A 15" vertical wire will not have 3 dB of gain. At best, it would be unity gain (0 dBd) before matching losses at one frequency, probably around channel 10 or so. Plus, unless your channel 10 has circular or elliptical polarization, there will be additional losses due to being improperly polarized. All stations in the US use horizontally polarization unless vertical components are added to their signal.
For the US stations, you need a highly directional UHF antenna with a steep D/B ratio. Unfortunately, your proximity to your local transmitters may result if the tuners are being desensitized to the point that reliable reception of the weak, distant signals may not be possible.
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