Malicious broadcast or signal hack: What to look for?

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Has anyone used Wireshark to forensically prove a signal hack

no, I have not and based on your assumption to use a TV transmitter, Wireshark is the wrong tool for such an endeavor.

The reasons are:

1. You need a capture device that is able to capture TV signals.
2. You need a decoder that can decode 'TV signals'. Wireshark does not have such a decoder.
3. Wireshark was mainly built to dissect Ethernet/IP packet. While it can dissect a lot of other protocols now, why do you assume, that such a uber attacker would use a known encoding scheme, when he want's to leak data via a TV signal

Long story short: I'm sorry, but there is no way to use Wireshark for the type of forensic analysis you described.

Regards
Kurt