Wednesday, September 21, 2016

TV, LAN, & intercom over one cable

The buildings on this property have shared a single MATV system installed several years ago.  This consists of two long RG11 drops and sundry splitters, taps, and amplifiers.  Over the years, the system structure has changed several times.  For many years before DTV conversion, music and security camera feeds were inserted into the network with modulators.  Slowly, the modulators and processors were removed and all that remained was a simple antenna distribution system.

Property & Drop Map

The off-air TV/FM signal isn't the only thing shared between the buildings in the diagram.  While the antenna is distributed from building A, the WAN connection is at building B.  For years, building A has been linked to the computer network over wifi with the aid of directional antennas.  Furthermore, buildings B and C share a voice intercom I'd discussed in a previous post.  This intercom was realized using a parallel drop of twisted pair cable.  The recent goals were to extend the computer network to point C and to replace the deteriorated UTP cable used by the intercom.

Network schematic between buildings B and C

While the wireless bridge to A works well enough, reaching C via wifi is difficult.  There is restricted line of sight and the obstacles in the signal path are all large metal structures.  While the 500' link can be established with makeshift waveguide antennas, I opted to try running it over the coaxial cable instead.  While I'm sure there are legitimate solutions for running ethernet over coax, the products I saw were either too expensive or appeared to be misrepresented Thinnet adapters.

Like usual, I used a low-cost and low-risk approach.  Parts should be inexpensive, common, and should have some secondary utility should plans change.  Using two ASUS RT-N12/D1 router/repeater/AP devices, a 802.11g/n link was made over the coax.  Satellite diplexers are used to combine/split the 2.4GHz signal from the MATV signal.  I made a simple adapter to connect the RP-SMA antenna port on the AP to the 75 ohm network.  The adapter serves to adapt the dissimilar connectors, but it also uses a simple L-pad impedance matching network and a single capacitor as a DC block.  The odd resistor values shown in the schematic are a product of convenience given my limited stock of SMD resistors.  Simply put, 100R || 1K = 91R; 75R || 100R = 43R. I gutted some old CATV channel traps to use as enclosures.  The RG316 cables with RP-SMA connector were stripped from old equipment.  Signal strength at C is about -57dBm, which is about on par with what can be expected of the drop.  The free antenna on the access point at B provides extended coverage to the shop and outdoor area.  The free antenna on the access point at C provides wireless coverage for any mobile devices in the area. 

Inner strand damage caused by jacket cracking in UTP cable

The UTP drop used by the intercom was originally a bundle of scrap indoor cable pulled from a PBX somewhere.  It was never expected to last outdoors for as long as it did.  Eventually, the jacket deteriorated and the cable would become waterlogged.  Due to the way the intercom is arranged, shunt conductance in the line does more than attenuate the voice signal; it causes the intercom to ring incessantly when on-hook.  Numerous modifications were made to the previously detailed intercom in order to make it robust against these faults, but ultimately it's difficult to work with a line that has roughly 1 ohm shunt resistance when it rains

The voice intercom project followed a similar course.  Two power inserters were used as diplexers to couple the voice and DC from the intercom.  Again, as a matter of physical and electrical interfacing, I made a pair of low-pass filters with screw terminals to attach to the indoor portion of the intercom network.  I felt that adding the LPF was prudent, as the DC path of the power inserters has a cutoff frequency of a few MHz.  I didn't want stray RF interfering with anything.  The LPF knee is set at about 20kHz, but in reality, it doesn't really matter so long as it encompasses the ~3.5kHz voice band allowed by the old telephone hardware.  There are versions of these power inserters with additional DC port LPF filtering, but I have no idea what the cutoff frequency actually is.  These may be an option and may avoid the need for external filters, but as with most things, if you're out to apply a product for an off-label use, you're going to have to perform some experiments. 

LPF filters for intercom connection

It's worth noting that this change means that the intercom signal path is now sensitive to ground loops, whereas before the remote end of the UTP was floating.  Attention has to be paid to grounding within the intercom base station and on the coax drop.  In my installation, the DC side of the base station's power supply is floating.  The coaxial drop has a hard ground at only one end.  The far end at C is grounded through a TVSS diode.  This breaks the ground loop created by the shield, but allows any surge currents to shunt to ground.  I imagine two antiparallel diodes would work just fine as well. When all ground loops are opened, the intercom is remarkably clearer over the coaxial drop than it ever was over UTP. 

Drop feed at B showing diplexers, fabricated adapters, and ground block
While the power inserters and diplexers are being used marginally beyond their specified bandwidth, they work well enough; after all, most of the fittings in the 2.4GHz path were only rated to 1GHz until the last few days. The components for this approach are inexpensive and easily obtainable.  The details of the adapter construction are not terribly critical.  It all works with an ugly simplicity that impresses me.

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