Originally Posted by holl_ands
compensates for the tuner's 7-10 dB Noise Figure.
Yeah, I checked the DTT7685x tuner (used in many CECBs) datasheet and it has a noise figure of 6dB typ, 8dB max. FM rejectin on CH06 was only 2-5dB. hdtvprimer
gives some noisefigures for different preamps at channel 30:
Channel master 7775 (discontinued) 2dB, winegard AP-2880 2.6dB, Radioshack 15-1109 3.6dB, 15-1170 6.0dB, 15-1108 7.0dB. Stark Electronic
gives noise figures of CM-7776 (disccontinued) 2.8dB, CM-7777 2.8dB, and CM-7778 3.0dB at an unspecified frequency. Denny's Antenna Service
lists the winegard AP8700 as 2.8dB VHF/UHF, HDP 269 <3dB, AP8275 (UHF 2.8dB VHF 2.9dB, and AP 4800 UHF 2.7dB.Todd Emslie
lists the Wineguard AC-4990 as 2dB, RDX UA-900 (discontinued?) at 1dB, wineguard AP-4800 at 2dB, Televes MRD 2dB, Research communications 98030 0.5dB (expensive), Alcad BR-105 1.5dB.Research Communications
9620 (UHF) 0.4dB and 9254 (UHF/VHF) 0.4dB ($320 with outdoor housing with waterproof cable glands).
And a google search for something like "TV preamplifier noise figure" turned up some homebrew preamps using a transistor with 0.5dB noise figure, though actual performance was not as good as 0.5dB.1.5dB
and 0.6dB at 1296MHz
but extends down to UHF TV and maybe VHF (50 ohm design). In sensitive applications, like radio astronomy, preamps are sometimes cryogenically cooled. The noise produced in electronic components is roughly proportional to temperature. Running your preamp at liquid helium temperatures (hard to do because of self heating, not to mention equipment that costs tens of thousands of dollars) gets you 37dB, and at liquid nitrogen temperatures 11.6dB.
Many components, however, do not function properly at cryogenic temperatures in addition to other practical issues, though Peltier coolers with 68 celsius degree temperature drop could give you 2.3dB (20*log10(225/293)) improvement, or 5.4dB with double stacking (but operating well below temperature rating for most components), while consuming a lot of power and lightening your wallet, provided you are still receiver noise limited. A 0.5dB noise figure amplifier is already acting as if it was at 40K equivalent temperature.
So, if you live in a rural environment where receiver noise might be the limiting factor then a good preamp might give you a 3dB improvement and a premium amp might give you 6dB improvement and a crappy amp no improvement. If you have a digital TV with a poor tuner, you might get another 3dB. Impractical draconian measures might possibly give more performance but once you exceed receiver noise figure (6-10dB) or ambient noise, you gain nothing.
First, use the best antenna you can. A preamp can't improve on the signal to noise ratio from the antenna, it can only prevent it from being further degraded.