Am transmitter site – Page 2 – Engineering Radio

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:

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:

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

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.

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

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 Ethernet cable protection unit

At the base of the tower, the DC power cable and the Ethernet cable go though 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 LPU’s 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

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

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

This principal 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 effect 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 copper shield with an optional aluminum spiral armor. This cable looks very robust.

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. 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 which passes the DC power but keeps the AM RF from following the DC power cable to 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 which 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 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 where blown open. Those where replaced and the tower lighting chokes where 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 decided 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.

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.

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.

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 condition was somewhat known about, 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 where moved ahead.

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 wave length above ground. That means a minimum of 78 to 157 feet at 1,490 KHz. The utility pole installed is 35 feet AGL.

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.

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.

The Answer to Ailing Copper

I don’t know how things are in your neck of the woods, but here in the Northeastern US, our old copper TELCO networks are on their way out. This is a problem for broadcasters who still rely on POTS lines (Plain Old Telephone Service) for transmitter remote controls, studio hot lines, etc. The vast majority of my transmitter site access is through dial up remote controls. There are a few locations that have web based remote controls, but to be honest; the phone part of my smart phone still gets a lot of use. There are several locations where the old copper is just failing outright and not through a lack of effort by the repair techs. Generally, the copper pairs get wet and develop a loud hum, which makes the remote control unit either hang up or become unresponsive to touch tone commands.

The best course of action is to get some type of VOIP line installed. Here is the rub; many transmitter sites are nowhere near a cable system. Several times, I have contacted the cable company to see if they will provide a VOIP phone line at a certain site. The response is usually; sure, we can do that! However, it will cost you (insert some ridiculous amount of money) to extend the cable to your transmitter site.

LAN extensions to the transmitter site are a useful for a number of reasons. More and more transmitters are equipped with web interfaces as are processors, UPSs, transmitter remote controls, security cameras, etc. LAN extensions can also be used for backup audio in case of STL failure. Finally, an inexpensive ATA (Analog Telephone Adaptor) and DID line can replace a POTS line for a lot less money. One example; voip.ms has the following plans as of this writing:

Plan type	Per month per DID number (USD)	Incoming call rate (USD) per minute	Outgoing call rate (USD) per minute
Per minute	$0.85	$0.01 (USA)	$0.009
Unlimited	$4.25	$0.00	$0.009
Toll Free (800)	$0.99	$0.019	$0.009

Any of those plans surely beats the standard TELCO rate of $40-50 per month per line.

Design criteria for a wireless LAN system needs to take into account bandwidth, latency and reliability. Each VOIP phone call takes anywhere from 28-87 Kbps depending on the protocol being used. If the wireless LAN is being used for other things such as back up STL service, access to various GUI’s, etc then the total bandwidth of all those services need to be considered as well. Do not forget ethernet broadcast traffic such as DHCP requests, ARP, SMB, etc which can also take up a fair amount of bandwidth.

For LAN extensions, I have been using a variety of equipment. The older Moseley 900 MHz LAN links still work, but are slow in general. The Ubiquiti gear has proven to be both inexpensive yet reliable, a rarity to be sure. There are several links to various transmitter sites running on various types of Ubiquiti gear, usually without problem. One simply needs to remember to log into the web interface once in a while and make sure that both ends have all the firmware updates installed. They are cheap enough that a couple of spares can be kept on the shelf.

The following diagram shows how I replaced all of the copper pots lines at various transmitter sites with VOIP:

Diagram of LAN extensions to various transmitter sites

List of equipment:

Nomenclature	Amount	Use	New or used
Ubiquiti Rocket M5	3	AP and station units	New
Ubiquiti AirMax 5G-2090 90 degree sector antenna	1	AP point to multi-point antenna	New
Ubiquiti Rocket Dish 5G-30	2	Station antennas	New
Ubiquiti ETH-SP-G2	3	Lightning protection	New
Trastector ALPU PTP INJ	6	Lightning protection out door units	New
Cambium PTP-250	2	Point to Point link	Existing/Used
Motorola Canopy 900DA PCDD	1	AP point to multi point	Existing/Used
Motorola Canopy 900DA PCDD	2	Station	Existing/Used
Microwave Filter #18486 diplexer	3	Diplexer 900 MHz ISM band and 944-952 STL band	Existing/Used
Cisco SPA122 ATA	9	Dial tone for remote controls	New

The main studio location has the gateway to the outside world. This system is on a separate subnet from the automation and office networks. From that location a point-to-multipoint system connects to the three closest transmitter sites. This setup uses the Ubiquiti Rocket M5’s with various antenna configurations. Then, from one FM transmitter site, there is an existing 5.8 GHz path to another set of transmitter sites. This uses Cambium PTP-250s.

The next hop rides on the STL system, using Motorola Canopy 900 MHz radios and Microwave Filter Company #18486 dilpexers. These are long paths and the 900 MHz systems work well enough for this purpose. The main cost savings comes from reusing the existing STL system antennas which negates the cost of tower crews to put up new antennas and or rent on the tower for another antenna.

There is a smaller sub system many miles away that is connected to the outside world through the cable company at the AM transmitter site. Unfortunately, due to the distances between the main studio and those three stations, there was no line of site shots to these sites available on any frequency.

When installing the 5.8 GHz systems, I made sure to use the UV rated, shielded cable, shielded RJ-45 connectors and Lightning Protection Units (LPUs). Short cuts taken when installing this equipment eventually come back in the form of downed links and radio heads destroyed by lightning.

Regardless, I was able to eliminate seven POTS phone lines plus extended dial tone service to two sites that previously did not have it before. In addition to that, all of the transmitter sites now have Internet access, which can be useful for other reasons. All in all, the cost savings is about $310.00 per month or $3,720.00 per year.