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Signal Booster (RF Amplifier) question

578 Views 10 Replies 5 Participants Last post by  tommyy2
I'm splitting the incoming cable 4 ways right now (3 Replays and a cable box), and I'm noticing a light static on the signal.

I have at my disposal 15db, 18db and 20db RF amps (or signal boosters, whatever you want to call them), as well as a 35db unit(with 20 db adjustment, so it can go from 15-35db) but I don't want to open the sealed boxes unless it's right for me (I'm one of those picky people who won't resell an open box as new :))

Anyway, is there a cheap and easy way for me to tell exactly what level of amplification I need? I've heard that too much boost can ruin a picture as much as too little.
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I usually just want to shoot for enough boost to cancel out what I'm losing to the split.

A lossless splitter would degrade the signal about 3.0103 db at each of the two taps. People generally call this 3.5 db, probably to account for the loss due to the splitter (rather than the split).

That's for a 2-way split. a 4-way split should double that - figure 7 db loss at each of the 4 taps.

Assuming there isn't too much cable between the amp and where each tap terminates, I'd think that 10 db should be plenty for a no-loss 4-way split. Naturally, each foot of cable adds to the loss. Figure out what kind of cable you're using and see if the manufacturer is kind enough to provide loss/meter figures at various frequencies (the higher the freq, the higher the loss).

The general rule of thumb is to put the amp as close to the original signal source (that is, your cable company) as possible, and don't use more gain than you'll need due to losses inherent in your distribution system.

Another obvious question is what the bandwidth of the AMP is (that is, what frequencies it really amplifies), and how much gain you get at any given frequency.

Then there's the whole issue of tilt compensation, but it seems a bit more involved than I feel like right now.

Basically, if the split signal doesn't look too bad, then too much gain will hurt. Personally, I can't see any reasonable use for a 35 db amp unless you're dealing with a marginal signal to begin with, and you have a small motel you're feeding.

Right now, I'm using the much-touted (by me) ChannelVision 4-way dist amp that as far as I can tell provides 15 db of gain, and it shows signs of having too much gain. I probably should sprung for the 8-way 15 db amp and just terminated the unused taps, just to cut down on the gain.
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Originally posted by toots
a 4-way split should double that - figure 7 db loss at each of the 4 taps.
Using this number, (and don't worry about cable length, the Incoming cable sits maybe 4 feet from the Replays!), what strength of amplifier would you reccomend? 7db gain? or 7 x 4 = 28db?
7 db in a perfect world. More reasonably, 10 db, but I guess 15 is the minimum.

How to know if you're getting too much gain:

1) Someone mentioned herringbone pictures. Sure, I've seen that on overamped or poorly amped feeds

2) Loss of sync in excessively white pictures

3) Text overlays (like ads) cause the audio to buzz

Among others.
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Thanks for the quick reply! Sounds like my 15db will do the tricl then. One last quick question - if it's a bit TOO strong, can you put any sort of trap or something before the amp to lessen the incoming signal by 3-5 db or something? Just to take the edge off? This (if they exist) would seem to let us fine tune a bit more.
Ah, thanks to the wonders of e-mail notification of replies...

You can buy attenuators to put inline to cut some of the gain. I think I'd prefer to put the attenuators after the amp, but there are arguments to be made on both sides, particularly depending on the quality of the signal coming in.

My experience with distribution amps, or RF amps of any sort, has generally been pretty bad, with only one bright success point.

To me, the quality of the amp is what's most important, but holding all other things equal, too much gain often results in a signal that looks worse than before you started.

smarthome.com have a bunch of good toys in this area, albeit not very economically priced.

The neighborhood electronics vendor who shall remain nameless, but for purposes of this discussion, we'll call "Radio Stuff" sells distribution amps that I find absolutely execrable.
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The common figures of 3.5 dB of loss for a 2-way splitter and 7.5 dB of loss for a 4-way are based on old 400 MHz cable systems. While the ratings still remain, modern cable systems with 100 or more channels plus digital channels use frequencies of 750 to 1 GHz or more, making loss at the highest frequency a bit greater (8 or 9 dB) for a 4-way splitter. Since RG-6 cable only loses about 3 dB per 100 ft. at channel 13, unless you have 75 ft or more connected in after your ground block it should make little difference.

Not only that, but not all splitters are created equal. I have in my junk box 4-ways that are rated at 500 MHz, 900 MHz, 1.5 GHz, and 2.5 GHz. Sometimes (but not always) those with a lower frequency rating will drop off sharply on the higher channels.

I would first check that you have a splitter that can carry the full bandwidth of your cable system. If you don't, you'll see increasing snow on higher channels. If you buy one rated for satellite, that should do the job, as they will pass L-band (950-1.5 GHz) and very likely everything below that.

I would also experiment with what the picture looks like directly (no splitter) and with the splitter in place. If there is degradation on the lower channels you are likely a good candidate for an amp. Check 2-6, 7-13, and probably the 70's to 80's (the 90's can be misleading because most of them are actually between channel 6 and channel 14).

Radio Shack makes some very good amps. My favorite is the one that has a pot to vary the level from +15 to +25. They are usually rated from 5 MHz to 950 MHz and are surprisingly clean. You can also usually return them if they don't help. If it actually makes the picture worse, try a 6 dB pad or so (RS carries these as well) just ahead of the amp, as that is an indication that you may be overdriving the front end.
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You know, "surprisingly clean" for RS is still pretty dirty. I've never seen an RS dist amp that didn't crap on the signal more than it boosted it.

Your other information is very good. I just don't trust any cheap single-band RS amp to even play in the same neighborhood as "useful."
You did it Ms. 1000 (or Dr. 1000 to me). Congrats! I hope this doesn't mean you're going to stop posting again!

And I agree with your comments about RS amps as well; since the Replays are so sensitive to the quality of the signal, I've found that I had to use an expensive amp (in my case, a channelvision) for these purposes or the amp does more harm than good.
I'm going to pick up the Radio Shack 15-1113, which is adjustable with an FM trap (which I've heard can clear up static) from 10-20db. This should at least let me know if 15 db isn't strong enough (or way too strong). It's $25, and I plan on returning it unless it produces a crystal clear image (unlikely, but who the heck knows).
I tried two different variable amps (1- radio shack and 1-??) and neither of them was worth snit. I varied them from min to max, filter out/in while watching the picture as it went from very bad to horrible (noise-snow etc). Both amps returned. I tried non variable radio shack amp (10db ??) and picture looked pretty good (big improvement), tried Philips 12db amp (walmart-??) and it looked even better (radio shack went back).

I use ATT cable, expanded basic (was TCI) to 4 cable ready tv's (two 20 inchers, one 20 year old nec 26 inch, one sony 36 inch xbr)and 3 replays. The cable feed is into a four way splitter outside the house then outside wiring to two bedrooms, an exercise room, amd a rec room. In the bedrooms and the rec room the feed is then split 2 to 1 between the tvs and the replays. I drilled some more holes and ran the main feed into my house through the amp then back out and into the four way splitter.

The picture with original TCI setup (well before replay #1) was very bad; had them out several times. Basic / expanded basic stations that were tuned by their box had grain bigger than Kansas (HORRIBLE), I found that by using the tuner in the tv (xbr) picture was drastically better, the other sets were also much improved; some stations i/e 95 was pretty good 3 was still poor but greatly improved over their box and the rest were between these two in appearance. The digital stations (premiums) all had soft focus, some better many worse (none looked as sharp as 95 on the xbr using the tv's tuner), plus regular pixilation problems on many digital stations often occurred (overcompression ??). TCI replaced their box and god knows what else but they never got it looking good (I may have been more critical after getting the 36 inch set).

Eventually I dropped the digital service, and just kept their expanded basic service. Got dish network for premiums only (in rec room and 1 bedroom), and got drastic improvement over what tci digital cable premiums had been. The basic stations stayed pretty good to poor.

Then I started with replay (and splitting the signals further), picture got below my tolerable levels again so I started trying

to amplify it. I never got one of those expensive amp/splitter combinations because the splitter is outside in the rain/snow/etc. Of the four amps I tried, both of the fixed amplification ones produced very big improvement, both of the variable ones were so bad I wonder if their variable nature is introducing a problems or if they were both just bad.

Anyway with my present "el cheapo" 12 db amp and 7 drops the picture is much better than it had ever been, it's also better than any of my neighbors. Channel 95 and many others are excellent(amazingly so), most are very good but channel 3 (cbs) and a few others could still use a little improvement. Perhaps if I find a returnable 15 db amp with no shipping charges (local purchase) I'll try another.

Anyway, try one, try a bunch, I think it's all FM anyway.

Good luck

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