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-   -   When a preamp is needed and cable length (http://forum.tvfool.com/showthread.php?t=14293)

stvcmty 7-Mar-2014 8:18 PM

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).

***

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.

tripelo 9-Mar-2014 12:21 AM

Quote:

Originally Posted by stvcmty (Post 42522)
Without preamplifier: NM at TV = NM in the air + antenna gain – cable loss – splitter loss

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

As applies to the input of TV tuner, the above is pretty much correct.

There is a bit more.

Primarily, the signal-to-noise ratio (S/N) at the tuner demodulator is what matters. Signal-to-noise ratio is related to what is referred to as ‘noise margin’.

Often a large contributor to S/N is the tuner noise figure. Typical tuners have noise figures that range from ~6 to 8 dB.

So, even with a very short transmission line, adding a preamp can improve overall S/N ratio.

This improvement comes about via the gain of the preamp. The preamp raises the signal (actually signal plus preamp & other noise) to high enough level that the tuner noise becomes a smaller fractional contributor to total noise.


Quote:

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.
Yes.

Another topic that also could be quite technical.

If the preamp overloads, it generates harmonics and additional intermodulation (IM) distortion.

Harmonics occur at multiples of fundamental (2x, 3x, etc)

IM involves other products of frequencies (frequencies & frequency multiples added and subtracted.

Depending on frequencies involved, the harmonics and IM may, or may not, interfere with desired signals.

------------------

With typical preamps and typical tuners, the tuner is more likely to be the first to overload.

Similar to preamps, harmonics and IM are generated within the tuner.

Low gain preamps help to avoid overload, but in some (maybe few) cases low gain may not be optimum for S/N improvement.

To optimize, one must know the signals and noise levels throughout and select gain accordingly.

informel 7-May-2014 1:20 AM

I saw some spec on preamp that shows the maximum input for 16 channels so it does not overload, I may want to add a preamp or distribution amp, since I was installing antennas a long time ago, I still have my field strench meter and I measure the level of each incoming channel, do I simply add all level togheter and check with the spec from the manufacturer

ZippyTheChicken 6-Jul-2014 5:08 AM

I recently purchased an antenna off of Amazon for $33
its small about 2.5 x 2.5 feet.. but comes with a rotator that is not accurate and a preamp

It has one cable ~30feet of coax from the antenna to the control box and then two coax outputs to provide signal to two tvs..

the amp could be in the antenna or the control box but i suspect its in the antenna.

If I point it at stations 55 miles away with the amp off I will see maybe 3 channel numbers showing in the scan results ... but no actual visual tv broadcast

If I turn the Amp on I will get over 35 stations with full signal and visual tv... my tv does not display signal strength.

So to say the preAmp won't pull signal out of the air??????? maybe not in a scientific standard.. I am guessing there is some signal being picked up by the antenna that the tv can't find... but turning the amp on and off is basically the same as unpluging the antenna.

informel 8-Jul-2014 5:36 PM

Quote:

Originally Posted by ZippyTheChicken (Post 44756)
I recently purchased an antenna off of Amazon for $33
its small about 2.5 x 2.5 feet.. but comes with a rotator that is not accurate and a preamp

It has one cable ~30feet of coax from the antenna to the control box and then two coax outputs to provide signal to two tvs..

the amp could be in the antenna or the control box but i suspect its in the antenna.

If I point it at stations 55 miles away with the amp off I will see maybe 3 channel numbers showing in the scan results ... but no actual visual tv broadcast

If I turn the Amp on I will get over 35 stations with full signal and visual tv... my tv does not display signal strength.

So to say the preAmp won't pull signal out of the air??????? maybe not in a scientific standard.. I am guessing there is some signal being picked up by the antenna that the tv can't find... but turning the amp on and off is basically the same as unpluging the antenna.

The Antenna is next to the Antenna, when you say that you are turning the amp off, you are in fact disconnecting the power supply to the antenna so you will only receive very strong signal.

In fact a preamp will block most of the signal when not powered. If you could bypass the preamp, you would receive more channels compare to a preamp with no power.

ADTech 8-Jul-2014 8:45 PM

Informel is correct.

An UN-powered amplifier usually behaves like a ~30 dB attenuator. Only those signals that, at the antenna's terminals, has ~30 dB more power than the bare minimum, will make it through the dead amp and be received by the TV set.

ZippyTheChicken 10-Jul-2014 6:18 AM

actually not really because recently i connected the antenna directly to the tv without the amp and there were no visible channels ... then i realized i forgot to connect the amp...

but yeah


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