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My office is a converted garage about 30 feet from my house.

I have a coaxial cable already running from my house to the office.

I want to attach a Ethernet cable from my DSL modem, in my office, to a computer in my house.

Since I already have the coaxial cable installed, I bought two coax to Ethernet converters (NVA-P-860-BP78 Video Balun), but when I hooked them up I am getting no signal. It was my understanding that an Ethernet signal could travel over a coaxial cable.

Is that wrong? Any ideas why this isn't working?

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    "The purpose of a passive video balun is to send a video signal and power through a Cat5e cable, which can be transmitted up to 1000 ft in length" - from the description of your referred converter. It does a different conversion than you need. – Máté Juhász Feb 1 '17 at 21:21
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    Using wifi is good enough in most cases instead of buying adapters and/or rewiring. Wifi ac can be comparable or better than Gigabit ethernet – phuclv Feb 2 '17 at 6:07
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    If you have a modern house, consider PowerLine. It doesn't work as well everywhere but it's very low cost and a lot easier than messing with coax (I know, I once had a coax network). Problem with coax is that it's unwieldy, expensive and no one uses it. So you are on your own pretty much. – Stijn de Witt Feb 2 '17 at 16:14
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    @LưuVĩnhPhúc, "_Wifi ac can be comparable or better than Gigabit ethernet _" Not for a single host. In aggregate, and under perfect conditions, maybe. In real life, hardly. Also, Wi-Fi is half duplex. – Ron Maupin Feb 2 '17 at 18:23
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    @RonMaupin I said it "can be", not that it is. However wifi is enough for most applications except file sharing, so unless speed is really critical then investing for ethernet might not worth it. – phuclv Feb 3 '17 at 0:59
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You bought the wrong thing.
These converters convert analog video-signal (on coax) to UTP (unshielded twisted pair) so you can forward the analog signal from a CCTV camera via existing UTP network cable to somewhere else and there convert it back to analog video to display on a monitor or feed it to a VHS.
camera <-> coax <-> UTP <-> coax <-> monitor
You can't use them in reverse to do: PC <-> UTP <-> coax <-> UTP <-> router.

Ether does run over coax. Actually it started there before we had UTP cables.
But for that you needed ethernet cards with a coax plug (and some other stuff as well, it isn't a plain point-to-point connection). And anyway it used a different type of coax than television does. Television coax won't work with these old network cards.
These things went the way of the dodo around the year 2000 and good riddance too. It was very slow compared to modern network technology.

What you really need is a MoCA (Multimedia over Coax Alliance) converter.
See this page for certified equipment and vendors: http://www.mocalliance.org/products/index.htm

Be advised that MoCA stuff is probably more expensive then pulling a CAT5E or CAT6 cable to the garage. (If the existing coax is in a pipe you may be able to attach an UTP cable to one end and use the coax to pull the UTP through the pipe.)

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    Note that if you use the coax to pull new CAT cable through, you can also pull a string and then pull the coax back, if you still need it. – StayOnTarget Feb 1 '17 at 21:40
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    To be more specific: Ethernet over coax used 10base2 (10Mbit, shared-medium), over cables with 50 ohm impedance and BNC connectors. Video coax cables are 75 ohm impedance, IIRC, and definitely use a different connector. You could probably still find a 10base2 PCI card, but I totally agree that would be a silly thing to invest time and money into. – Peter Cordes Feb 2 '17 at 3:00
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    There are converters which allow you to extend 1 Gbit ethernet over coax. But a quick search on amazon suggests, it might be cheaper to replace the cable. – Josef says Reinstate Monica Feb 2 '17 at 8:55
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    Bear in mind the converters will still also probably need better-grade coax than just something that happened to already be running, since that'll probably be bargain-basement brown TV coax put in by builders for the cheapest price possible. I doubt ethernet would run very well over that. Only chance of better quality coax is if it were put in for satellite, or by someone who doesn't spare expense. I highly recommend you either run CAT5E/CAT6, or look for a wireless solution. – Muzer Feb 2 '17 at 10:20
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    @PeterCordes: There was 10base5 too. But it is even less likely that the OP has 10base5 coax installed :-) – Martin Bonner supports Monica Feb 2 '17 at 13:06
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Presumably you have power in the garage (or why have network connectivity out there) so maybe powerline adaptors might be worth looking at. It won't use the coax but is that mandatory?

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    powerline adapters are often much more expensive and slower than modern wifi and ethernet devices – phuclv Feb 2 '17 at 12:00
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    Much more expensive is relative. You can get them for 30-40 bucks or so? Order a pair online and plug them in... if it works right those are the best 40 bucks you ever spent... (and you probably can't beat it if you count your own time at minimum wage). If they don't work right... ah well try other rooms they may come in handy somewhere else. I used them with moderate success (in fact, this post is going a short segment over powerline before making it to the SO servers). – Stijn de Witt Feb 2 '17 at 16:19
  • @StijndeWitt minimum wage is not the same everywhere. 40$ might be half a month's (or more) salary in many places, although probably not in the OP's. With the cost for a pair of low end 200-300Mbps powerline adapters you can buy a high-end wifi ac AP (just checked and found out they're cheap on amazon but again not the same everywhere). Moreover powerline apdaters can't work over circuit breakers, which an external garage might have – phuclv Feb 3 '17 at 1:14
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    I've had mixed results with powerline adapters. Sometimes they don't work if the rooms or sides of the house are on different rings/circuits. There's a good chance the garage will be on a different one to the house. – Matthew Lock Feb 3 '17 at 3:19
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A better answer may be to investigate using a WiFi router in your office and just use WiFi to connect up within your house. 30 feet really isn't very far for modern WiFi routers (especially if you get one that's high power) even through walls and structure. May be a more turn-key solution than trying to route Ethernet across your coax cable.

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    router? why install a full blown router? can't the OP just add an access point? – anna328p Feb 2 '17 at 20:46
  • How to find one that's high power? Don't the regulations limit the power and practically all routers use the maximum power permitted by the regulations? – juhist Feb 2 '17 at 21:52
  • @DmitryKudriavtsev: Buying a router is often the cheapest way to get a good access point, AFAIK. Leave the WAN side unconnected and just plug stuff into the LAN side, and use it as an AP. – Peter Cordes Feb 3 '17 at 11:05
  • @PeterCordes I'm pretty sure most so called "extenders" are actually access points. – anna328p Feb 3 '17 at 16:49
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Assuming the coax you have installed is RG59 or RG6 type, I suggest using MoCA. The current generation of MoCA adapters achieve gigabit speeds, and the signal coexists with cable TV if you have that. I'm currently running these in three rooms in my house with great success.

See this answer where I give a complete description of the setup, with a diagram, that shows how it integrates with cable TV.

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