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 with out 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 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
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 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
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 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.

A few pictures

Some things I have been working on lately:

A nice row of transmitters
A nice row of transmitters

Finishing up a transmitter site rehab.  The BE FM20T is nearly 20 years old.  The BE FM2C transmitters are new.  There is also a rack of new fiber equipment and CODECs.  This site has good utilization; there are three stations on one tower with a shared STL antenna and generator.

Energy Onix ECO-6
Energy Onix ECO-6

Energy Onix ECO-6 tube type transmitter.  One of Bernie’s better designs, a grounded grid tube with solid state driver section.  This one needed some fans replaced and a new tube.

AM transmitter site.  Looks like these vines have not been cut in a couple of years.
AM transmitter site. Looks like these vines have not been cut in a couple of years.

I wonder how much the guy tensions have changed…

Noticed this after some particularly strong thunderstorms
Noticed this after some particularly strong thunderstorms

The reason why you do not use a POTS line phone during a thunderstorm.

USS Slater radio room
USS Slater radio room

I took a tour of the USS Slater, a museum ship in Albany, NY.  The museum has painstakingly restored the ship to its WWII configuration.  The main transmitter is the RCA TBL-8 seen in the left/center of this picture.  This unit put out 200 to 400 watts CW or 150 watts AM phone.  During the hostilities it was turned off as allied ships observed radio silence unless they were sinking (and sometimes even then).

A little ChiFi tube type RIAA phone preamp.
A little ChiFi tube type RIAA phone preamp.

I have been fooling around with this little 6AK5 preamp.  I find it works very well and sounds better than the built in phone preamp on my Kenwood VR-309.  The FU-29 tube amp did not come with a phone preamp.

This is a short video clip of an audio processor at one of our transmitter sites. The fancy lights around the control knob are designed for the program director. They are saying “Buy me… Buy me…”

Status of AM revitalization

It has been about five years since the AM revitalization initiative was first proposed by the FCC and about five years since the first rules changes took place.  Those rules changes included:

  1.  FM translators for AM stations
  2. Allowing stations to use MDCL (Modulation Dependent Carrier Level)
  3. Changing some of the antenna radiation efficiencies requirements
  4. Changing some of the allowable interference towards other stations requirements
  5. Loosening some rules regarding proofs, MOM, night time coverage over city of license, etc

Things that were not addressed:

  1. Receiver quality and technical advances
  2. Ambient noise levels on Medium Frequency (among other) bands
  3. HD Radio or any other digital modulation scheme

Things that were discussed then changed subsequently as a separate initiative:

  1. The main studio rule, which was eliminated for all broadcasting stations

What has been the net effect of these changes?  Has any of this revitalized AM radio?  The net effect has been approximately more of the same.  There have been many stations that have applied for and received licenses for FM translators.  Those stations, in most cases that I am aware of, receive some benefit of extra revenue because of this.  Stations with carrier power levels of 10-50 KW have taken advantage of MDCL technology to save some money on their electric bill.  Nothing wrong with that.

For stations that use a directional antenna, proofs of performance and other DA matters with the FCC have become slightly easier.  Medium Frequency (MF) directional antennas are very large, require a lot of land, are expensive to build, license and maintain.  I know of several stations which have downgraded from a class B station with a directional antenna to a class D station with a single tower and greatly reduced night time power.   Those downgraded stations certainly benefit from an FM translator.

I have heard from more than one AM station owner who says after four years, they are going to “turn in their AM license and just keep the FM.”  I am sure that they are not informed regarding translator rules.  Perhaps, however, the FCC will allow this in the future; a sort of back door commercial low power FM station classification.

The AM band zenith occurred in November of 1991, when there where 4990 licensed AM stations in the United States.  As of June 30, 2018, the total stands at 4633.  That is a decline of 357 stations.  There are currently 90 AM stations listed as silent.  That represents a decline of approximately 9 percent or less than 1/2 of one percent per year.

The last number of AM stations actually transmitting HD Radio that I found was approximately 110, which differs from the iBiquity (and FCC) number of 240.  The FCC data base includes stations which are currently dark, or stations which where transmitting HD Radio at one time but have since turned it off.  Either way, it is a small percentage of licensed stations.  As of this time, AM HD Radio appears to be a non-starter.  In other parts of the world, Medium Frequency DRM seems to be doing well.  The difference seems to be that the DRM operation is all digital and the digital carriers have a much higher power level than that of the hybrid AM HD Radio being used here.

Of those 4633 standard broadcast stations, approximately 260 belong to iHeart radio, Cumulus owns approximately 120 and Townsquare owns approximately 80.   That accounts for 460 stations.  The remaining 4000 or so stations currently on the air are owned by medium sized corporations or individual owners.  The reason for the distinction; I have noticed that the large corporate owners tend to concentrate resources and effort on those licenses that will make the best return, e.g. FM stations.  Of course, there are a few exceptions to that trend, often in major markets.

Of those 4000 or so remaining AM stations, most seem to be treading water.  They are making enough money to stay on the air.  There are a few AM stations that are doing remarkably well.  Those are the ones with primarily  local content.  The vast majority of AM stations are running some type of syndicated talk.  News/talk and sports radio are the two most common formats.  Conservative news/talk seems to be the bread and butter.  Liberal news talk has been tried, but none have succeeded.

Last May, the Supreme Court overturned the Professional and Amateur Sports Protection Act of 1992.  That federal law prevented gambling on outcomes of professional and college sports games.  With the overturn of that rule, individual states can now legalize sports betting.  It will be interesting to see what states allow legalized sports gambling and whether that has any effect on the various sports radio formats.  I can see where individuals and odds makers may want to get good inside information regarding team dynamics and so on.  The sports network that can furnish such information may be in a good position to carve out a niche.

Music can and does sound good on AM when it is done correctly.  There is a great misconception that AM fidelity is poor.  That is not necessarily so.  There are a good many AM receivers these days which have much better bandwidth than the previous generation receivers.  I am noticing that car radios in particular sound much better.  Yes, there are still problems with electrical noise and night time interference.  There are still technological improvements that can be made for analog AM on the receiver side.

In summary; the revitalization efforts have benefited some AM stations in some areas.  The truth is, that many AM stations have been let go for so long that there is no saving them.  Other AM stations that are still viable are making a go of it.  In nautical terms; there is six feet of water in the hold, the pumps are working and the ship is not sinking… for now.

The inglorious task of AM antenna array maintenance

AM radio stations are rough customers. They frequently operate on the margins, both in terms of ratings and revenue. Their transmitter plants are complex and very often have been on a reduced maintenance schedule for years, sometimes decades.  Those of us that understand the operation of AM transmitter plants and all their quirky behaviours are getting older.  I myself, feel less inclined to drop everything and run off to the AM transmitter site when things go awry.  Seldom are such efforts rewarded, much less acknowledged.  Station owners are also finding that their previous demands are unrealistic.  For example, time was that any work that takes the station off the air had to be done after midnight.  These days, I can tell you, I will not be working at your radio station after midnight.  You can find somebody else to do that work.

Thus, today, we took this particular AM station off the air from Noon until 3 pm to diagnose and repair a problem with the four tower daytime array.  Once again, this involved a shift in common point impedance and a drastic change in one tower’s current ratios.

Antenna Tuning Unit, mice have made a mess
Antenna Tuning Unit, mice have made a mess

In all fairness to the current owner, this ATU reflects years of neglect. At some point, mice made a home in here and created a mess. The ATU smells of mouse shit, piss and mothballs.  It is full of mouse droppings, grass seeds and fur.  All of the ATUs in this array are in similar condition.

Paper wasp, inside ATU
Paper wasp, inside ATU

It was warm enough that the wasps were active, if not a little bit lethargic.

Broken stand off insulators in ATU
Broken stand off insulators in ATU

This coil is being held up by the tubing that connects it to other components. When the ATU was built, no nylon or cork bushings were used between the insulators and the wall of the ATU they were mounted on. Heat cycling eventually did all of the insulators in.

Catwalk to the other towers
Catwalk to the other towers

Catwalks to the other towers. At least the swamp grass has been cut this year, it is only four feet tall instead of ten.

Tower base
Tower base

The tower bases are all elevated above the theoretical maximum water level. The ATUs are also up on stands with platforms build for maintenance access.

ATU Work "platform"
ATU Work “platform”

I cannot even blame the current owner, who has spend considerable money to make repairs and upgrades to this site. It is very difficult and very expensive to catch up with deferred maintenance. Sadly, most AM stations we encounter have similar or worse problems.

I think it is too late to save many of these AM stations.  The technical issues, lack of revenue, perceived poor quality, lack of good programming are all taking their toll.  At this point, the hole is so deep there is no hope of ever getting out.  The FCC’s faux interest in “revitalization” followed by two years of stony indifference seems to be a final, cruel joke.