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he system was however impedance mismatched (50/75 at load),
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That causes some really funky S11 traces. Only way to eliminate them is to (generally) use a min loss pad then calibrate using a 75 ohm call kit at the F-connector plane. Unfortunately Keysight wants about $5G for a 75 ohm cal kit, so I don't have on here (our designer has one at his lab).
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But isn't the governing factor here the apparent power at the load? In terms of raw dBm, the VHF signals are largely on-par with those in the UHF band for my geographical location
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You have to have both adequate power and adequate signal quality for the ATSC signal to be decoded. The apparent power of the signal at the tuner input is only half the equation, so to speak. Excessive VSWR at the antenna creates mismatch loss which must be treated as insertion loss in terms of absolute power. You still have to keep under consideration the effects of the reflected signals when they get to the tuner's input and how they affect the demodulation scheme.
Another factor to consider indoors is that, as well as pure signal penetration of construction materials by the incoming signals, there may be window openings that admit (or, if the window has a low-e coating or a metal window screen, block) incoming signals which may substantially increase the randomness of the signals inside the living space.
Frankly, after discussing all the things that could go wrong, I'm about ready to declare this to be a bumblebee which, as everyone knows, shouldn't be able to fly.
