Quote:
Originally Posted by Electron
Bob called to tell me that with the reception of HDMS9100 take a look at these figures. For reception of WJW on channel 8 the NM(dB) is +18.4 and the antenna gain with out the built in preamp is -6.4 so that makes the signal strength 12 NM(dB). The amplifier gain is 16 dB on the VHF channels and then subtract the 3.7 noise figure , making the amplifier gain 12.3 dB. So while it is true that a amplifier does not make signal , the amplifier will at least carry the 12NM(dB) signal from the antenna through the coax with no loss of the 12NM(dB). So 12 , while not a real hot number , it is Ok. . . And then if WJW 21 is on the air , the NM(dB) # is 14.4 . The antenna gain is +2.5 making the number 16.9 . The UHF amplifier gain is 20 dB and then subtract the 2.9 of noise and that leaves 17.1 dB. So the 17.1 dB amplifier gain will at the very least carry the 16.9 signal through the coax to the tv with 16.9 at the tv. The FCC reference point number for reliable reception is 0 NM(dB).
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Thanks for discussing this.
Looking at the predicted NM of WJW, real channel 8, we agree it's 18.4 dB per TV Fool.
We agree that the vendor specified gain of the HDMS9100 is -6.4 dBd in the high-VHF band.
We agree that the net NM at the antenna output, ahead of any amplifier would therefor be 12 dB.
At this point we seem to take different paths. I subtract the noise figure of the amplifier, 12 dB - 3.7 dB = 8.3 dB NF net at the output of the amplifier.
Assuming the amplifier is operating within it's linear region, the desired input signal & noise is amplified equally, by the gain ratio of the amplifier. The amplifier adds it's own noise to the combination of input signal+noise resulting in the total noise output increasing by the amplifier gain + noise figure. The net result is a lower noise margin at the output of the amplifier.
(I think we both agree that the amplifier gain is sufficient to overcome cable and splitter loss on the output side of the amplifier.)
If Bob says that you simply subtract the NF from the gain of the amplifier, I respectfully disagree. (Is Bob an Antennacraft engineer?)
Re. "The FCC reference point number for reliable reception is 0 NM(dB)", Using 0 NM as the design target leaves no margin for noise, fading, multipath or imperfections in the receiver. There are many examples in this forum where people have much better NM predictions, yet have trouble obtaining reliable reception.
http://transition.fcc.gov/Bureaus/En...7/fc97115a.pdf
pp A3, section III implies that noise in the low VHF band requires at least 13 dB better net NM (per TV Fool prediction corrected for antenna gain and system losses including amplifier NF) than the UHF band.
Noise in the high-VHF band is implied to be 5 dB higher than the UHF band.
Please correct me if I'm wrong.
Again,
Thanks for discussing this.