Piggie

Seems I explain this once a month.

The antenna receives a A db of signal. But also receives X of noise. This then gives you Y which is equal to A/X or signal to noise ratio of the signal.

If you don't have an amp, then you loose L in the downlead and any splitters, etc getting to the TV.

So the TV then has y-L of signal available.

Here the signal goes into the receiver. In the receiver there are obviously RF circuits that process the signal.

There is always thermal noise from the electricity passing through the electronic devices. that creates noise. This is why very low noise amps used in quantum physics are kept at very low temperatures. So not only is the amount of noise added a function of how the electronics are made, but their temperature. So if you know a little chemistry or physics this noise is actually movement of the electrons in the receiving device. Hence if it's hotter they move more and if there is more current they move more, then the factor of how the device is made plays a substance role.

Most TV receivers and CECBs add about 5 to 8 db of noise right away to the signal to boil it down.

So now at the TV we have for signal

y-l-N where N is the noise in db added by the receiver.

So to receive a signal, it has to be strong enough that after we loose some in the feed system , y, then it has to over come the noise, N it will decode. Remember also that Y contains noise.

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Now add an amp that is NOT in overload.

The antenna still receives y db amount of signal, that doesn't change.

It goes directly into the amp assuming a perfect connection to it (not in the real world but simplifies things).

The amp adds noise to the signal since it is a electronic device just like the receiver explanation above. Lets call this noise, Namp.

Here is where you must understand a little about RF reception that goes a little deeper.

The antenna does receive y amount of signal. But actually it's y amount of signal above the ambient noise the antenna also receives. So it's not a simple amount but has to be thought of as an amount above another amount. So y is actually an expression of the Signal to Noise received by the antenna.

This y goes into the amp. The amp amplifies both the signal and the noise. So despite the amp rated at a gain of G db, and the signal comes out stronger by G db, so does the noise, so y remains the same, remembering that y is really a ratio, not an independent value.

The amp also adds Namp of noise. So in a sense it makes y worse by N as the signal is amplified. Here is the first part of why a lower noise amp is better. The lower the noise of the amp, the less noise is added. So no matter what any time you add an amp, y goes lower, because remembering again it's a ratio of signal to noise, the noise or denominator in the ratio grows.

But all is not lost, as now there is more signal to get through the losses through the feed system that again includes any coax, or splitters, etc.

So now with the amp, we have the y increased by G, but reduced by Namp. But G adds so much gain it over comes the loss in the feed system.

note that the ratio y actually gets worse through the amp.

The idea here as you stated that Gamp over comes the l or loss of the feed system, which hopefully it does.

But now we have more signal from Gamp, but y is actually less because of the amp noise. It's the gain of the amp that overcomes the loss of the feed system.

One can think of Y ratio riding on the extra gain of the amp.

When the signal gets to the receiver we hope that the Gamp is more than the loss of the feed system, but not by too much.

So lets pretend the loss of the feed system equals the gain of the amp.

So at the receiver the Gamp minus the L loss of the system is then zero.

So we have the y the antenna received intact at the receiver, but with the noise the amp added.

So one would think, wow, why put in an amp? you increased the noise?

Well there is more.

Remember above I said the receiver had noise? And that noise figure of a receiver is 5 to 8 db. But since we put an amp in the system the front end of the receiver is actually the amp, it is just physically distant by the length of the feed line.

A fact you must buy out right is the first stage of any receiver's noise figure sets the noise figure for the entire receiver.

So without an amp we had Y-L over coming 5 to 8 db of noise.
Now with an amp we have Y with 3 db noise added by the amp but since it's the first stage of the receiver now, the receiver lowers it's noise to the noise of the amp.

So if the amp has noise of 3 db and the receiver 6 db, with the amp, we lower the noise figure of the entire system by 3db. The is the same as doubling the signal or doubling the size of the antenna.

Now if you put the amp at the receiver and it has a lower noise then it lowers the noise of the receiver why amps into noisy receiver help right at the back of the receiver.

BUT! when placed at the back of the receiver, the signal into them is weaker, but with the same amount of noise, so y is much lower. This is because not only does the feed system lower the signal but actually adds a little bit of noise itself, since it too is a circuit.

Hence, the close to the antenna you place the amp, the better the signal to noise ratio exists to preserve. Also the lower the noise of the amp, the lower the noise of the system (the system being the antenna, amp, feedline, and receiver.)

There may be some redundant information here, spelling errors, but this was written as a draft.