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Old 7-Mar-2014, 8:18 PM   #1
stvcmty
Senior Member
 
Join Date: Mar 2013
Posts: 135
When a preamp is needed and cable length

There is a lot of discussion about preamps. The quick summary is a preamp canít pull signal out of the air, all it can do is push a signal through the distribution system. If a preamp has less noise than the first transistor in a tuner, it may help overcome noise in the tuner.

I looked at the loss for RG6 cable for 50 to 700 MHz. I assumed a typical preamp added 3 dB of noise on VHF (RF 2 to 13), and 2.6 dB of noise on UHF (RF 14 to 51). I then looked at the length of coax for each channel where loss in the coax was equal to the noise added by a typical preamp. All of this assumes no loss in the ground block, no splices, and no splits, just the antenna system feeding one TV.

For a preamp to make sense for VHF low (2-6), the cable run needs to be at least 150 feet.

For a preamp to make sense for VHF high (7-11), the cable run needs to be at least 100 feet.

For a preamp to make sense for UHF (14 to 51), the cable run needs to be at least 50 feet.

I do not know if 3 dB of noise is reasonable for VHF-low. I think 3/2.6 should be reasonably representative of the TVPREAMP1R and the 10G221/10G201 for VHF high/UHF.

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The above is for 1 TV. If splitters and multiple TVís are going to be used, the NM of the weakest desired channel after antenna gain, cable losses, and splitter losses needs to be calculated. If it is negative, a preamp may help. If the antenna gain + the desired stations NM is negative, assuming the TV fool model is accurate, a preamplifier will not help, because the preampís noise would be subtracted from the NM, digging a deeper hole.

Without preamplifier: NM at TV = NM in the air + antenna gain Ė cable loss Ė splitter loss

With preamplifier: NM at the TV = NM in the air + antenna gain Ė preamplifier noise
(Assuming the selected preamp has enough gain to overcome cable loss and splitter loss in the system).

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Overload is another problem. If a preamplifier is overloaded by signals in the air, it will make noise at harmonics of the overloading signals that could drown out desired channels.
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