[spidey]

So a friend of mine has a large OTA in attic and when the OTA line is connected to the TV seems like hes getting 50 - 60% signal. When he connects the OTA to the HR20 all he gets is 771 errors ( hasnt tried getting signal readings from it yet ).

First question is really OTA related, if we were to try a power amp should it be closer to antenna or receiver.

Second if we eventually diplex in the OTA would were should the AMP be located.

 

[carl6]

First question is really OTA related, if we were to try a power amp should it be closer to antenna or receiver.

Closer to the antenna. You want to amplify signal, not noise.

Second if we eventually diplex in the OTA would were should the AMP be located.

Closer to the antenna, before any diplexers. Please do some research on diplexing OTA with the newer HD systems - there are severe limitations on what can be done.

Carl

 

[Scott in FL]

Closer to the antenna. You want to amplify signal, not noise.

The reason you put amplifiers at the antenna is so that the internal noise of the amplifier is insignificant compared to the atmospheric noise from the antenna. This is probably what Carl meant. I'm just trying to explain.

The signal at the antenna is made up of the carrier and noise. The ratio is C/N, and is one of the most important parameters (more so than absolute signal strength). The amplifier itself generates noise, and its noise adds to the atmospheric noise. If the atmospheric noise is attenuated too much, like at the end of a long cable run or after a splitter, the amplifier's noise become significant. If the atmospheric noise is high, than the amplifier's noise is insignificant. So you want an amplifier at the antenna or before any splitters.

The C/N will remain constant (assuming the amplifier does not contribute too much noise). That's one of the problems with amplifiers: they can only degrade the C/N, not improve it. Amplifiers can not "pull in" distant stations. So you should only use an amplifier to make up for a long cable run or splitters. (If the absolute level is too low, then an amplifier can help but it's usually low C/N that gets you).

Amplifiers can also overload, oscillate and produce intermod (IM). That's why you should never use an amplifier unless you need one.

Hope you don't mind me jumping in. This is one of my favorite subjects.

 

[spidey]

The reason you put amplifiers at the antenna is so that the internal noise of the amplifier is insignificant compared to the atmospheric noise from the antenna. This is probably what Carl meant. I'm just trying to explain.

The signal at the antenna is made up of the carrier and noise. The ratio is C/N, and is one of the most important parameters (more so than absolute signal strength). The amplifier itself generates noise, and its noise adds to the atmospheric noise. If the atmospheric noise is attenuated too much, like at the end of a long cable run or after a splitter, the amplifier's noise become significant. If the atmospheric noise is high, than the amplifier's noise is insignificant. So you want an amplifier at the antenna or before any splitters.

The C/N will remain constant (assuming the amplifier does not contribute too much noise). That's one of the problems with amplifiers: they can only degrade the C/N, not improve it. Amplifiers can not "pull in" distant stations. So you should only use an amplifier to make up for a long cable run or splitters. (If the absolute level is too low, then an amplifier can help but it's usually low C/N that gets you).

Amplifiers can also overload, oscillate and produce intermod (IM). That's why you should never use an amplifier unless you need one.

Hope you don't mind me jumping in. This is one of my favorite subjects.

Thanx for the info. I think he needs an AMP since TV only shows 50 - 60 % and HR20 isnt seeing anything to old onto

 

[carl6]

I meant what I said. The signal level (of the signal you are trying to use) is higher at the antenna than it is at the end of a coax run. You could easily have anywhere from 1/2 to 1/100th of the original signal level at the far end of the coax. Signal to noise level, attenuation levels of various types of coax, velocity factor of the coax, and the quality of the amplifier used (and how much inherent noise it has/introduces) are all part of the equation. An amplifier will amplify whatever signal is fed into it (including noise). If your signal to noise ratio is high (lots of signal, little noise), then the result of the amplification is still a high signal relative to a low noise. If your signal level is much lower, and you amplify it, you are still amplifying both signal and noise and the result will be higher signal and noise.

The inherent noise of the amplifier should not be an issue if you are using quality equipment.

Carl

 

[houskamp]

plus you don't want to amplify an noise picked up running thru your house..

 

[Scott in FL]

I meant what I said. The signal level (of the signal you are trying to use) is higher at the antenna than it is at the end of a coax run. You could easily have anywhere from 1/2 to 1/100th of the original signal level at the far end of the coax. Signal to noise level, attenuation levels of various types of coax, velocity factor of the coax, and the quality of the amplifier used (and how much inherent noise it has/introduces) are all part of the equation. An amplifier will amplify whatever signal is fed into it (including noise). If your signal to noise ratio is high (lots of signal, little noise), then the result of the amplification is still a high signal relative to a low noise. If your signal level is much lower, and you amplify it, you are still amplifying both signal and noise and the result will be higher signal and noise.

The inherent noise of the amplifier should not be an issue if you are using quality equipment.

Carl

Carl,
An amplifier can not increase the S/N ratio, no matter where it is placed in the line. You said it yourself: an amplifier will amplify both the signal and noise.

It is the absolute values of signal and noise that matter, along with the noise figure of the amplifier. At the antenna the signal and noise will be high, and the internal noise of the amplifier will be insignificant. After 20 dB of coax loss the signal and noise will be 20 dB lower, and the amplifier's noise contribution becomes significant.

I am assuming the coax does not pick up any external noise, which should not be the case with any decent quality coax.

Velocity factor has nothing to do with it.

 

[NR4P]

First recommendation, get the antenna out of the attic. You may pick up 20db signal. That alone could solve your problem.

If you can't do that and decide to purchase an amp, Do not purchase at Home Depot, Lowes or Radio Shack. Get something from a notable company that has a low-noise figure (meaning it doesn't introduce alot of noise). One I recommend is in my signature.

 

[Scott in FL]

I meant what I said. The signal level (of the signal you are trying to use) is higher at the antenna than it is at the end of a coax run. You could easily have anywhere from 1/2 to 1/100th of the original signal level at the far end of the coax. Signal to noise level, attenuation levels of various types of coax, velocity factor of the coax, and the quality of the amplifier used (and how much inherent noise it has/introduces) are all part of the equation. An amplifier will amplify whatever signal is fed into it (including noise). If your signal to noise ratio is high (lots of signal, little noise), then the result of the amplification is still a high signal relative to a low noise. If your signal level is much lower, and you amplify it, you are still amplifying both signal and noise and the result will be higher signal and noise.

The inherent noise of the amplifier should not be an issue if you are using quality equipment.

Carl

Here's an example, very typical for near fringe UHF OTA...

At the antenna:
-100 dBm signal (or carrier)
-120 dBm atmospheric noise
S/N = 20 dB

Through a 20 dB amp with a noise floor of -140 dBm, located at the antenna:
-100 dBm input signal
-120 dBm input atmospheric noise
-140 dBm amplifier noise floor
-80 dBm output signal
-99.95 dBm output noise
S/N = 19.95 dB

Now move that same 20 dB amplifier to the end of a coax line with 20 dB of loss:
-120 dBm input signal
-140 dBm input atmospheric noise
-140 dBm noise floor
-100 dBm output signal
-117 dBm output noise
S/N = 17 dB

When the amp was at the antenna, its -140 dBm noise floor was insignificant compared to the -120 dBm of atmospheric noise. The S/N was only degraded by 0.05 dB.

Move that amp to the end of the coax line, and the atmospheric noise is now attenuated 20 dB, to -140 dBm, and the -140 dBm noise floor becomes significant.

Its simple math:
-120 dBm plus -140 dBm = -119.95 dBm (insignificant increase in noise)
-140 dBm plus -140 dBm = -137 dBm (3 dB increase in noise).

That's why you put the amplifier at the antenna.

 

[carl6]

We are saying the same thing using different words. I believe we are in agreement.

Carl