Installations – Page 8 – Engineering Radio

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

GatesAir FLX-40 one year in

I was at the WEBE transmitter site recently and took the time to look over the transmitter we installed last year:

GatesAir FLX-40 transmitter, WEBE Bridgeport, CT

Overall, I would say that this transmitter has been very reliable. We had to install a UPS for the exciter and HD Radio exporter, but that is not a big deal. During the first power outage, the exciter went dark first. It took longer for the transmitter controller board to lose power, in the interim the controller turned the transmitter power all the way up. When the generator came online 10 seconds later, the transmitter returned to operation at 41.5 KW. This, in turn, caused one of the other field engineers to freak out and nearly lose his mind (stay away from the brown acid, FYI).

I installed the UPS a few days later.

WEBE TPO 35.3 KW with HD Radio carriers on

The transmitter power output is 35.3 KW, which is getting into the semi-serious range. The reflected power goes up when it gets warm out and goes down in colder weather. Over the winter, it was running about 50 watts. Even at 138 watts, that represents 0.004% reflected power. The TPO forward goes to the 6 bay, 1/2 wave spaced antenna side mounted, 470 feet (143 meters) AGL. The station covers pretty well.

WEBE Pump station, pump is running 2/3 speed and fans are running at about 1/2 speed

Overall, I would give the liquid cooling system an A grade. The transmitter still dumps a fair amount of heat into the room from the RF combiners and PA power supplies. Most of the heat, however, ends up outdoors. Previously, we had two Bard 5-ton AC units running almost full-time. Now, only one AC unit cycles on and off except for the hottest days of the year. The outside temperature when this picture was taken was 81 degrees F (27.2 C).

Next year, we will have to send a sample of the coolant to be analyzed.

I have had good experiences with the GatesAir FLX/FAX series transmitters. I would recommend this to a friend.

The GatesAir FLX-40 transmitter

The GatesAir FLX-40 transmitter is my first liquid cooled transmitter installation. Previously, I have installed an air cooled Nautel NV-40, a V-40 and a couple of BE FM-35T/20T units. The WEBE transmitter site in Bridgeport, Connecticut is an interesting facility.

Smoke Stack, Bridgeport Energy, Bridgeport, CT

This coal fired power plant smoke stack which currently holds up the six bay, half wave spaced Shively antenna. The old BE FM35A transmitters are getting little bit long in the tooth. Thus, we picked one to scrap, the other will be kept for backup service.

Scraping 34 year old BE FM30A transmitter

We saved a whole bunch of parts to keep the other FM35A on the air in backup service.

The power supply cabinet with that 500 pound plate transformer was the last to go.

On second thought, that plate supply transformer is a good spare to have

The FLX-40 came on a large truck. Fortunately, we were able to open the side gate at the power plant and get the truck to the front door of the transmitter building easily. The transmitter consists of two large cabinets, each with two 10 kilowatt power blocks. There is also a pump station and an outdoor heat exchanger.

FLX-40 in place, cabinets bolted together

This transmitter design is based on the Harris digital TV transmitters.

The pump station and heat exchanger are the same systems used for TV transmitters. Liquid cooled units require a bit more planning on the installation end. The coolant piping should have a high spot from which everything else slopes down hill.

I put a 1/4 to 12 inch pitch on everything. Of course, there are several low points, the heat exchanger, pump station and bottom power blocks.

Holding steady at 18 PSI for 24 hours — Holding steady at 16 PSI for 24 hours

After assembling the cooling system, we pressure tested it for 24 hours.

Installation debris in the coolant line strainer

Following that, we flushed the system with distilled water for several hours before we filled it with 40/60 glycol/water mix. Record low temperature in Bridgeport is -7 F (-22 C), thus a 40/60 mix will give protection down to -15 F (-26 C). The more water in the coolant, the better heat transfer capacity it has.

At the highest point in the system, there is a sight glass and an air purge valve

The pump station is controlled by the transmitter, which speeds up the pumps according to how much heat needs to be moved. In turn, the pump station control the fan speed on the heat exchanger outside.

The pump station runs with one motor most of the time. The other pump motor will run in the event of failure or if there is not enough flow through the power blocks. Each of the four power blocks has a flow rate meter on the return line.

Heat Exchanger Fan motor controllers, Variable Frequency Drive modules

Variable Frequency Drive (VFD) fan motor controllers show them running at half speed.

GatesAir 50 KW heat exchanger mounted on concrete pad behind the building. Air flows out from the motor side.

One of several shipping containers with modules and other parts for the FLX-40

As with most things, some assembly required. The RF modules needed to be placed in the power blocks according to their serial numbers on the test data sheet. This insures that the information on the test data sheet matches the installed transmitter configuration. The power combiner between the two cabinets as well as the reject load and directional coupler all need to be installed.

RF modules with large aluminium heat spreaders. Coolant flows through each module.

On the air!

We ran the transmitter for several hours into the antenna yesterday afternoon. The coolant system is still purging air, so we periodically needed to add water/antifreeze to the pump station to keep the pressure between 12-18 PSI. Eventually, the TPO will be 34 KW with the HD carrier(s).

All in all, I would say that this was a fun project. The liquid cooled transmitter had a few extra steps during the installation process, but not too difficult.

Part 101, Private Fixed Microwave Service

I have been tasked with installing one of these systems for a sixteen-channel bi-directional STL. This system was first mentioned here: The 16 channel bi-directional STL system. As some of you pointed out, the unlicensed 5.8 GHz IP WLAN extension was the weak link in this system. It was not an interference issue, however, which was creating the problems. The problem was with layer two transparency in the TCP/IP stack. Something about those Cambium PTP-250s that the Wheatstone Blade hardware did not like and that created all sorts of noise issues in the audio. We installed the Wheatstone Edge Routers, which took care of the noise issue at the cost of latency. It was decided to go ahead and install a licensed link instead of the license-free stuff as a permanent solution.

Thus, a Cambium PTP-820S point-to-point microwave system was purchased and licensed. The coordination and licensing took about three months to complete. We also had to make several changes to our network architecture to accommodate the new system. The PTP-820 series has a mast-mounted radio head, which is the same as the PTP-250 gear. However, for the new system, we used three different ports on the radio to interface with our other equipment instead of the single port PTP-250 system. The first is the power port, which takes 48 VDC via a separate power cable instead of POE. Then there is the traffic port, which uses Multi-Mode fiber. Finally, there is the management port, which is 1GB Ethernet and the only way to get into the web interface. The traffic port creates a completely transparent Ethernet bridge, thus eliminating all of the layer two problems previously encountered. We needed to install fiber transceivers in the Cisco 2900 series switches and get those turned up by the IT wizards in the corporate IT department.

Andrew VLHP-2-11W 11 GHz microwave antenna — Andrew VHLP-2-11W 11 GHz microwave antenna

The radios mount directly to the back of the 24-inch 11 GHz Andrew antenna (VHLP2-11) with a UBR100 interface. The waveguide from the radios is a little bit deceptive looking, but I tried not to overthink this too much. I was careful to use the O ring grease and conductive paste exactly where and when specified. In the end, it all seemed to be right.

Cambium PTP-820S mounted on antenna — Cambium PTP-820S mounted on Andrew antenna

Not wanting to waste time and money, I decided to do a back-to-back test in the conference room to make sure everything worked right and I had adequately familiarized myself with the ins and outs of the web interface on the Cambium PTP-820 radios. Once that was done, it was time to call the tower company.

One side of these is mounted on the studio building roof, which is a leased space. I posted RF warning signs around the antennas because the system ERP is 57.7 dBm, which translates to 590 watts at 11 GHz. I don’t want to fry anybody’s insides, that would be bad. The rooftop installation involved pulling the MM fiber and power cable through a 1 1/4-inch EMT conduit to the roof. Some running back and forth, but not terrible work. I used the existing Ethernet cable for the management port. This will be left disconnected from the switch most of the time.

Cambium PTP-280S 11 GHz licensed microwave mounted on a skirted AM tower

The other side is mounted at about 85 feet AGL on a hot AM tower. I like the use of fiber here, even though the tower is skirted, the AM station runs 5,000 watts during the daytime. We made sure the power cables and Ethernet cables had lighting protectors at the top of the run near the dish and at the bottom of the tower as well as in the transmitter room rack. I know this tower gets struck by lightning often as it is the highest point around for miles.

PTP-820S RSL during aiming process — PTP-820S RSL during the aiming process

Aligning the two dishes was a degree of difficulty greater than the 5.8 GHz units. The path tolerances are very tight, so the dishes on each end needed to be adjusted in small increments until the best signal level was achieved. The tower crew was experienced with this and they started by panning the dish to the side until the first side lobe was found. This ensured that the dish was on the main lobe and we were not chasing our tails. In the end, we achieved a -38 dBm RSL, the path predicted RSL was -36 dBm so close enough. This means the system has a 25 dB fade margin, which should be more than adequate. While were aligning the transmitter site dish, a brief snow squall blew through causing a whiteout and the signal to drop by about 2 dB. It was kind of cool seeing this happen in real-time, however, strangely enough, the tower crew was not impressed by this at all. Odd fellows, those are.

Currently brushing up on FCC part 101 rules, part C and H. It is always good to know the regulatory requirements of any system I am responsible for. As AOIP equipment becomes more mainstream, I see many of these types of installations happening for various clients.