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Old 18-Oct-2019, 2:27 PM   #13
Retired A/V Tech
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,708
Originally Posted by BrooklinOTA View Post
My quess is that the RCA preamp causes an initial 2.6db of signal loss as the signal from the antenna goes into the unit. Is that accurate?
Not exactly.

The preamp does not cause a signal loss of 2.6 dB. What would be the point of using a preamp if it caused a signal loss of 2.6 dB?

It does, however, add its own internal noise, called the Noise Figure or NF, to the signal that reduces the SNR of the signal coming from the antenna terminals. The greatest affect of this added noise would be on the weakest UHF signals. The preamp also makes all signals stronger before they travel down the coax to the TV.

But, all is not lost, because the Noise Figure of the preamp is usually lower than the Noise Figure of the tuner in the TV that comes after the preamp, so there is a net improvement of the antenna System Noise Figure.

Since the preamp is just after the antenna, it is at the beginning of the antenna system. This means that it primarily determines the System Noise Figure according to the Friss Formula.
An important consequence of this formula is that the overall noise figure of a radio receiver is primarily established by the noise figure of its first amplifying stage. Subsequent stages have a diminishing effect on signal-to-noise ratio. For this reason, the first stage amplifier in a receiver is often called the low-noise amplifier (LNA).
Here is an example of with and without preamp that shows how adding a preamp might make it possible to receive weaker signals:

If you can not measure it, you can not improve it.
Lord Kelvin, 1883

Last edited by rabbit73; 19-Oct-2019 at 12:23 AM.
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