Lightning protection for WLAN links

More and more wireless LAN links are being installed between the transmitter and studio.  Often these links are used for network extension, remote control, site security, VIOP telephony, and sometimes even as a main STL.  These systems come in several flavors:

  • Moseley LAN link or similar system.  Operates on unlicensed 920 MHz (902-928 MHz) band.  Advantages: can use existing 900 MHz STL antennas, can work reliably over longer distances, transmitter and receiver located indoors.  Disadvantages: slow, expensive
  • ADTRAN TRACER or similar system with indoor tranceivers and coax fed antenna systems.  Operates on unlicensed or licensed WLAN frequencies.  Advantages: fast, transmitter and receiver located indoors, can be configured for Ethernet or T-1/E-1 ports.  Disadvantages; expensive
  • Ubiquiti Nano bridge or similar system where tranceiver is located in the antenna, the system is connected via category 5/6 cable with POE.  Operates on unlicensed or licensed WLAN frequencies.  Advantages; fast, relatively inexpensive.  Disadvantages; equipment located on tower, difficult to transition base insulator of series fed AM tower.
  • Ubiquiti Rocket or similar system where the antenna and tranceiver are separate, but the transciever is often located on the tower behind the antenna and fed with category 5/6 cable with POE.  Operates on unlicensed and licensed WLAN frequencies.

For the first two categories of WLAN equipment, standard lightning protection measures are usually adequate:

  • Good common point ground techniques
  • Ground the coaxial cable shield at the tower base and at the entrance to the building
  • Appropriate coaxial type transmission line surge suppressors
  • Ferrite toroids on ethernet and power connections

For the second two types of WLAN equipment, special attention is need with the ethernet cable goes between the tower and POE injector or switch.  Shielded, UV resistant cable is a requirement.  On an AM tower, the shielded cable must also be run inside a metal conduit.  Due to the skin effect, the metal conduit will keep most of the RF away from the ethernet cable.  Crossing a base insulator of a series excited tower presents a special problem.

The best way to get across the base insulator of a series excited tower is to use fiber.  This precludes the use of POE which means that AC power will be needed up on the tower to power the radio and fiber converter.  This my not be a huge problem if the tower is lit and the incandescent lighting system can be upgraded to LEDs.  A small NEMA 4 enclosure can house the fiber converter and POE injector to run the WLAN radio.  Some shorter AM towers are no longer lit.

Another possible method would be to fabricate an RF choke out of copper tubing.  This is the same idea as a tower lighting choke or a sample system that uses tower mounted loops.  I would not recommend this for power levels over 10 KW or on towers that are over 160 electrical degrees tall.  Basically, some 3/8 or 1/2 inch copper tubing can be wound into a coil through which a shielded ethernet cable can be run.  Twenty to twenty five turns, 12 inches in diameter will work for the upper part of the band.  For the lower part, the coil diameter should be 24 inches.

In all cases where CAT 5 or 6 cable is used on a tower, it must be shielded and the proper shielded connectors must be used.  In addition, whatever is injecting power into the cable, ether POE injector or POE switch must be very well grounded.  The connector on the shielded Cat5 or 6 cable must be properly applied to ensure the shield is grounded.  A good video from Ubiquiti, which makes TOUGHCable, on application of connectors to shielded Cat5 cable is here:

In addition to that, some type of surge suppressor at the base of the tower is also needed. Tramstector makes several products to protect low voltage data circuits.

Transtector APLU 1101 series dataline protector
Transtector APLU 1101 series dataline protector

These units are very well made and designed to mount to a tower leg. They come with clamps and ground conductor designed to bolt to a standard copper ground buss bar.

Transtector APLU 1101 series dataline protector
Transtector APLU 1101 series dataline protector

There are various models designed to pass POE or even 90 VDC ring voltage.

Transtector APLU 1101 series dataline protector
Transtector APLU 1101 series dataline protector

This model is for POE. The circuit seems to consist mostly of TVS diodes clamping the various data conductors.

As more and more of these systems are installed and become a part of critical infrastructure, more thought needs to be given to lightning protection, redundancy and disaster recovery in the event of equipment failure.

On being responsible

No two days are alike. Sure, there are days that are similar in nature, office work, filing, FCC compliance, etc.  However, there is always something different, some new problem, person, fault, error, client, site or situation to deal with.  It helps to be well versed.

So, when the tower climbers started climbing a 1,000 foot (304 meter) tall tower to find a damaged section of transmission line, I thought; Just a routine day.

Even when they encountered a hornet’s nest at 50 feet (15 meters) AGL, still, fairly routine:

Tower climber applying bee spray to paper wasp nest
Tower climber applying bee spray to paper wasp nest

Tower climber A received a nasty bee sting to his left arm. He climbed part way down the tower and is in the lower part of the picture hugging the tower face. Tower climber B moved up and killed the nest with Wasp and Bee killer.  All is well and work resumes, right?  Except, no.  Tower climber A is apparently allergic to bees.  He states he is not feeling well and his arm begins to swell up.  He comes down the tower and I start looking for Benadryl.

Now, we have a problem.  This is a mountain top tower site, there is a long dirt road with a locked gate at the bottom of the hill.  There is almost no way an ambulance will be able to find its way up here.  The tower climber says the he has not been stung in many years.  I also notice his face is beginning to swell up.  Right, so lock the door, in the truck and get to the bottom of the hill as fast as possible.  It took about five minutes, but at the bottom of the hill, we were in a much better position if things got worse and an ambulance needed to be called.  Fortunately, his condition was the same, so we drove to an urgent care facility were he was treated.

Benadryl, something else to add to the go bag.

Always keep ahead of the situation.  Even if we drove to the bottom of the hill and his symptoms completely disappeared, it still would have been the right decision.

The Shukhov Tower

A very interesting bit of broadcasting history in Moscow may disappear forever.  Designed and built by Vladimir Grigoryevich Shukhov, the Shukhov Tower was completed in 1922.  Since that time it has served as a AM broadcasting and later and FM broadcasting tower.  In the picture, one can see what looks like a massive FM panel antenna at the top.  According to this website: www.shukhov.org, the tower is in very poor shape and is slated to be demolished.

The tower itself is described as 160 Meters (525 feet) tall, hyperboloid steel lattice structure. The design is unique in that it is very strong, yet uses approximately 60-70 percent less steel than a comparable four legged structure like the Eiffel tower.  An amazing feat of engineering for its day, when everything was calculated and drawn by hand.

Shukhov Tower, Moscow, FSR
Shukhov Tower, Moscow, FSR.

The antenna is a little hard to discern, however, it looks like a horizontally polarized six or eight around 4 bay FM antenna. Could also be low band VHF TV.

Shukhov Tower antenna
Shukhov Tower antenna. Courtesy Wikipedia

Unfortunately, time is running out and little or nothing is being done to protect the steel structure from the elements. The last paint job was more than twenty years ago. The land it currently occupies has some value, and there is talk of putting up a high rise development in its place.

Article from the New York Times; An engineering landmark faces demolition in Moscow.

There are lots of videos on youtube and pictures from the wikipedia article.  It is an interesting bit of history, if it can be saved it will be a very close run thing.

North Adams Tower Collapse

High winds seem to be the culprit in the collapse of two towers in North Adams. According to the Motorola system technicians, it happened at about 12:30 am Sunday morning, which is when all their link loss alarms started going off.  The larger, self supporting tower broke from it’s mounting plate and tipped over into the smaller guyed tower next to it. Effected are WUPE-FM and W226AW (WFCR New England Public Radio) as well as NEPR new station WNNI which has not officially signed on.

Cellular service for ATT, Verizon and Sprint/NEXTEL were all knocked off line as well internet services and E911 dispatch.  Those services are coming back on line, with temporary modular cell units en route. N

Here are some pictures:

North Adams Cell Tower
North Adams Cell Tower
WUPE-FM antenna on the ground
WUPE-FM antenna on the ground
WUPE-FM antenna
WUPE-FM antenna
WUPE-FM STL dish
WUPE-FM STL dish
Base of WUPE-FM (formerly WMNB) tower
Base of WUPE-FM (formerly WMNB) tower
WNNI antenna
WNNI antenna
WUPE-FM WNNI and W266AW transmitter building
WUPE-FM, WNNI, and W266AW transmitter building
North Adams Cell Tower
North Adams Cell Tower
North Adams Cell Tower
North Adams Cell Tower
North Adams Cell Tower
North Adams Cell Tower
North Adams Cell Tower
North Adams Cell Tower
Tower base mounting plate, apparent failure point
Tower base mounting plate, apparent failure point
Tower base mounting plate
Tower base mounting plate
Tower Base Mounting Plate
Tower Base Mounting Plate

For pictures of the towers during happier times, refer to this post: Filtering for co-located FM transmitters.

Restoration work is underway with WUPE-FM expected to return to air at low power by Monday afternoon.

Update:

WUPE-FM was returned to air at low power by about 1pm on Monday 3/31.  We took an unused Shively 6812 antenna that was tuned to 94.1 MHz and retuned it to 100.1 by cutting 1/4 inch pieces from the end of the elements until it was on frequency.  It took a bit of doing, but with a network analyzer, we were able to get it to 1.2:1 SWR with symmetrical sidebands.  Running 600 watts, it covers the city of license and then some.

WUPE-FM temporary antenna
WUPE-FM temporary antenna, Shively 6812

The STL antenna is a survey antenna mounted on the side of the building. In this configuration, with the leaves off of the trees, we are getting about 250 uV signal, which is pretty good.

WUPE-FM temporary STL antenna
WUPE-FM temporary STL antenna

The site is now crawling with insurance investigators, cell site technicians, North Adams fire department, Berkshire County Sheriff’s officers, tower workers, etc.  After we finished this work, we cleared out to make more room for everybody else.  Estimated restore time for W266AW is Wednesday 4/2.

Planning for the replacement tower is already in progress, I’d expect it to happen fairly quickly. The next step for the broadcasters is to put up a 70 foot utility pole and get a full powered antenna for WUPE. This should happen in the next two weeks or so. That will serve as the temporary facility until the new tower is constructed.