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Discussion Starter · #1 ·
I read some time ago where someone was using two preamps to boost signal up to improve weak signals.The reason I ask there's two LP stations I like to watch at 50 plus miles,but they are rather snowy(with color).I have the amps but not sure there will be any detrimental effects to any of the equipment.


Any thoughts?
 

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It probably won't do any good, and probably will cause problems.


The first amplifier in the chain sets the noise figure of the system, the second one will amplify both the signal and the noise of the first amplifier. Placing 2 amps in cascade will probably cause the second one to overload, placing intermodulation components into the channels you are trying to receive (overload).


The only place 2 amps would be useful is if you are going through a long enough piece of coax cable to where an amplifier might enhance the noise figure (on the order of hundreds of feet of RG-6).


If you have 2 amps, the best bet would be to place the one with the lowest noise figure at the antenna, if you don't know the noise figures, experiment with them to find the one that gives the best picture.


Bob Smith
 

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Stay away from two preamps in series! That's really asking for trouble.


You can achieve more gain by switching to a better antenna, going to lower loss feedline, and using a low-noise preamp with good gain.


Generally, improvements to the antenna and feedline yield the most bang for the buck.


KC
 

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RF preamps are typically mis-understood. They cannot boost weak signals from the antenna. They only compensate for cable and splitter loss. The signal to noise ratio is determined at the antenna. If there is insuffiecent voltage on the antenna, what are you going to amplify, noise?


Here is an example. It takes a 6 foot or larger dish to get a signal from a C band satellite because it's power output is only 5 watts. DBS satellites have several hubndred watts output and thus can be received with only an 18" dish. If preamps were a solution, why aren't C band dishes 18" too with just a bigger preamp?


Ah, but two or more antennas in parallel will boost distant signals. However to do this is a bit complex. The antennas must be phased or mulipath will result. Ham radio operators have been using this trick since the 1920's.
 

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Quote:
Originally posted by Glimmie:

It takes a 6 foot or larger dish to get a signal from a C band satellite because it's power output is only 5 watts. DBS satellites have several hundred watts output and thus can be received with only an 18" dish. If preamps were a solution, why aren't C band dishes 18" too with just a bigger preamp?
While ultra-low wattage used to be the norm, most C band satellites today have power output in the 10-30 watt range. Still, not nearly the output of most DTH Ku birds. However, part of that large output is required to minimize environmental effects (moisture, etc.) to which the shorter wavelength of the Ku band is significantly more susceptible than is C band. It also affords the use of smaller dishes for most areas which are cheaper and cosmetically more appealing.


The three largest contributors to C band dish size are localized EIRP, satellite adjacency, and dish sidelobes. Effective Isotropic Radiated Power (EIRP) is a measure of power that a satellite directs toward its boresite on the Earth. One's proximity with respect to the boresite will impact the original signal strength with which the on-dish electronics can work (similar to proximity to an OTA transmitter). This holds true for DTH birds as well.


However, because C band satellites are frequently located adjacent to one another (2* spacing) and use the same frequencies, their signals can conflict because of antenna sidelobes and pointing errors. This, despite alternating polarity to minimize such interference. The smaller the dish, the harder it becomes to reject adjacent satellite interference. I have heard of 4' dishes successfully used to receive the NASA channel on GE-2/9. However, GE-2 is as strong as one of its neighbors and the other neighbor is vacant on that frequency. When you have a weak bird sandwiched between two strong birds it becomes difficult to defeat adjacent interference with a small, or poorly designed, antenna. This is exacerbated when the EIRP is moderate to poor.


I have used line amplifiers to mitigate line loss (in one case I run over 200'). As Glimmie points out, if I wanted to boost the signal, I would have to collect more signal from the source (either a larger antenna or a complex computer-controlled multi-dish configuration - not to mention expensive and divorce prone).



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Discussion Starter · #6 ·
Thanks to all of you on your well informed suggestions!


When I get the Parascope back on the tower next week I'll see if that helps.I'd try stacked arrays but I don't think the tower would stand the extra "real estate".


BTW,Does anyone know where I can find good data on screening a Parascope? Like beamwidth,side-lobe rejection,forward gain and front-to-back ratio figures in real db terms.


Has anyone tried this?


Thanks!

 

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Quote:
Originally posted by Glimmie:
RF preamps are typically mis-understood. They cannot boost weak signals from the antenna. They only compensate for cable and splitter loss. The signal to noise ratio is determined at the antenna. If there is insuffiecent voltage on the antenna, what are you going to amplify, noise?

Technically it's true that the SNR due to thermal noise is determined by the antenna gain, but you may still get a better 'effective' SNR with a preamp. How? If the preamp you are using has a better noise figure than the tuner in your STB or TV and has sufficient gain to swamp out the noise figure of the tuner then you will get a better SNR at your demod IC. Typically tv tuners range from about 6-10dB NF while you can get preamps with as little as 3dB NF.


So if you're hoping for more than a 3-7dB gain in what shows up on your TV screen you're going to need a higher gain antenna (or higher location) 'cause a preamp can't get you more than that (the noise figure difference between the preamp and tuner) regardless of how much gain it has.


 

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MAXHD,


I use 2 preamps in series. If you want to try it you first need to get a signal meter. Also you will likely need good filters, as I have added to select just the channel you want to amplify. The gain from doing all this is very slight. It works for me because I am on the edge of a cliff signal wise, and my tuner is not very sensitive.


I see a lot of comments about signal to noise. I don't believe this is the whole story here. Signals can come in with a good signal to noise ratio and yet still work better slightly if made stronger. I think this is due to equalizer performance and multipath issues, more than anything else. The tuners seem to have a preferred signal level for best equalizer performance. This may not be true for all tuners. And this issue would only apply to digital tuner and stations.


As to screening the Parascope, I measured about a 3 db increase in gain and F/B ratio with mine. I used 1/4 inch Aluminum mesh.


And for sure, I would love to take out my second preamp, and get a better antenna instead, but so far I haven't found one.


 

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Sorry, but it's all about the noise figure of the first amplifier. Noise figure is the excess noise an amplifier creates relative to a resistor at room temperature. A perfect amplifier would create no noise of its own, only gain. Gain with a high noise figure is worse than no gain at all. I first discovered this as a 10 year old kid, someone gave me an old Blonder Tongue MTV amplifier with about 8 tubes in it and 40 dB of gain. Try as I might, all I could do was make the signal from my conical antenna snowier.


A preamplifier helps if the combination of the noise figure of the tuner plus the attenuation of the deadline and any splitters in the line is higher than the noise figure of the preamplifier, and the preamplifier has sufficient gain to overcome the cable/splitter loss.


A directly in front of the receiver will help sensitivity only in the difference between the receiver's noise figure (about 10 dB), and that of the preamplifier (about 2.0 dB for a really good one). In this case, you will achieve an 8 dB improvement in ultimate signal to noise ratio.


This must all be weighed against the possibility the amplifier will overload and cause spurious intermodulation components to fall upon the desired signal and cause a lower SNR than would be achieved without the amplifier. Low level/low noise figure amplifiers are optimized for that purpose and are usually not very good in high signal environments. This is why cascading 2 identical amplifiers is usually not a very good idea. The time 2 amplifiers would help is in the case where the first one sets the noise figure, and has enough gain to overcome the noise figure of the second amplifier which has the necessary power output to drive multiple receivers/long feedlines. In MATV systems this is done with an antenna mounted preamp (low noise) along with a distribution amplifier (high output).


Antennas, btw, give free/0 dB noise figure gain. Every dB of gain out of a terrestrial antenna is real gain and will be realized as an increase in SNR at the receiver. There are exceptions to this for satellite antennas, since the design of the antenna may receive noise from the warmer earth, but not for terrestrial antennas.


Amplifiers should be thought of as part of a system, not as a magic bullet to pull signals out of thin air.


Bob Smith
 
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