AM Towers – Engineering Radio

WKIP; removal of the taller tower

This is the original tower for WKIP, but not the original antenna. It was put up circa 1960 or so and like many towers from that era, has hollow legs. Thus, after 60 years or so, it is deteriorating from the inside out.

This was part of a two-tower directional array. It is odd that a class C station on 1,450 KHz would have a directional antenna at all. Even stranger still, it was directional daytime, non-directional night, both at 1,000 watts. The reason for such an odd situation; the station was co-owned with WGNY in Newburgh and the daytime coverage contours would have overlapped without a directional array. The taller tower is 215 degrees tall with top loading. During the daytime, the pattern goes to the North and it covered very well.

Vertical Bridge, the tower owner, decided it was time to replace the aging structure with a monopole. They are completing the project this summer. Our part is to move WKIP to the shorter tower and put up a temporary FM antenna for the translator. Once the project is completed, WKIP will operate from the shorter tower (which is 85 degrees) permanently, getting rid of the now unnecessary directional antenna on a class C channel. The translator antenna will move back to the monopole, once it is put up.

Problems… Yes, we have a few of those…

First, the short tower had a broken guy wire. Actually, the guy wire was fine, but the lowest grip connecting to the equalizing plate was rusted through. It is fortunate that this was discovered because the upper guy wire was getting ready to let go too. Northeast Towers was able to replace all of the grips on that set of guy wires and re-tension the tower. They did a full investigation of all of the other anchors prior to any climbing. This is in a swamp, which has flooded several times over the last few years.

Tower #2, guy wire repaired, Scala FMVMP translator antenna mounted

Next, the temporary FM translator antenna was hung on the tower. It was thought that the 3/8 sample line from the old AM sample system could be used as a temporary transmission line for this system. Unfortunately, that line turned out to be 75-ohm cable TV drop line and was not suitable for transmission of VHF. We had about 600 feet of leftover 3/8 sample line (Cablewave FCC 38-50J) from a decommissioned AM site, so we used that instead. It has quite a bit of loss on VHF, however, for temporary use, it will work.

Next, it seems this black rat snake had taken up residence in the ATU cabinet. The bottom of the ATU was full of mouse nests going back many years. One of our employees dutifully cleaned out the mouse nests unknowingly under the watchful eyes of this snake. Only after he was done, did he see the snake coiled up on the disused current meter shunt. There was a mild freakout for several minutes, but the snake left on his own and we got back to work. The black rat snakes are helpful to have around, but perhaps best if he stays outside of the ATU. We will seal up the entryway for the coax, which seems to be where all the critters are coming in.

This Kintronic Isocoil was mounted to the back of the ATU with unistrut. Even though this is a temporary installation, I have found that sometimes temporary things can last much longer than anticipated. Besides, it was easier than trying to use pressure treated 4 x 4 lumber.

Next, we measured the ATU with the fancy machine (Agilent E5061B network analyzer). In theory, the ATU input should be 50 ohms to match the incoming transmission line. No, instead it was 38 Ohms -j20.

So, a little bit of a retune was required. With the fancy machine, we were able to get it to 52 ohms -j9 or so. This is good enough for now, there will be numerous cranes in the air and the station has an STA to run at 250 watts for the project’s duration. After the new monopole is up, we will measure the base impedance of the tower and tune up the ATU for 50 ohms and then return the station to full power at 1 KW.

Smaller crane, used to assemble the larger cranes

The old tower coming down:

Top section and top loading wires separated

Two cranes were used; one to hold and lower the tower section, the other to lift two tower workers to cut away the sections. The tower was deemed unsafe to climb, therefore it had to be removed like this. It was also unsafe to drop because of the proximity to the studio building and the other tower, which is being retained.

You get the idea. These tower sections and guy wires were cut up and put in a scrap metal dumpster. They will be recycled into something else.

Now, they will work on removing the old tower base and putting up the monopole. Once that is done, we will tune up the AM on the short tower and get it back to full power.

WSM Nashville, Tennessee

We recently went on vacation in Tennessee, which is a great state. Most of the time was spent hiking around various state parks, investigating interesting places, or eating at various restaurants. I highly recommend the state parks, there are many and they are all good. All of that being said, I could not resist the temptation to swing by WSM on the way back to the airport. I am happy I did.

Historical Marker, WSM Brentwood, Tennesee

The site is right off of I-65 and easy to get to. There is a public dog park that is behind it, which is a convenient place to park.

WSM Blaw-Knox tower, transmitter building

The Blaw-Knox tower is impressive. The site is well-maintained overall.

WSM tower base, transmitter building and Aux tower

The main tower is fed with open wire transmission line. The aux tower is off the right. It was nice to stop and walk around the site taking pictures. The Brentwood Police Department even stopped by and welcomed us to the neighborhood.

WSM open wire transmission line, looks like 450 ohms

It is always interesting to stop by some of these more famous stations. It would be nice if more sites were recognized as historical places, given the role that radio played in 20th-century US society. Both of my parents grew up during the Great Depression. According to their stories; life was tough, it was a struggle to feed the family and pay rent, but they did have a radio in the living room which was switched on every night after dinner.

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 60’s, 70’s and 80’s, 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 a little 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.

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.