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There's a tutorial somewhere that explains the Noise Margin figure. Applying that to your situation, the MANT has very little, if any gain at UHF, and probably -6 to -10dB gain at VHF. These figures add to your NM. The internal amplifier probably has a 5dB noise figure that subtracts from NM. The 18dB serves only to overcome cable loss in this case. Combining these values to the NM of VHF stations WOIO (14dB) and WJW (20dB), and you get near 0dB and 6dB net noise margins. Protecting against random fades usually requires having a 10dB or higher NM, you can see why you're getting the results you have.
Here's a radical thought: since you have plenty of UHF signal, the MANT provides enough net MN to work reliably. Since the antenna is small, with little directivity, it is picking up Youngstown and Cleveland UHF acceptably, save for WYTV. Why not build a simple bow-tie antenna scaled for VHF, mount it on the tower towards Cleveland, and join it with the MANT with a USVJ? You'll have only $15 or so invested in such a solution.
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