loss)
If you have three- or four-port VNA you can easily get this information measuring S-Parameters. By connecting the test port of the bridge to port 1, the forward coupling to port 3, and the main thru arm to port 2 we can determine directivity with this equation:
Directivity = S31/(S21*S23)
If you do not have a three- or four-port VNA, you can use a two-port VNA to make the measurements as long as you load the other port with a good termination.
Thank you to both of you for the reply/help!
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