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The effects of multipath depend on frequency. That is, given the same scenario, what affects one channel may not affect another. Also, the antenna you have is one that comes recommended by many, the AntennaCraft HBU-33. A couple of suggestions:
* try changing the aim of the antenna a little bit. If signals are bouncing around up there, there may be an aim that penalizes/boosts one signal path over the other, settling down reception.
* move the mount to a different location. In my case, a close-by CFL on the floor below zapped VHF reception, and I suspect ceiling fans with dusty motors might also be an issue. Also, if multipath is really the issue, then a move and re-aim would help in the same way as the first idea.
* avoid that RadioShack amp. A pre-amp's role is to overcome cable losses, so the benefit in our case will just be a few dB. The extra gain in the amp does nothing to improve reception quality. Cheap amps, like the RadioShack model, also have higher-than-average internal noise - upwards of 7dB that SUBTRACTS from signal-to-noise ratio. I dare say that if you put the RS amp in, you will end up worse, not better.
A short explanation: the number to pay attention to on TVFool plots is the NM, or noise margin. Decoding a digital TV signal requires a minimum signal-to-noise ratio. NM is the measure of signal with respect to this minimum, so a 0 MN could be decoded if there were no losses. All signals fade, so a more apropriate target is +10 for NM to ensure reliable reception.
Things that impact NM:
* antenna gain (unamplified) - adds to NM, so a 7 dBi antenna picking up a signal with an NM of 25 will provide a signal with an NM of 32.
* cable loss - there's no avoiding this, and it subtracts from NM. If the loss in a cable run is 5 dB, then the above signal will have an NM of 27 at the converter box.
* pre-amp noise factor - this is the amount of noise generated inside the amp. Subtracts from overall NM, so if an amp with a 7 dB noise figure (truly bad - good ones run about 3) were to be installed in this situation, the NM before the cable run will be 25. Pre-amps zero out cable losses, so the NM at the converter box would be 25 also.
* roof shielding - at least 10 dB typical, could be more, and it subtracts from NM. In this example, that means the signal to the antenna would be no better than a 15 dB NM, likely worse.
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