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Coaxial cable - splitting the signal - best way?


ualdriver's Avatar
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05-17-11, 01:40 PM   #1 (permalink)  
Coaxial cable - splitting the signal - best way?

Hello-

Hopefully I am posting on the correct forum.

I have Comcast cable in the NW suburbs of Chicago if it matters.

I have a main coaxial cable coming into my house. I have 12 TV jacks + 1 cable modem that I want to feed a signal to, but I think I am running into signal strength problems.

Here's what I did. Should I be doing this a better way?

The main cable from Comcast comes into the house. I immediately split it with a splitter (Regal DS2DGH10).

After the splitter I send one cable directly to the cable modem. I want the cable modem to get the "best" signal so I give it that first split. The other split goes to a elabs bidirecitonal RF amplifier*, which was recommended to me to boost the signal for the splitters down the road.

The RF amplifier only has one out, so I have a short length of cable and a two way splitter, with each side feeding another two, 8-way splitters. One splitter is a Channel Vision 8 way splitter** The TV's on that splitter seem to get good signal strength, although at around 7-9pm on some of the network HD channels I get some pixelation and garbled sound, which is what led me to this investigation in the first place.

The second 8 way splitter is transmitting horrible signals to its 4 TV's. The sound is fine but the picture is snowy. I replaced it with a 4 way splitter and the picture is better, but not as good as it could be. I took a sample TV from this 8 way splitter and connected it to a jack on the other 8 way splitter and the picture from that splitter is much better.

So, too many splits? Are my splitters of poor quality? Do I need another amplifier? I am not sure what the best way to feed 12 TV's and one cable modem from one input from Comcast is.

Also, what the heck do the MHz and db numbers mean on these splitters and the RF amplifier??? Is a big number good, bad, or.....? I have no idea what I'm looking at and perhaps I am mismatching stuff?

Any help appreciated. Thanks for your time. Sorry it's long!

*on RF amplifier it says Return (5-40MHz): 0dB, RF Signal (54-1000Mhz): +20db
**on Channel Vision 8 way splitter is says 5Mhz-1Ghz 11db out

 
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DaDruid's Avatar
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05-17-11, 05:47 PM   #2 (permalink)  
As a former cable TV contarctor I will do my best to explain and help where I can. Lets start with the education first. TV signal is typically measured in dB. Comcast's standard is that you should have no less than 0 dB at the first wall plate connected directly to the outside line. On your splitters, the dB number shown is how much dB you loose every time you hook a TV (for example) to it. So, say you have 5 dB coming into the house and you hook a two way splitter (typically 3 dB loss on one leg) you now have 2 dB going to the TV. Now you have two more TV's you want to hook up. Well, you connect another two way splitter to the spare port on the first two-way splitter and you are now loosing 6 dB at TV number two which menas you have -1 dB coming to that TV. With a cable box that is not so bad because box's boost a little before they go to the TV.

Also, I mus not forget to add, that the further you get away from the supply (or comcast input) you also loose signal due to the size of the cable. Typical house wire is RG6. As you move more than 200 ft in distance you need to chnage from RG6 to RG7 and so on. Each cable has a maximum distance for use. Another thing, crimp/twist on fittings are trouble. You want to use compression fitting fi you can find them and the tool. Remember, when doing fittings, you do not want to see any braid sticking out of the back of the fitting, and the whitish/clear center conductor must be flush with the inside bas of the fitting (where you see the stinger. Also your stinger (pin that goes into the TV must be almost even with the threads (it can be a little longer though).

The MHz on your splitters and amplifiers typically refer to the sned/recieve function of them. Will they send/recieve the data when connected in the path of your modem. Typically not a big deal on TV signal.

I hope this has helped some, I know it is long, but a lot to explain on how the TV signal and everything works together.

With that being said, basically having two 8-way splitters is a bad idea. Even with an RF Amplifier, there is not enough signal at the end to support all the TV's. Your modem was done correctly, as this is how I have my modem currently. You can cascade amplifires to boost the the furthest TV's out. I would reccomend try taking a 3-way and changing that out at the Comcast input. Put your modem on the 3 dB leg and it shoudl be fine. Connect one of the amplifiers to the 7 dB leg and another amplifier to the second 7 dB leg and then do 8-ways of those amplifiers and see if it improves. This way you have close to the maximum signal you can coming into the amplifiers and then being boosted back out.

 
ualdriver's Avatar
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05-17-11, 08:28 PM   #3 (permalink)  
Posted By: DaDruid As a former cable TV contarctor I will do my best to explain and help where I can. Lets start with the education first. TV signal is typically measured in dB. Comcast's standard is that you should have no less than 0 dB at the first wall plate connected directly to the outside line. On your splitters, the dB number shown is how much dB you loose every time you hook a TV (for example) to it. So, say you have 5 dB coming into the house and you hook a two way splitter (typically 3 dB loss on one leg) you now have 2 dB going to the TV. Now you have two more TV's you want to hook up. Well, you connect another two way splitter to the spare port on the first two-way splitter and you are now loosing 6 dB at TV number two which menas you have -1 dB coming to that TV. With a cable box that is not so bad because box's boost a little before they go to the TV.

Also, I mus not forget to add, that the further you get away from the supply (or comcast input) you also loose signal due to the size of the cable. Typical house wire is RG6. As you move more than 200 ft in distance you need to chnage from RG6 to RG7 and so on. Each cable has a maximum distance for use. Another thing, crimp/twist on fittings are trouble. You want to use compression fitting fi you can find them and the tool. Remember, when doing fittings, you do not want to see any braid sticking out of the back of the fitting, and the whitish/clear center conductor must be flush with the inside bas of the fitting (where you see the stinger. Also your stinger (pin that goes into the TV must be almost even with the threads (it can be a little longer though).

The MHz on your splitters and amplifiers typically refer to the sned/recieve function of them. Will they send/recieve the data when connected in the path of your modem. Typically not a big deal on TV signal.

I hope this has helped some, I know it is long, but a lot to explain on how the TV signal and everything works together.

With that being said, basically having two 8-way splitters is a bad idea. Even with an RF Amplifier, there is not enough signal at the end to support all the TV's. Your modem was done correctly, as this is how I have my modem currently. You can cascade amplifires to boost the the furthest TV's out. I would reccomend try taking a 3-way and changing that out at the Comcast input. Put your modem on the 3 dB leg and it shoudl be fine. Connect one of the amplifiers to the 7 dB leg and another amplifier to the second 7 dB leg and then do 8-ways of those amplifiers and see if it improves. This way you have close to the maximum signal you can coming into the amplifiers and then being boosted back out.
DaDruid!

Thanks, that was excellent! Can you answer some more questions please?

I get the dB now. I want something >0dB at each TV.....is there anyway I can measure the dB reaching a particular jack in the house? Some sort of tool or something?

How many dB's does Comcast typically have coming into the house on the source cable so I can compute all the dB splits?

After your explanation about the fittings, I have been doing them all wrong. I have been cutting the cable to length and then just using those screw on fittings. And I always left the stinger longer thinking it would make a "better" connection. I need to cut those back even with the threads.....got it.

Now about the splitters. I have a few of them. I have a Monster Cable 8 way splitter that says "OUT -12dB" at each output. Does that mean whatever dB I have going in, each TV on one of the 8 "outs" subtracts 12dB from the input? Another 8 way splitter says 11db and the other says -7.4dB. So I guess the best splitters have the smallest dB number at each output, right (i.e. -12dB is worse than -2dB).

One final question: Can you recommend a good RF amplifier that I should buy? Also some good quality splitters that keep these dB losses to a minimum?

Thanks again for your time.

 
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05-17-11, 09:02 PM   #4 (permalink)  
Here is a useful tutorial for home TV signal distribution: HomeTech Solutions :: Video Distribution Tutorial (RF)

As for the splitter that isn't sending good signal to its loads, I would suspect a faulty coax connector or damaged coax. There might be a nail or a staple penetrating the cable. The cable crimped by a door or window can also cause problems. A simple test would be to swap the two splitters, Also, you aren't doing yourself any favors by using an 8-way splitter to feed only 4 outputs. The added insertion loss for the 8-way over a 4-way is eating a lot of your signal that would be better used going to your TV's.

Pay attention to the method for calculating losses and gains in the system in the tutorial. As you work your way through it, you may discover a problem area you overlooked.

If you need more explanation of terms or concepts, just holler.

 
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05-17-11, 09:50 PM   #5 (permalink)  
Posted By: tldoug Here is a useful tutorial for home TV signal distribution: HomeTech Solutions :: Video Distribution Tutorial (RF)

As for the splitter that isn't sending good signal to its loads, I would suspect a faulty coax connector or damaged coax. There might be a nail or a staple penetrating the cable. The cable crimped by a door or window can also cause problems. A simple test would be to swap the two splitters, Also, you aren't doing yourself any favors by using an 8-way splitter to feed only 4 outputs. The added insertion loss for the 8-way over a 4-way is eating a lot of your signal that would be better used going to your TV's.

Pay attention to the method for calculating losses and gains in the system in the tutorial. As you work your way through it, you may discover a problem area you overlooked.

If you need more explanation of terms or concepts, just holler.
OK, thanks very much tldoug. That will give me something to do tonight and tomorrow

 
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05-18-11, 06:00 AM   #6 (permalink)  
Every doubling of the signal results in a loss of 3dB plus an "insertion loss", which is typically .5dB (1/2 dB).

Splitting from one to two = -3.5dB. A four-way is a double split, so you lose -6.5dB. An 8-way is -11dB. The numbers vary slightly between manufacturers. RG6 cable also has a per-foot insertion loss, and it's worse that the high frequencies than it is at lower frequencies. A 100-foot run, for example, will attenuate 55MHz (channel 2) by 1.5dB. At 1GHz it will attenuate about 6dB. Equalizers are available for long runs, but they are usually not necessary in a typical home.

Your system is wired properly. You need to simply make up the losses created by the insertion of those splitters. At the input, you have 0dB. The first splitter cuts the signal to -3.5dB. The cable modem is fine with that. However, on the TV side, the second (2-way) cuts it by another -3.5dB to a total of -7dB. (8-way) cuts it yet another -11dB to a total of -18dB. You've inserted a +20dB amplifier to compensate, which is on the money. However, any little anomaly in the cabling or connections can drop that perfect signal like a ton of bricks.

Cable modems typically use the "return path" and those frequencies above channel 70 (up to 1,000MHz or 1GHz), so all of the devices in your TV cabling must be rated from 5MHz to 1,000MHz. This includes the connectors. Use only compression-type connectors, never hex crimps or screw-on. Don't bend or kink the cable. If you need to angle it, make sure there's a nice radius in the bend. The white insulator (the "dielectric") inside the cable is easily damaged, and that insulator is critical in maintaining the necessary impedance across the frequency bandwidth.

Meters are available to measure signal strength, but they cost far more than they are worth for a DIYer. The meter I use was $2,600.

 
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05-18-11, 09:25 AM   #7 (permalink)  
Posted By: Rick Johnston Every doubling of the signal results in a loss of 3dB plus an "insertion loss", which is typically .5dB (1/2 dB).

Splitting from one to two = -3.5dB. A four-way is a double split, so you lose -6.5dB. An 8-way is -11dB. The numbers vary slightly between manufacturers. RG6 cable also has a per-foot insertion loss, and it's worse that the high frequencies than it is at lower frequencies. A 100-foot run, for example, will attenuate 55MHz (channel 2) by 1.5dB. At 1GHz it will attenuate about 6dB. Equalizers are available for long runs, but they are usually not necessary in a typical home.

Your system is wired properly. You need to simply make up the losses created by the insertion of those splitters. At the input, you have 0dB. The first splitter cuts the signal to -3.5dB. The cable modem is fine with that. However, on the TV side, the second (2-way) cuts it by another -3.5dB to a total of -7dB. (8-way) cuts it yet another -11dB to a total of -18dB. You've inserted a +20dB amplifier to compensate, which is on the money. However, any little anomaly in the cabling or connections can drop that perfect signal like a ton of bricks.

Cable modems typically use the "return path" and those frequencies above channel 70 (up to 1,000MHz or 1GHz), so all of the devices in your TV cabling must be rated from 5MHz to 1,000MHz. This includes the connectors. Use only compression-type connectors, never hex crimps or screw-on. Don't bend or kink the cable. If you need to angle it, make sure there's a nice radius in the bend. The white insulator (the "dielectric") inside the cable is easily damaged, and that insulator is critical in maintaining the necessary impedance across the frequency bandwidth.

Meters are available to measure signal strength, but they cost far more than they are worth for a DIYer. The meter I use was $2,600.
Wow, two grand for a meter! I guess I will not be getting one of those!

These crimp on ends vs the screw ons......I think I have only seen the screw ons before. Do I need a special tool for the crimp ons? Are they available at the local Home Depot?

So the theme I seem to be getting is that if I am going to split, I need at least another amplifier. Can anyone recommend a good one, or does it matter?

 
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05-18-11, 10:24 AM   #8 (permalink)  
Yes...HD will have them, but they might be more expensive than possibly an electrical supply house or online sources. You need to get the right connectors for the type of cable you are using. The tools aren't cheap, but they won't break the bank for the basic models...the fancier ones are about $60 retail as I remember


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05-19-11, 04:43 AM   #9 (permalink)  
HD has a decent compression-connector tool for around $20. A coaxial cable stripper is also a good idea. They make life easy and won't damage the dielectric or shield. They're around $10. Again, you don't want to use hex crimp connectors.

Before you spring for another amp, redo all of the cables. The first thing I'd do is swap the cables going to the 8-way splitters to see if that makes a difference.

 
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05-19-11, 09:25 AM   #10 (permalink)  
All through Rick:

Thanks guys. I'll look for that tool during one of my frequent runs to HD. I already bought another amp!

 
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