Since 2003 or so, I have been experimenting with
alternative STL systems, and have written about some of my experiences in these
pages. On a recent project, however, I took the opportunity to go a different
direction with part of the link.
A 90 micron fiber with some
of the buffer stripped off. A 1/4-watt resistor is shown for scale. Tiny stuff,
and it will draw blood!
These “alternative STLs” have, for the most
part, taken the form of some kind of 802.11 unlicensed link, a Part 101
licensed microwave system or both. All provide some quantity of Ethernet bandwidth
between one end of the link and the other, and we have employed various codecs
to transport audio from end to end in either or both directions.
Because they carry data, each of the links is
fed with a piece of Cat-5 or Cat-6 cable. That is good in one sense because
Cat-5 and Cat-6 cable is a lot cheaper than coaxial cable or waveguide, but
it’s bad in a couple of others because there is a 100-meter length limit on
such cables, and it’s tough to get them across the base insulator of a
series-fed AM tower in AM station applications.
Those limitations led me to a relatively
inexpensive solution: employing a short-hop 802.11 link to get from the top of
the tower to the transmitter building with the data. There are many options
commercially available, but my favorite is the Ubiquiti NanoBridge M5 link.
It’s inexpensive (less than $200 for a complete link), reliable and has a lot
more bandwidth than any long-haul microwave link you’re likely to use. This
configuration requires power on the tower very near the microwave and 802.11
link antennas, which is probably not a big issue or expense, and it also
requires a small Ethernet switch at that location.
Over the decade or so that I have been using this kind of
link, those Ethernet switches have been the number one point of failure. They
don’t fail often, but let’s face it — in this application, they’re operating in
a tough environment. These simple consumer-style switches are mounted outdoors,
probably in a poorly ventilated NEMA enclosure, with ambient temperatures
ranging from 10 below zero to 140 above … and don’t forget about the occasional
magnetic field spike from a lightning strike on the tower! It’s amazing they
work at all!
my new project, I was looking for a way to do away with the on-tower Ethernet
switch and the 802.11 link, thus reducing possible points of failure in the
system by three or so active devices. Because I was working with a series-fed
AM tower, part of a 50 kW array, I had to deal with the issue of crossing the base
insulator without adversely affecting the impedance of the tower. My options
were few, but one kept jumping out at me: multimode fiber.
Contents of the Corning
Except for the short piece of fiber in the old
Continental “Power Rock” series of transmitters, my experience with fiber-optic
cables was limited to gimmick Christmas decorations. I knew it was possible to
pass a lot of data over a long piece of fiber, but I knew nothing of how to
convert Ethernet to optical, put on connectors, make splices or any of that. It
was time to do some investigating.
I started talking to people, surfing the Web and learning all I could about it.
Multimode fiber is certainly nothing new to the folks who work with data. I
found some of my amateur radio friends who regularly use it as a transmission
medium within large data infrastructures. As such, there is a lot of
information on fiber-optics out there. That’s the good part. The bad is that
none of the information fit my specific application.
where we engineers get to do our thing — figure out how to apply different
technologies to meet the needs of particular situations.
Rear of Dragonwave 11 GHz
transceiver showing power connection (top) and fiber connection (bottom).
AN EDUCATION IN
The microwave radios I planned to use were made
by Dragonwave, and a brand-new option at the time was for multimode fiber inputs
and outputs. It was so new, in fact, that evidently none had yet been shipped.
provided me with the information I needed — the type of fiber, connectors, etc.
required; and with that in hand, I began searching for equipment that I could
use on the other end of the piece of fiber to get the blinky red light back to
Ethernet ones and zeros.
search turned out to be short. Cisco and other manufacturers of high-end
gigabit switches offer units with GBIC (gigabit interface converter) ports, and
for under $40, there are any number of GBIC fiber-optic transceivers available.
Buy one, plug it into the switch’s GBIC port and you’ve got yourself a gigabit
switch with a fiber port on it.
next part of my education came in learning how to put connectors on multimode
fiber. That was quite a learning curve. On the recommendation of a friend who
works with fiber, I purchased a Corning fiber-optic connector installation
toolkit, something that set me back some $1,400. It included all kinds of tools
used to work with fiber, the connector jig, crimp tool and continuity tester
with several different connector adaptors.
I had to learn first to be very careful, very
gentle. Fiber is incredibly robust, but nick it and it’s extremely brittle.
Removing the “buffer” (what you and I would think of as “insulation”) from a
90-micron fiber is a delicate operation that can easily end with a broken fiber
and ruined installation.
I also had to learn that everything had to be
very clean. You clean the bare fiber with a special cloth and cleaning solution
before putting on the connector. And you don’t cut fiber, you “cleave” it,
essentially scoring it at the desired cut point and then breaking it. Weird.
Interior of the NEMA
enclosure on the tower containing the surge suppressor, –48V power supply and
loop-through for the fiber. The –48V power supply employs a coaxial-type
connector. The coaxial m-f connection was secured with electrical tape by the
tower rigger to keep it from vibrating apart.
Once I got the hang of it, though, putting on
the connectors wasn’t so hard. My third or fourth effort worked (the connectors
I wrecked were $12 each!). For a test, I used a pair of Cisco switches with
GBIC fiber transceivers to connect my laptop to the office network. I was
amazed when I was able to connect to the Internet and pass data over that piece
of fiber! But that experiment proved to me that it would work, and it showed me
that I could successfully put the connectors on.
IT ALWAYS PAYS
TO TEST FIRST
I had the Dragonwave transceivers shipped to my
office in Colorado and my next step was to configure them and connect them with
fiber to the LAN.
I got that done and tested, and then programmed in the
frequencies to the Dragonwaves. Next, I ran the power all the way down and
actually pushed data through the microwave link and fiber, just to make sure it
would work (it did). Better to confirm all that on the ground than try and
troubleshoot it 300 feet in the air!
I shipped the Dragonwave transceivers, power supplies and a 1,000-foot roll of
duplex multimode 90 micron fiber-optic cable to California for the
installation. The tower crew installed the antenna, a four-foot dish, on the
tower along with a NEMA enclosure for the –48V power supply for the radio. The
enclosure provided a pass-through for the fiber-optic cable to the radio.
My initial plan was to use a solid run of duplex
multimode fiber from the transmitter building all the way to the top of the
tower, and indeed, that would be the ideal way to install it. But after much
thought and discussions with the tower crew, we decided it would be too
difficult to pull a cable with delicate connectors installed in a piece of
3/4-inch rigid conduit up a 300-foot tower. If a connector were damaged,
it would all have to come back out — there’s no way I could install a connector
hanging from a harness at the top of a tower!
Fiber-optic cable splice
employing LC-connectors and a dual coupler inside the ATU. The duplex cable,
with buffer and outer jackets, resembles zip cord.
The plan, then, changed a bit. I opted to
terminate the fiber-optic cable from the transmitter building (some 500 feet
away) inside the ATU where it would be spliced to the cable coming down the
tower. The conduit from the transmitter building terminated into the bottom of
the ATU cabinet, so that part was easy. For the run from the tower, I set a
6x6x4 PVC J-box right on the transom plate of the tower, terminated the
conduit run from the top into that box, then ran a piece of Seal-Tight to the
lighting control box on the tower to pick up 120 VAC.
cross the base insulator with the fiber, I used a length of 100 percent PVC
non-metallic Seal-Tight, connecting one end into the J-box on the transom plate
and the other end into the underside of the ATU cabinet. I had some
reservations about this (there would be 15 kW or so across that base insulator
and thus across the piece of Seal-Tight), and I expressed them to Bobby Cox of
Kintronic Laboratories. Bobby replied that KTL has been using PVC to
manufacture isocouplers for years with no issue. That made sense, so I stopped
worrying. So far so good with all the RF!
With the splice made inside the ATU, I applied
power to the Dragonwave transceiver on the tower, and just like that, the GBIC
port lit up with a valid gigabit connection and we were immediately able to
push management data to the transceiver. In short order we had the path aligned
with the predicted signal strength at each end, and we were pushing data
between studio and transmitter in both directions.
PVC J-Box on transom plate.
The conduit coming in on the top left is from the NEMA box on top of the tower
and contains both 120 VAC and fiber-optic cables. The top right conduit carries
the 120 VAC power conductors to the lighting control box on the tower. The PVC
conduit on the bottom contains the fiber cable and crosses the base insulator
to the ATU.
link now carries STL audio, remote control, security, video surveillance and
telephone to and from the site. I sure feel a lot better without an Ethernet
switch on the tower and an unlicensed RF link between tower and transmitter
building. The removal of those additional possible points of failure changes
Going forward, I plan to make fiber a primary
design element in my AM and FM STL links. It solves both the base insulator and
length limit problems with Cat-5/6. And besides that, I think it’s pretty cool.
Cris Alexander is director of engineering at Crawford
Broadcasting, a longtime RW contributor and a past recipient of SBE’s Broadcast
Engineer of the Year award.