Installing a WISP on an AM broadcast tower

This is an interesting project currently underway at one of our client’s AM sites.  They have decided to go all in and create a WISP (Wireless Internet Service Provider) for the community around the AM tower.  I thought it would be interesting to explore this topic, as there are not many opportunities for AM towers to lease vertical real estate.

First a few basic ideas.  For an AM broadcaster, (aka medium wave or standard broadcast band) the entire tower is part of the transmitting antenna.  There are two types of towers; series-excited and shunt-excited.  A series excited tower has a base insulator, like this:

AM tower with base insulator
AM tower with base insulator

A shunt tower usually has a series of wires called a skirt, separated from the tower by standoffs, which go to the top of the tower or nearly to the top of the tower. The base of the tower is grounded, like this:

AM tower with out base insulator
AM tower without base insulator

A shunt-excited tower has distinctive advantages for co-location opportunities in that the tower itself is grounded, greatly simplifying placing additional antennas on the towers.  That is not to say that antennas can not be installed on series excited (insulated) towers, it just requires an extra step of using isolation coils.

In all cases, the tower should have a structural study done to insure that the additional antennas do not overload the tower and cause structural damage or collapse.

In this case, the tower is new and was designed for the extra load.

The plan is to create a sectorized wireless internet system using four 90-degree panels, each with three access points.  A tower-mounted sixteen-port switch is mounted behind the panel antennas and the switch communicates with the ground-mounted router through two fiber optic cables.  A 54-volt DC supply powers the switch, access points, and point-to-point radios mounted on the tower.  There are two fiber runs, one is for subscriber traffic and the other is for radio management.  This system is using Ubiquiti gear.

Ubiquiti 90 degree sector antennas and radios
Ubiquiti 90-degree sector antennas and radios

A word or two about Ubiquiti gear.  Ubiquiti specializes in cheap equipment manufactured in China.  That is a double-edged sword.  On the plus side, if anything breaks or gets damaged by lightning or whatever; throw it out and install a new one.  On the negative side, I  have seen Ubiquiti gear do some strange things, particularly after a firmware upgrade.  The newer stuff seems to be better than the older stuff.  All that being said, as this is a brand-new operation and seems to be a proof of concept, then the Ubiquiti gear will be fine to start with.

Going up
Going up

The tower crew made quick work of installing the sectorized access points.

Tower crew waiting for equipment lift
Tower crew waiting for equipment lift

Going up the face of the tower, there are the aforementioned fiber cables, the 54 VDC power cable, and one backup Ethernet cable.  All of the Ethernet jumper cables used to connect the access points to the switch are UV-rated, shielded Cat 5e, and use shielded connectors.  This is very important on a hot AM tower.  Due to the skin effect, the shield on the shielded cable protects the interior twisted pair conductors from the high AM RF fields present on the tower.

Transtector LPU 1101-1158
Transtector LPU 1101-1158 Ethernet cable protection unit

At the base of the tower, the DC power cable and the Ethernet cable go through high-quality lightning protection units.  These are Transtector 1101-1158 Ethernet and 1101-1025 48-volt outdoor DC power units.  Even though the DC power supply is 54 volts, the 48-volt LPUs will function adequately.  The TVSS devices used in the LPU circuit are rated for 88 volts maximum continuous voltage.

Transtector 1101-1025 48 VDC lightning protection unit
Transtector 1101-1025 48 VDC lightning protection unit

In addition, I made a service loop on the DC cable with also creates an RF choke.  Several (12-14) turns of cable 18-20 inches (45 to 50 cm) in diameter act to keep the induced RF at the input terminals of the LPU low so the protection devices do not fire on high modulation peaks.  This also helps to keep the AM RF out of the 54 VDC power supply in the rack.

Making ethernet jumper cables, TIA/EIA-568B
Making ethernet jumper cables, TIA/EIA-568B

The backup Ethernet cable has a similar setup.  Regarding the Ethernet cable and induced RF, this station runs 1 KW.  As long as the shielded RJ-45 connectors are applied properly and the tower-mounted switch is grounded along with the LPU, then all of the RF should be on the very outside of the cable shield (due to the skin effect).

Base of AM tower with WISP equipment installed
Base of AM tower with WISP equipment installed

This principle also applies to lightning strikes.  Although lightning is DC voltage, it has a very fast rise time, which makes it behave like AC on the initial impulse of the strike. The voltage induced on the shield of the cable will not affect the twisted pairs found deeper within the Ethernet cable.  Of course, all bets are off if there is a direct strike on a piece of equipment.

AM stations running powers more than 1 KW, Superior Essex makes armored shielded cable called BBDG (the new trade name is EnduraGain OSP).  This cable comes with a heliax like a copper shield with an optional aluminum spiral armor.  This cable looks very robust.

Enduragain OSP armored shielded Category cable
Enduragain OSP armored shielded Category cable

On series excited towers (those with an insulated base) fiber optic cable can be used to cross the base insulator without any problems, as long as there is not any metal in the cable (armor or aerial messenger).

LBA Group TC-300 tower lighting choke
LBA Group TC-300 tower lighting choke. 180 turns #12 AWG enamel wire on 6-inch coil form.

DC power can cross the base insulator using something called a “Tower Lighting Choke.”  This device is a set of coils wound around a form that passes the DC power but keeps the AM RF from following the DC power cable to the ground.  These work relatively well, however, lightning protection units still need to be installed before the DC power supply.

Last Walk across The Island

Yesterday I took, what I hope to be, my last walk across Pleasure Beach Island in Bridgeport, Connecticut. The task at hand was repairing the antenna array for WICC. There turned out to be several issues that were addressed in turn.

WICC tower feed point, courtesy of NECRAT

The trouble started when the feed line between the ATU and the tower became disconnected during a storm. That consists of a 1-inch copper pipe extending from the ATU feed through an insulator up to a brass plate suspended between the four tower legs by hard-drawn single 0 copper wire. The feed line separated at the brass plate which, unfortunately, is approximately eighteen feet in the air.

North Tower feed point connection, cold soldered

The feed line was repaired, but not effectively. By the looks of the picture, the brass plate never got hot enough to accept the solder.

After the feed line was re-repaired, other issues became apparent. The base impedance of the tower was still off and the array was still way out of tolerance.

It was noticed that several bypass capacitors on both of the tower lighting chokes were blown open. Those were replaced and the tower lighting chokes were checked for shorted turns. While it is always nice to replace burned-out parts, this did not correct the problem.

Finally, we were back at the base of the tower with the defective feed point and a decision to grab the pipe and give it a good shake to see if it came apart again. It did not, but then I realized that that tower was supposed to be back in the circuit and I did not receive any RF burns for my carelessness.

We dug into the ATU and discovered that the input capacitor was marginal and there was a large crack in it. The output capacitor seemed to be completely open. The base current that we were seeing on the base current meter was being induced by the other tower. It all began to make sense.

Bad Capacitor

The parts were ordered and shipped and I made another trip out to install them myself.

Thus, on this particular day, I had my tool bag, an OIB-3 with fresh batteries, my cordless drill, drill bits, and three type 294 mica capacitors. I took the drill because the new capacitors were quite a bit larger than the old ones, so I needed to move the stand-off insulators to remount them.

Pleasure Beach pier, foggy day

The walk from the end of the dock to the transmitter site is approximately 900 meters or 0.55 miles, according to google maps. On a nice day, it is a pleasant walk. On not-so-nice days, it can be less so. It was foggy with light drizzle. Not enough to get wet right away, but enough to get slowly soaked while working on the ATU repairs.

WICC square base self-supporting towers, manufactured by Milliken Tower, circa 1924

With the new capacitors installed, I needed to adjust the array back into tolerance, which didn’t take too long. I made a short video of the station running at full power showing the antenna monitor readings for both the day and night patterns. Then packed up and headed back to the dock.

My ride is here

I wanted to take a set of monitor points, but the FIM-41 had been moved to another location. That was fine, I was getting pretty uncomfortable in my wet clothes, so I headed home.

Goodbye, WICC.

The Temporary AM antenna

One of those things that I have written about before, but seems to be common these days as older AM towers need to be replaced. One of our clients had just such a tower. Erected in 1960, the hollow-leg stainless tower was rusting from the inside out. When the tower crew came to put up the translator antenna, they discovered that there was a hole in one of the legs and climbed back down.

The tower’s condition was somewhat known, there were braces installed several years ago at certain levels to keep the tower standing. The new owner had planned to replace the tower eventually, so those plans were moved ahead.

Temporary Wire antenna, WKNY, Kingston, NY
Temporary Wire antenna, WKNY, Kingston, NY

A temporary utility pole was installed near the transmitter building and a wire was strung to another customer-owned pole about 170 feet away. At 1,490 KHz, that proved to be a pretty good length. The issue with these medium wave temporary antennas is always the height above ground. In order for the radiation resistance to be somewhat reasonable, the antenna needs to be at least 1/8 to 1/4 wavelength above ground. That means a minimum of 78 to 157 feet at 1,490 KHz. The utility pole installed is 35 feet AGL.

WKNY temporary ATU

Thus, the wire antenna has a fairly low resistance, with loads of inductive reactance. Something on the order of 20 ohms, +j480. Since this is temporary, we reused the existing ATU that was designed for the series excited tower. With a capacitor installed on the incoming wire to cancel out some of the inductive reactance, a simple T network was configured to match the 50-ohm transmitter output to the 20-ohm antenna.

In the end, we were able to run about 400 watts into the wire, which covered the city of license fairly well. While the new tower was being erected nearby, we had to reduce that to about 100 watts to protect the tower workers from the hazards of non-ionizing radiation.

WKNY new tower build

The new replacement tower has been constructed. It is the exact same height as the old tower but has a twenty-foot pole on top instead of a normal tower section. The pole was installed to mount the translator antenna. In addition to that, there will be other wireless services installed on this tower.

WKNY will have a six-wire skirt installed in the next few days. As this tower is close to 160 degrees at 1,490 KHz, the skirt can go anywhere from 60 to 120 degrees up the tower.

WKIP

This was the radio station that I listened to (or rather, my parents listened to) when I was a very young kid.  From this source, things like school closings, weather, lunar landings, news, sports and traffic could be heard.  At one point, there was a guy called the “Traffic Hawk,” (real name Don Foster) who flew in a Cessna 172 east and west over main street in Poughkeepsie advising drivers of any slow downs in the area.  That’s right, Poughkeepsie, New York, population 30,000, had it’s own eye in the sky, broadcasting live from the aircraft overhead.  Actually, I think he also flew up and down South Road (US Route 9) in the vicinity of the IBM plant, which employed quite a few people in those days.

There was also a guy who tried to break the Guinness Book of World Records by staying awake the longest, this happened several times.

For me, it was the school closings.  I hated school with an absolute passion.  Everyday, I would ride the school bus and say a little prayer; “…please God, make it today.  Make the boiler stop working, or the electricity go out.  Make the kitchen catch on fire or the roof cave in.  You are a great and mighty God and I don’t ask for much.  Please destroy my school today.”  Alas, God did not seem interested in this.

Anyway, back to the topic at hand.

WKIP first signed on in 1940 with the studios and transmitter located at The Nelson House, 42 Market Street, Poughkeepsie.  That building is long gone and the location appears to be the parking lot for the Dutchess County Office building.  Being neighbors with some influential guy from Hyde Park made for a nice dedication speech:

It signed on with a power of 250 watts on 1,420 KC on June 6th, 1940. Soon thereafter, it changed frequency to 1,450 KC as a part of the AM band shift brought about by NARBA.

Over the years, the station went through several ownership changes. The first major technical change came in 1961, when the station transmitter site moved to its current location, then called Van Wagoner Road, now Tucker Drive. The station increased power to 1,000 Watts and installed a directional antenna for daytime use.  It is one of those rare nighttime non-directional, daytime directional stations.

The directional antenna consists of two towers; tower one is 180 degrees tall (103.4 Meters or 340 feet) with 35 degrees of top loading.  That is used for both the day and nighttime array.  Tower two is 85 degrees tall (48.8 Meters or 160 feet) and is used only for the daytime array.  This pushes the major lobe of radiation towards the north.  I don’t know the reasoning behind that, but somebody spends a good amount of money to make it so.

Here is an air check from the early 1980s.  Weather on that day was “Sunny, cloudy, whatever… take your pick.”

Good old Steve Diner.

Today, the station looks like this:

The 1961 WKIP transmitter building with tower
The 1961 WKIP transmitter building with tower

When I was growing up, my cousins lived within walking distance of this. We used to come over and throw rocks at the tower when the station was unmanned on Saturdays and Sundays. At least, I think it was unmanned because no one ever came out and yelled at us.

WKIP backup transmitter, phasor and main transmitter
WKIP backup transmitter, phasor and main transmitter

Mid-1980s MW-1A still runs. The BE AM1A is the main transmitter. The phasor is the Original 1960s Gates Phasor.

This video shows how the studios used to look before they were rebuilt by Clear Channel Circa 2002 or so. At about the 2:02 mark, you will see the room pictured above as it looked in 1990.

The space between the video above and the picture below looked bad with nothing in it. It looks better now.

WKIP clock
WKIP clock

That clock is a collector’s item and belongs in a museum.