Installations – Page 7 – 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:

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.

A tale of five signals

I am currently finishing an interesting project involving putting up two translators on a diplexed AM tower which also holds a mobile phone/data tenant as well. All-in-all, this seems to be a very efficient use of vertical real estate.

The AM stations are WMML and WENU in Glens Falls, NY. The AM stations are diplexed using a Phasetek diplexor/ATU.

Diagram showing WENU/WMML tower with W250CC/W245DA antenna installed

Diplexor diagram, WENU/WMML Glens Falls, NY

The translators are W250CC and W245DA which are using a NICOM BKG-77/2 two bay 3/4 wave spaced antenna mounted at 53 meters AGL. The translators use a Shively 2640-04/2 filter/diplexor which is a broadband input port in addition to the translator input ports. Since these translator signals are only 1 MHz apart, the higher-power Shively filter was installed because it has better rejection characteristics. The broadband input port allows the NICOM antenna to be used as a backup for any of the three FM stations; WKBE 107.1, WNYQ 101.7, or WFFG 100.3. Two transmitter sites for those stations are mountaintop locations which are very difficult to get to in the wintertime. Having a backup site available takes some of the pressure off during storms or other emergencies.

Shively 2640 -04/2 filter for W250CC and W245DA

The NICOM FM antenna was mounted on the tower when W250CC went on the air in October 2016. When it was installed, the base impedances for both AM stations were measured. For some reason, WENU 1410 KHz seems to be more sensitive to any changes on the tower, thus the WENU ATU needed a slight touch-up. When working on diplexed AM systems, it is also important to make sure that both trap filters, which are parallel resonant LC circuits, are tuned for maximum rejection of the other signal. During this particular installation, nothing was added to the tower and no change in the base impedance for either station was noted.

Shively Filter, connected to transmitters and antenna

As a condition of the construction permit, measurement of spurious emissions of all stations sharing the common antenna needed to be completed to ensure compliance with FCC 73.317(b) and 73.317(d). I made careful measurements of the potential intermod products between the two translator frequencies. This measurement was completed with my TTI PSA6005 spectrum analyzer.

The primary concern here is mixing products between the two transmitters. Both transmitters are BW TXT-600 with low pass filters before the output connector. There are three frequencies of interest;

(F1 – F2) + F1 or (97.9 MHz – 96.9 MHz ) + 97.9 MHz = 98.9 MHz
F2 – (F1 – F2) or 96.9 MHz – (97.9 MHz – 96.9 MHz) = 95.9 MHz
F2 + F1 or 97.9 MHz + 96.9 MHz = 194.8 MHz

That, plus harmonic measurements out to seven or eight harmonics of the fundamental frequency should be enough to demonstrate compliance with FCC out-of-band emissions standards. Being that this site has LTE carriers, it is very important to measure the harmonics in those bands. Mobile data systems often use receiver pre-amps, which can amplify harmonics from the FM band and make them look out of compliance. Having a base set of readings to fall back on is always the best course in case the “out of tolerance” condition gets reported to the FCC.

Measurements on these frequencies must meet the emissions standards outlined in FCC 73.317 (d), which states:

Any emission appearing on a frequency removed from the carrier by more than 600 kHz must be attenuated at least 43 + 10 Log¹⁰ (Power, in watts) dB below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.

Harmonic frequencies to be measured:

Harmonics for 96.9 MHz fundamental	Harmonics for 97.9 MHz fundamental	Comments
193.8	195.8
290.7	293.7
387.6	391.6
484.5	489.5
581.4	587.4
678.3*	685.3*	US LTE Band 71
775.2*	783.2*	US LTE Band 5
872.1*	881.1*	US LTE Band 5
969.0	979.0

*Frequencies that fall within the mobile data LTE bands. Traces were recorded and saved for these frequencies.

All of that information, once compiled is attached to the FCC form 350-FM, which, once filed grants Program Test Authority.

BW TXT-600 V2 translator transmitters under test and measurement