Testing antennas is a non-trivial task. But before you go down that path, make sure you've isolated the problem first. It is often the case that signal loss is caused by a bad wire, bad connection, bad electronics, or water inside of something rather than the antenna itself.
If the antenna is not physically damaged is some way (bent or broken elements), then it's pretty rare for an antenna to spontaneously go bad. It is much more likely that the problem is somewhere else in the chain.
If possible, try to use the process of elimination to narrow down the source of the problem. Check connectors to make sure they're snug (and not over-tightened). If any of the connectors were put on the cable manually, check to make sure they are making good electrical contact with the cable. If any parts/connections are exposed to the elements, make sure they are weather-proofed. Check for sharp bends, cracked, or crushed cable. Swap out sections of cable, connectors, or splitters where possible.
If you still can't isolate the problem, you can check the longer cable runs with continuity or loopback tests to make sure there aren't any unexpected shorts or open circuits.
The most vulnerable spots in most antenna setups is at the balun, or places where there are connectors, splitters, or electronics (e.g., pre-amps) involved.
If the converter box itself is questionable, you can try it with a different (known working) antenna setup, or another location (e.g., friend's/relative's house).
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