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Originally Posted by Radio Wrangler
The antenna will see cold sky in half its field of view, and outdoor temperature ground in the other half, so overall its noise output is colder than the 290K assumed in noise figure definition. (This is why it's still worth pursuing noise figures down to 1dB and below)
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True for UHF, but at VHF there is more noise coming from somewhere else? I seem to recall reading somewhere that at 100MHz there is little point in going for a noise figure below about 3dB.
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In later FM tuner designs the Q of varactor diodes is dramatically lower than that of mechanical variable capacitors, and they become the dominant limitation on resonator Q, no longer the inductor. So the losses of the input filter increase leading to more noise disadvantage unless heavier coupling is used to widen it. A wider filter lets more off-channel signals in to add up and clobber the first active device. Worse, the varactors themselves are non-linear. Narrower filters run their resonators at higher Q's and that scales up the signal voltages across the diodes. Intermod products rise disproportionately fast.
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When I designed a DIY FM tuner (with varicaps) I think I went for Q=40 single-tuned from the antenna into the dual-gate MOSFET RF stage and Q=80 double-tuned into the diode DBM. I have no idea how optimised that is, but it seems to work OK.