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Old 3-Dec-2013, 6:45 AM   #4
GroundUrMast
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Join Date: Oct 2010
Location: Greater Seattle Area
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You can estimate the losses in your cabling and splitter(s). Worst case, RG-6 should have no more than 6 dB loss per 100' at the top end of the UHF band. 2-way splitters should not have more than 4 dB insertion loss, 4-way splitters should not have more than 8 dB insertions loss and 8-way splitters should have no more than 12 dB insertion loss. Many splitters will be labeled, indicating loss at each port.

Simply add the length of cables starting at the antenna, all the way through to the tuner you are estimating distribution losses for, then (Total Cable Length / 100') * 6dB + Insertion Loss of the Splitter(s) in the Path = Estimated Antenna to Tuner Distribution System Loss

Given that the base thermal noise power of a 6 MHz channel is -106 dBm (http://en.wikipedia.org/wiki/Johnson...3Nyquist_noise), we can estimate the equivalent power level of the noise in a tuner if we know it's noise figure... -106 dBm + Tuner NF. In your case, assuming an estimated NF of 6 dB, the equivalent baseline noise level in the tuner is -100 dBm.

The antenna has a baseline noise level of -106 dBm for the same 6 MHz channel, so if a preamplifier is placed in the system, close to the antenna, both the signals and noise received along with the baseline noise of the antenna will be amplified. For a 20 dB gain amplifier with a 3 dB NF, we would expect signal + received noise to be increased by 20 dB, and we would also expect the baseline noise of the antenna to be elevated to a power level of -106 dBm + 20 dB = -86 dBm

Presuming that the preamplifier has a lower noise figure than the tuner... As long as the loss in the Antenna to Tuner Distribution System does not attenuate the amplified baseline noise level of the antenna to a level equal to or less than the equivalent baseline noise level in the tuner, the preamplifier NF dominates the net system NF.

In other words, if we had 14 dB of Antenna to Tuner Distribution System loss, the power level of the amplified baseline noise of the antenna would be attenuated to -86 dBm - 14 dB = -100 dBm which is equal to the tuner equivalent baseline noise level in the tuner of -100 dBm -- which would make the superior amplifier NF of no benefit. If we could reduce the distribution losses by 3 dB or more, the low NF of the preamp would be at the threshold of fully dominating the net NF of the system.
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Last edited by GroundUrMast; 3-Dec-2013 at 6:47 AM. Reason: sp.
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