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Supervising coax cable connections?


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#1 Quixote_1

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Posted 15 April 2018 - 02:56 AM

Hi,

I have a couple of questions that I can't seem to answer on my own by searching online, so I thought I'd pose them here to see if anyone can offer me answers, or a nudge in the right direction.

 

1: I have coax cable entering my dwelling from 2 points for internet, one at the front and one at the back. Supposedly (according to the technician that installed the second wire) they go to the same place on the pole. Is there any reason I can't connect both of them to a reversed Y-adapter so that if one wire is severed, the other will continue to allow my router to remain connected uninterrupted?

 

2: Does there exist a device or a way to rig up something to monitor coax connections for tampering without interfering with the signal that's being transmitted over that connection? I'd like to be able to use a couple of zones on my Elk-M1G so that if any potential thieves try to cut off communication from outside, my system will recognize that and set off the alarm, then send me a text.

 

Note: I am not looking to install a cellular solution since I don't want to pay an additional fee every month.

 

Thanks in advance for any pointers.

 

 



#2 cobra

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Posted 15 April 2018 - 08:29 AM

2: Does there exist a device or a way to rig up something to monitor coax connections for tampering without interfering with the signal that's being transmitted over that connection? I'd like to be able to use a couple of zones on my Elk-M1G so that if any potential thieves try to cut off communication from outside, my system will recognize that and set off the alarm, then send me a text.

Maybe start with this question...  On coax, it's basically connected or not.  There's not much to 'tampering'.  I can't imagine you can detect anything other than the line is broken or it's not broken.  Usually with alarm systems, this is why you have a backup communicator.  Once the line is broken, you can't send a text message to notify you over that line.

 

Why do you have a second line?  If it is a backup in case one is cut, then you will need some sort of equipment on each line that can tell you when it goes inactive.  This may be more expensive than just adding a secondary cellular communicator.  Who is the internet provider?


Edited by cobra, 15 April 2018 - 08:30 AM.


#3 Quixote_1

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Posted 15 April 2018 - 08:54 AM

...Once the line is broken, you can't send a text message to notify you over that line.

 
Precisely why I want to have two lines connected and monitor them. ;)
The main problem I understand is that these lines are not like a home electrical circuit where you could simply slap in a nice heavy-duty relay and wire the NC connections to a zone on the system. The other problem is that there will be two lines in play.
 

Why do you have a second line?  If it is a backup in case one is cut, then you will need some sort of equipment on each line that can tell you when it goes inactive.  This may be more expensive than just adding a secondary cellular communicator.  Who is the internet provider?

 

The second line was installed when I moved in here so that I could set up my modem and router in my bedroom where I initially decided they would be best located. The original line came in at the other end of the apartment. I've since decided that I'd like to set up a wiring closet that is around the middle of both entry points of the wires and extend the wires to that closet, then run Cat5e, etc. from there to the other spots I need it. I want to set up both wires to be connected in unison because I've noticed that at least one of them could be easily severed from a location that would be relatively discreet.

The internet provider is a third-party company that sells bandwidth from the main local cable company, so the wiring basically belongs to the cable company.



#4 RAL

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Posted 15 April 2018 - 11:13 AM

The two lines almost certainly don't go to the same place on the pole.  Rather, they go to separate taps on a splitter or to separate amplifier outputs.

 

I think if you try and merge them back into a single line, you'll cause problems with the signal.  The cable has signals that are in the GHz range, meaning data pulse widths are around 1 nanosecond.  Electrical signals travel on copper wire at a speed of about 1 ns per foot.  So unless the lengths of coax are precisely matched, they will be out of phase with each other, causing interference.



#5 Quixote_1

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Posted 15 April 2018 - 11:26 AM

Makes sense.
Now I'm wondering if there are any clever workarounds, like using a modem for each cable, then connecting them to some sort of switch before the router.
If that's even possible, I could then look at testing their connections through software rather than hardware. More complicated than I'd like, but I can't think of a better way to avoid using a cellular connection.

Edit:
Maybe I can use "Moca" hardware with a network switch?


Edited by Quixote_1, 15 April 2018 - 12:26 PM.


#6 RAL

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Posted 15 April 2018 - 01:56 PM

If you had two cable modems, then you might be able to make that work.  You would also need a router capable of handling dual WAN connections and failing over from one to the other.

 

But the router makes the failover transparent downstream.  So if one cable was cut, you might never know, unless the router is able to send out notifications of the failover.

 

Also, your cable company will probably charge you a significant amount for having two modems.   It might just be less expensive to go with a cellular solution and have a more robust backup.



#7 Quixote_1

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Posted 15 April 2018 - 02:08 PM

Hmmm.  This is sounding less and less feasible. I really need to avoid extra monthly fees.

Back to the drawing board, I guess.

Thanks for your help.


Edited by Quixote_1, 15 April 2018 - 03:10 PM.


#8 BraveSirRobbin

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Posted 15 April 2018 - 09:10 PM

Not saying this will solve your problem as you still have to perform the detection as was mentioned above, buy FYI, there are coaxial relays (75 ohm in your case).



#9 Quixote_1

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Posted 15 April 2018 - 09:17 PM

Very cool! Not cheap, but knowing that these exist may make it easier to come up with a concept.

These won't cause any interference on the line?

Thanks very much for the tip.



#10 BLH

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Posted 16 April 2018 - 05:33 AM

I suspect. Both coax lines go to a separate drop connection on the cable network at the same drop interface location.

The relay pointed out. Grounds the coaxial cable not selected. Not sure how that would effect the system.



#11 upstatemike

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Posted 16 April 2018 - 08:22 AM

My favorite source for coax relays is old junk VCRs. They used to have a relay to switch the RF output between the antenna/cable and the internal channel 3/4 modulator.



#12 Work2Play

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Posted 16 April 2018 - 11:15 AM

This whole concept sounds like a fantasy that doesn't make a whole lot of sense, but it sounds fun anyway.  What about a continuity test between the shielding of the two coax wire shields?  If the continuity drops to zero, one of the lines was cut.  Even couple that with the relay to try switching to the other?



#13 Quixote_1

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Posted 16 April 2018 - 08:38 PM

My favorite source for coax relays is old junk VCRs. They used to have a relay to switch the RF output between the antenna/cable and the internal channel 3/4 modulator.

 

This could be super useful information to me, so thank you. I'll keep my eyes open for people tossing them out on garbage day and pull one apart at some point.

 

This whole concept sounds like a fantasy that doesn't make a whole lot of sense, but it sounds fun anyway.  What about a continuity test between the shielding of the two coax wire shields?  If the continuity drops to zero, one of the lines was cut.  Even couple that with the relay to try switching to the other?

 

I agree that this is not a very conventional way of doing things, but I'm sure that most of the stuff we were doing back before Google, Amazon, et al got involved in home automation was considered fantasy and nonsensical by most. The continuity test sounds like another great idea for tampering detection. The only thing that I could see being problematic is that it seems that I'll have to be switching from one cable to another with a relay based on which coax is still connected. I don't think I can do that with continuity. I'm not very knowledgeable about electronics, but I guess I could be testing each one to see if the shield is grounded? Of course, that could be a stupid idea since it may be grounded through the modem even if the cable is severed. :wacko:






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