Installations – Page 7 – 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.

The Broadcast Electronics STX-5 transmitter

Another install, this time a new BE product. I am familiar with the BE FM “C” series transmitter. Those are pretty solid units and we take care of many of them.

This new version of the transmitter looks like it has a little bit of Elenos in its DNA. Perhaps I am wrong about that.

The STXe exciter is an all-purpose analog/digital unit that will do standard FM stereo, hybrid FM +HD radio, HD radio only, DRM+, or FM and DRM+. I like that. It gives the owner lots of options with regard to future planning. Frankly, I would love to see some DRM+ testing done in the US.

We have actually installed a couple of “C” series transmitters with the STXe exciter as well.

The rest of the transmitter consists of four RF amps and an output combiner all in a short rack. Frankly, if I were ordering one of these units, I’d order the taller rack. Not that I am getting old or anything like that, but stooping over to program the date/time, frequency, and power output introduced a slight discomfort in my lower back.

Running into the antenna. At 4.1 KW, 18 watts of reflected power is slightly high. This antenna has always had a little bit of reflected power.

"The chicken coop, " WHUC and WZCR transmitter building — “The chicken coop,” WHUC and WZCR transmitter building

The building I installed this in is nicknamed “The Chicken Coop,” likely because it used to be an actual chicken coop. I am not kidding. The site was originally just the AM station (WHUC). That station had a different transmitter building located some distance away which was fed with open transmission line. This building was put in place sometime around 1969 when the FM station signed on as WHUC-FM (now WZCR). It has seen better days, but we are working on fixing some of the issues with air conditioning and cleanliness.

Remains of open wire transmission line left over from orginal 1947 installation — Remains of open wire transmission line left over from original 1947 installation

The transmitter fired up without any issues and sounds much, much better than the QEI which it replaced.

The QEI transmitter had problems over the years, mostly burned-out resistors in the RF combiner network. It has since been scrapped.

Another FAX 5 install

At the risk of becoming redundant, here are a few pictures of a GatesAir FAX-5 install recently completed in Westerly, RI. This was installed in a recently vacated Verizon cell site next to the old transmitter building. The old transmitter building and the equipment contained therein had seen better days, to be sure.

UPDATE:

As requested, the only pre-installation photo I can find:

That photo was taken back in October 2018, when we first looked at the Verizon shelter as a viable alternative to the current transmitter site.

FAX-5 transmitter with fancy logo, placed in position

Transmitter in place, AC mains and RF connections made

Test mode, clamp-on AC current meter, measuring amps per leg at full power

FAX-5 transmitter and equipment rack, on the air

Transmission line, supported by unistrut

Delta coax switch and Electro impulse dummy load, salvaged from old installation

FAX-5 running into antenna for the first time

Overall, the transmitter sounds great. Much better than the old unit which had an AM noise problem.

If it wasn’t so far away, this would have been a pretty easy project. There were minor miscues along the way that added up. I will say that I learned a few good life lessons about the reliability and responsibility of people.