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Originally Posted by
AntAltMike
Been a while since I actively posted in this thread, but I'll dust off some old recollections and invite anyone who thinks I am senile to offer alternative numerical values.
I love to hear your old stories, and learn from them. The first meter I used was calibrated in microvolts, and then I graduated to dBmV units for my Sadelco meters. I think in dBmV with 0 dBmV as the magic reference level, but had to learn to think in dBm, because that is what the TVFool reports use. The conversion factor is 48.75, rounded to 49. or 50 for a quick estimate.
I think the Channel Master engineers gave much more accurate specs for their preamps than Winegard did, and the CM shielding was better.
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Did we simply use dBm at the same bandwidth for the digital noise floor.
Yes, the Thermal Noise Floor for a 6 MHz bandwidth signal is -106 dBm, digital and analog. Of course, if your ambient noise is higher than that (like on VHF-Low), you will need more signal to have the minimum required C/N for analog and SNR for digital.
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Can someone here do the ballpark calculations and estimate how weak a signal one might be able to sustain using the established thermal noise floor, S/N or C/N ratios?
For some reason, the umbers -106dBm and -102dBm are stuck i my head.
If you put a dipole in a shielded enclosure and connect to it with an F-81 through the wall of the enclosure, the noise level will be -106 dBm at the antenna terminals. If you take the dipole out of the enclosure, the minimum required digital signal will be -85 dBm in a location with low ambient noise. This is the calculation:
-106 dBm + 15 dB for minimum required SNR = -91 dBm
-91 dBm + 6 dB for tuner NF = -85 dBm
This is one way to look at it:
but this is probably more accurate, because the tuner noise is just above the thermal noise floor, and the minimum required SNR is above that:
If you add a reflector and directors, the noise will still be -106 dBm at the antenna terminals, but you have collected more signal to exceed that noise.
However, a TVFool report equates a 0 dB Noise Margin with -91dBm, because it assumes a preamp with a 0 dB NF is used; the difference between the listed NMs and the listed signal powers in dBm is a constant of 91.
When you use a preamp, the tuner NF becomes irrelevant, because it is buried in the amplified noise floor. Then, the preamp NF primarily determines the System Noise Figure as shown by the Friis formula.
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FOR OLD TIMERS: