Signal Amplifiers (Amps, Preamps, Distro Amps) - See Chart in Post #1

[majortom]

DTV Cascaded Noise Figure


Was tinkering with this online version again to convert to a Google Spreadheet Version,
and in the process made a few changes, highlighted below.

-Tryin to make it as customizable as possible without comprimising too much
formatting wise so it's readable online in a browser, and ur not scrollin all over the place, etc.
-Modified the default pre-amp gain to that of a winegard AP-8700 in uhf (what I have here).
-Modified to use a 4-way splitter (again no good reason other than that's what I have here)
-Trying to account for scenarios people may run in to.
The default values I think most closely resemble what I have here.
-decoupled the net comparisons between scenarios, so they'll change as the
user changes inputs.

Scenario Assumptions.

Scenario 1: Antenna->Balun->Downlead->Splitter->Coax Drop->Receiver
Scenario 2: Antenna->Balun->JMPR->Preamp->Downlead->Splitter->Coax Drop->Receiver
Scenario 3: Antenna->Balun->Downlead->Splitter->JMPR->Preamp->Coax Drop->Receiver-> (not likely anyone would connect this way, but just to illustrate why we'd want to put the pre-amp ahead of a splitter)
Scenario 4: Antenna->Balun->Downlead->Preamp->JMPR->Splitter->Coax Drop->Receiver

- In a future version, gonna try and take in to account the IP3 values in the Holl-Ands modified Pre-amp chart of Post#1, so people will be able to play around with the scenarios and see the tradeoffs between system noise figure and IP3.

Any other suggestions?

[threeflags]

I know that the typical installation is to put the pre-amp a few feet for the antenna. However I was thinking, for long cable runs, would using a second pre-amp make the signal stronger? Thanks.

[NineBall]

I would imagine that the 1st pre-amp would block the DC from the 2nd one negating it's use. You could use both a pre-amp and an amplified splitter with DC passthrough however.

Edit: That's all assuming that you actually need that much amplification. It would require really long runs and multiple splits to really justify this. Low initial signal strength at the antenna can often be overcome by just using a good, high-gain antenna in the first place with proper aiming and then a pre-amp if required.

[iblackford]

Many use distribution amps along with preamps in their systems, however, the 2nd amp can easily overload if it receives too much signal.

What are you trying to achieve?

Ivan

[stampeder]

That question has come up in this thread a few times in the past, and the concerns are the DC power (as mentioned by NineBall) and also the issue of amplifying noise beyond acceptable amounts. As iblackford mentions, the question of whether the 2nd preamp can handle the load of the first one is always an issue.

[wilspin]

I use a second amp that is variable. I had to trap some offending signals to make it work well. Also MT what is JMPR? Not sure if I understand what the benchmark is, scenario 1 system degraded -12.5 compared to what? So can I conclude scenario 2 is best? Nice work!

[majortom]

Wilspin,

Also MT what is JMPR? Not sure if I understand what the benchmark is, scenario 1 system degraded -12.5 compared to what? So can I conclude scenario 2 is best?

Sorry, my bad. JMPR -> short for Jumper, i.e. a short jumper cable.
by "benchmark" just showing mathematically Scenario 1's overall system Noise Figure is degraded by that 12.5 dB (cable / splitter losses, etc), relative to the noise figure of the Tuner itself(i.e if ur antenna was right next to ur TV, system noise figure would essentially be the TV's Noise figure, eh) . We're trying to make the crappy noise figure of our tuners, which we can't control (is what it is), insignificant when we install an LNA. So yep, Scenario 2, as you already knew, but there u can see why, mathematically.

If you really wanted to, you could modify one of the scenarios, say scenario 2, and replace the Losses after the splitter or somethin with ur variable 2nd AMP that u mentioned above. It shouldn't change things much, Noise Figure wise.

[threeflags]

Originally Posted by iblackford

Many use distribution amps along with preamps in their systems, however, the 2nd amp can easily overload if it receives too much signal.

What are you trying to achieve?

Ivan

Just wondering about this scenario. I just picked up a CM 7777 as it has the power injector feature which is more convenient over a Phillips All in One type that needs an outlet close to the amp. Thanks for all of the replies.

[wilspin]

Good choice 3flags try is as is see results before adding xtra amp.

[wilspin]

So I was concerned about signal loss and splitters on the weekend. Watching Fox Indy and it would break up periodically. The Signal would fall below 20%. I figured I could improve it by removing the 4x splitter and feeding only one TV. Much to my surprise signal was zero on all channels. What did I experience here?

[majortom]

Not sure without more detail, like how were all the other stations, ANA & DIG behaving at the same time. U mentioned u use a 2nd one but adjustable, did ya try removing that guy also? Did u inadvertantly remove the dc bias to ur pre-amp in the process?

[wilspin]

Here is the flow; ant /cm7777/wg200amp/tv. 4x split was on end of WG200. I did not try all channels but many, all were zero. Panicked, Put split back in, all was normal, watch TV. May try again when I have more time.

[majortom]

I guess what I meant was, while FOX (assuming WUTV UHF 14) was falling off, making ya wanna change something, what were all the other stations doing? Before ya changed anything.

[flavoie]

I think you are Overloading your tuner with too much signal
I suggest you try a variable attenuator to tweak the amount of attenuation needed before your tuner. You may find even more attenuation is better or worse than with your 7.5db splitter. This will confirm why you needed the 4:1 splitter attenuation, and even may tell you to add another splitter inline
After all I see you seem to be cascading 2 amps which can be a lot to handle without attenuator or splitters

[PanaMark]

Don't make the mistake I have made. I thought that I needed the same setup you have ie: pre amp then distro amp. Likely all you probably need is just the preamp. Fringe signals are just that, fringe.
If drop outs occur it is because there just is not enough signal making it to the antenna. No amount of amplification can help that.
Remember that the television's AGC (Automatic Gain Circuit) will be affected from overload.