Thanks for the details.
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Antennas placed at the tear drop.
At this position UHF channel reception is fine.
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Looks like the signal path is between two trees.
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VHF RF 12 gradually degraded over the last 6 months.
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Tree growth?
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Could I use AM radio as a interference detector?
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Yes, interference that is heard on the AM broadcast band is often strong enough to affect VHF-High TV reception. A battery operated portable can be used as a direction finder; its built-in loop antenna is directional. Tune to a clear spot at the low 550 end and then at the high 1600 end.
The last case I helped with was detected on an AM radio in a car as it drove in a road with the power line along side. The source of the interference was a battery charger affecting VHF-High. Other cases involved LED lights, and switchmode AC to DC power adapters for laptops.
Some of the sources of interference are constant; others time related.