To make a simple "sniffer cable", it is sufficient to shorten the inner pin of the connector. It creates a capacitive coupling.
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| BNC connector with shorted inner pin |
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| Top: 50 Ohm Load; right side: power input; bottom: "sniffer cable" |
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| The good match remains; the missmatch due to "sniffer cable" is low. |
The attenuation shows the properties of capacitive coupling.
