AM Towers – Page 2 – Engineering Radio

More AM work

I have been working on an AM station lately. WBNR signed on in 1959 and follows the now familiar AM trajectory; after making bank in the ’60s, ’70s, and ’80s, revenue declined, maintenance deferred, yada, yada, yada…

After a stint with a news-talk format, the station changed to “Real Country,” a few years ago. WAT! Music on the AM? Actually, it is doing quite well. The perception is that AM sounds terrible and nobody listens to it. The stock AM radio in my Subaru (made by Pioneer) sounds pretty good on AM. I have noticed that when I first tune a station in, it sounds narrow-banded, slightly better than a telephone. However, after a second or two, the bandwidth opens up and it can sound quite good. I have also heard this station playing at several local businesses. When we turn it off to do maintenance, the phone starts ringing. Clearly, somebody is listening…

This station is part of a three-station simulcast. The AM station to the north got rid of its directional antenna and added an FM translator a few years ago. That has made a big difference. Thus a translator was acquired for this station as well.

The translator was held up by an informal objection filed by Prometheus, Et. Al. as part of a blanket filing against all new translator licenses by the LPFM advocate. In any case, the Construction Permit has been on hand for a while, so the owner felt it was time to move forward with building out the new FM signal.

Installing the single-bay Shively 6812 antenna on the side of one of the nighttime towers triggered some other things. A bit of the deferred maintenance was addressed; new stockade fences around all the towers replaced the original fences put up in 1988. Those original fences were falling down.

4 Tower antenna system, WBNR, Beacon, NY

The antenna system for WBNR is actually quite elegant, perhaps even beautiful. A simple two-tower system for the daytime array and a separate two-tower system for the nighttime array. The nighttime towers are top-loaded, adding about 30.7 degrees in electrical height.

The CP for the translator required some extra steps because of the mounting on the night tower of the AM array. Before and after impedance measurements need to be taken on the tower in question. Another requirement of the CP, is a set of before and after monitor points need to be taken.

WBNR tower, with translator antenna side mounted at 390 feet AGL — WBNR tower, with translator antenna side mounted at 381 feet (116 Meters) AGL

While I was measuring the base impedance, I decided to measure all the towers instead of just the nighttime tower that has the translator antenna mounted on it. This is a good point of reference if any problems arise in the future. Often, this information can be found in the technical paperwork from the original license application. Those files can be a treasure trove of information. Unfortunately, it appears that a good portion of the original paperwork is missing.

The Phasor and ATUs are a late 80’s Harris product. They are actually in remarkable shape, all things considered. All of the RF contactors are Harris HS-4P motor-driven units. They are rated at 30 Amps, RF-RMS. I don’t think that they are supported by GatesAir. I have a small stock of spare finger stock and contact bars. I suppose, if I had to, I could make or adapt parts to repair.

Looking at the base currents and the base current ratios for both the day and night patterns (base current ratios are on the station license), the tower impedance has changed very little over thirty years. That is good news, especially with those 215-degree-tall nighttime towers.

The WBNR license application did contain an overall system diagram showing the Phasor and all the ATUs. It did not contain any component ids or other information. I scanned that in, created a vector graphics file, and expanded it to a 24 x 36 inch size. I was able to fit all the component values and other information on the diagram.

Schematic diagram WBNR day/night antenna systems

The other issue is the monitor point descriptions. They include statements such as “Point is marked with yellow and white paint on a tree,” or “In the northeast corner of the Texaco research facility parking lot.” Those references are long gone and I would prefer to use a set of GPS coordinates. Using the topographical maps from the proofs, I found each monitor point and then recorded a set of GPS coordinates for each. In the future, they will be much easier to find. If anyone is still doing monitor points, I would recommend this method.

Yet another problem; the phasor control system was damaged by lightning. The overly complicated Harris Phasor control card was replaced with something more straightforward and reliable. I designed a simple set of relays, one for daytime and one for nighttime, to change the antenna system over. The transmitter interlock goes through the relay contacts, so the transmitter PDM is killed while the power changes. Tally back from each of the towers is handled by a set of relays for each pattern, which is also interlocked with the transmitter. All of this prevents the RF contactors from switching hot, something that has caused some damage in the past.

W243EM is 100 ERP watts, non-directional with a 1 bay Shively 6812-1R antenna installed at 381 feet (116 Meters) AGL on one of the nighttime towers.

The transmitter is a BW Broadcast TXT-600. The power calculation is as follows:

ERP 100 Watts = 50 dBm

System gains and losses:

Transmission Line loss, 500 feet (152.4 Meters), RFS LCF78-50JA = -1.75 dB

Isocoupler loss, Kintronic ISO-170-FM = -0.8 dB

Antenna gain, Shively 6812-1R = -3.39dB

Total system losses and gains: -5.94 dB

TPO: 55.94 dB or 393 Watts

With all that work completed, the license application was filed to cover the construction permit. Once that was accepted by the FCC, program test authority was granted and the transmitter was turned on. Hopefully, with the translator on the air, the perceptions regarding listeners will change and the station can bill more.

I really enjoy working on Medium Frequency antenna systems. I don’t know why, but antenna systems in general are always fascinating to me.

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:

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.

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 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

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 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

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 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.

A few pictures

Some things I have been working on lately:

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 tube-type transmitter. One of Bernie’s better designs; a grounded grid tube with a 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.

I wonder how much the guy tensions have changed…

Noticed this after some particularly strong thunderstorms

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

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:

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

Things that were not addressed:

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

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

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 nighttime 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 were 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 database includes stations that are currently dark or stations that were 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 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 that 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 nighttime 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.