Anatomy Of A Megawatt Upgrade

KMSS lights up a new liquid-cooled transmitter system with just 30 days to TV’s most-watched event.

Recent transmission system upgrades in the United States have largely been driven through the spectrum repack, but these stations don’t cover the entire picture. The last time many TV stations not part of the repack installed a new transmitter was 15 to 20 years ago, for the DTV transition. That reality created an unsatisfactory situation for many TV stations that simply needed a new transmitter.

This was the case for KMSS-DT, a Marshall Broadcast Group-owned FOX affiliate serving the Shreveport, Louisiana market. KMSS was operating a 15-year old Acrodyne transmitter that had performed well over the years, but the stability of the single-tube model was quickly fading. Power reductions were often required to maintain reliability. This allowed KMSS to maintain video quality, but reduced signal coverage.

KMSS is a very high-power UHF station, transmitting at 1 Megawatt ERP on Channel 34. While power reductions allowed KMSS to reach most of the population, the station was no longer providing consistent coverage through its entire licensed contour. As the Acrodyne’s performance and reliability were fading, maintenance needs were escalating in the fight to optimize performance. Pluria Marshall Jr., MBG’s CEO, was not optimistic about the future as he considered the situation just before Christmas 2019. As their consultant, I was asked to consider their options, and what would be required for each.

Re-tube or Upgrade?

Concern grew as the highly-anticipated FOX televised Super Bowl LIV broadcast loomed. There were only two options to ensure that Shreveport-area viewers could clearly receive the event over the air, the first of which was a replacement tube. While the appropriate tube was available, there were few qualified professionals available to install and service the tube and cavity assemblies. Those that were contacted had legitimate concerns about guaranteeing reliable performance for such a high-ticket event, and very few were able to respond within a 30-day window.

The second option of a broader system upgrade with a new transmitter was deemed more desirable. This would not only ensure a reliable, market-wide broadcast for the most-watched TV event of the year – it would also bring KMSS into a modern RF environment with a high-efficiency, solid-state transmitter.

There was another problem to consider, and that was the limited number of high-power transmission vendors serving US broadcasters. The most reliable vendors in the mix had deadline-driven repack commitments to manage, with limited inventory available. Which supplier, if any, could deliver a non-repack transmitter under the 30-day deadline?

We spoke with all the vendors that could deliver a 30kW (TPO) liquid-cooled transmitter along with a mask filter with essentially no success. The latter was the bigger problem, as mask filters are made-to-order systems procured directly from antenna and RF suppliers. These companies had repack commitments of their own, with even less capacity to produce a new mask filter so quickly. The only way to address this was to determine if the existing RF system and mask filter at KMSS could be used.

The unique attributes of KMSS’s existing filter included a combined system to support UHF digital channel 34 and a previous analog channel UHF 33. The system was specially built to combine and route both signals into a common filter to the antenna without a switch. Conversations with most transmitter vendors led to dead-ends, either because they could not turn around a new transmitter in time or work with the existing mask filter.

The GatesAir Maxiva ULXTE-50 easily fit into the space planned.

The GatesAir Maxiva ULXTE-50 easily fit into the space planned.

One day after Christmas, one vendor, GatesAir, confirmed they could handle both challenges. The transmitter, a Maxiva ULXTE-50 UHF liquid-cooled model, was delivered on Friday, January 17. That gave us two weeks to prepare the site and bring the new system online.

Site Preparation

With little time to spare, we quickly brought Bo Hoover and his team at Technical Services Group into the project for installation. In advance of their arrival, we worked with KMSS engineer Steve Henry to clear space and prepare for installation. This included removing a large, HVAC unit that made space for the new transmitter available.

The removal of the old air handling unit was a blessing, as it was positioned directly behind where the ULXTE-50 was to be installed. The power density of the ULXTE-50 is particularly impressive, and it easily fit into the space we planned. The proximity to the air handler would have created a mechanical interference issue with the wiring trays and coolant lines that came into the facility from the outside heat exchangers.

A local electrical contractor was hired to upgrade the electrical system, add new breaker panels, and install a new surge suppressor supplied with the transmitter. A step-down transformer was added for the ULXTE-50, which operates at 240 volts (the previous Acrodyne operated at 480 volts). Steve moved an existing waveguide switch and dummy load to the input of the RF system, which helped the TSG team move straight into the installation phase while keeping the existing Acrodyne transmitter on the air. 

As a result, the transmitter cabinets, dummy load, and indoor liquid pumps were all arranged in a standard, space-saving layout, although we positioned the reject load further away due to limited real estate inside the building. The previously-installed waveguide switch was already in place to connect the dummy load with its heat exchanger.

Hiccup

Naturally, there is always a hiccup along the way, when we realized that a six-inch coax to waveguide connection tuned for Channel 34 was required to make the system work. The TSG team retrieved one from a recently decommissioned Channel 34 system. That solved what would have been a major problem due to a long manufacturing lead time for the coax-to-waveguide connection. This piece was quickly added along with thermal interlocks for the heat exchangers and the associated control systems, and soon the system was ready for testing.

We also reconsidered the usual ceiling-mount positioning of the dummy load, as there was a substantial amount of RF coax in the ceiling from an existing transmitter. (KMSS shares a transmitter facility with KSHV-DT.) We decided to mount the new dummy load adjacent to the existing transmitter’s dummy load. The location required three additional elbows to cleanly run RF, without any complicated layouts. 

The liquid-cooling system included outdoor heat exchangers that manage the heat transfer outside the building,

The liquid-cooling system included outdoor heat exchangers that manage the heat transfer outside the building,

Plenty of Room, Power and Time

Beyond its exceptional energy efficiency at high-power, the liquid-cooled ULXTE-50 also provided the greatest power density of any transmitter we evaluated. While KMSS has an official TPO of 25.2kW, we wanted something with a little more headroom. The ULXTE-40 would have supported the required TPO, but no more. There was also concern that insertion loss from the existing mask filter would ultimately reduce power output to below allotment. The ULXTE-50 solved this problem, supporting 32kW of power. That would provide plenty of headroom to overcome insertion loss as well as potential RF amplifier failures in the future.

The ULXTE cabinets were installed by Monday the 20th, and the indoor associate pump modules, low-pass band filters and outdoor heat exchangers were in place the following day. The heat exchangers were stacked next to the KSHV heat exchangers but were compact enough to share the concrete pad with plenty of separation. This ensured that the transmitters would not compete for air or otherwise blow air into each other’s systems. A traditional complement of well-positioned peripheral cables and hoses between all the units ensured a clean operation for both stations.

Since the AC transformer was not available until Tuesday, the local electrical contractors returned to the site to install that system on Wednesday. The system was fully connected at that point, with primary power lit up to gradually powerup the transmitter. That gradual commissioning process ensured that all circuits and wiring were correctly installed.

The transmitter commissioning was straightforward. We tested during the daytime but without the mask filter, and while the existing transmitter was on the air. There was enough electrical capability in the AC panels to run the Acrodyne and the dual-cabinet ULXTE-50 system, and run the latter straight into the dummy load using the RF switch on the input to the RF system. If not possible, we would have had to test one cabinet at a time or reduce the power of the Acrodyne transmitter that was still serving viewers.

Ready for Prime Time

The procedure of bringing the ULXTE-50 to air had few but significant challenges. We first ensured that each cabinet was functional and ran the combined cabinet power into the dummy load. Late one evening, we switched and tested the transmitter into the RF filter and made the necessary tuning adjustments.

The output of the mask filter goes directly into waveguide, which goes directly to the antenna. There would typically not be a switch there to switch the output to the dummy load. That meant that there was no capability to add directional couplers to feed samples to the ULXTE-50 and make pre-corrections.

We instead took the same samples provided to the Acrodyne transmitter and ran a separate line to the ULXTE-50. Once the initial tests were completed, the transmitter was routed into the RF system late on Friday evening. The necessary pre-corrections were performed, and the transmitter was tested into the antenna. The matches proved optimal with no tweaks necessary. We confirmed that the transmitter was operating well within its defined parameters and could easily make power.

Those tests were repeated for a few days without incident, and then repeated using the generator as the station power source. Once the interlocks were double-checked and confirmed as functioning properly, we were confident that the transmitter would be protected when the site was vacated. The ULXTE-50 was placed into operation on January 27th, less than one week before the Super Bowl.

Long-Term Viability

The transition from tube to solid-state technology brings long-term benefits from a labor perspective. The layout inside the cabinet provides plenty of room to work, and the amplifier modules and power supplies are accessible and easy to replace. There are five power blocks inside each cabinet, and these can be configured as five independent power blocks. We chose a streamlined implementation that uses a single, dedicated power connection for each cabinet. That configuration further simplified the installation process.

Spillage is a common concern with liquid-cooled transmitters, as it can create a massive cleanup effort. The ULXTE-50 design includes a redundant pump system inside the cabinet that all but eliminates that possibility. It’s yet another important point of reliability that comes from a thoughtful design.

There is also no longer that back-of-mind concern regarding an overnight trip to the transmitter site, and being woken from a peaceful sleep. If an amplifier fails, the transmitter keeps operating at maximum power, and can be hot-swapped while still on the air. Finally, monitoring the transmitter can be achieved over IP, using remote connectivity to the transmitter versus having to digest health and status readings on-site.

The transmitter has now been on the air for four months, and coverage has improved significantly. The ULXTE-50 runs at full power and covers the entire market, versus the 80-percent threshold KMSS was typically powered to with the tube transmitter. The integrated GatesAir Maxiva XTE exciter is stable and robust, and it is also ready for an ATSC 3.0 future along with KMSS’ elliptically polarized 27-bay ERI antenna.

Collectively, KMSS has transitioned into a new era of over-the-air efficiency with long-term viability, exceptional performance, and cost-reducing benefits through the entire RF infrastructure.

Best is President of Greg Best Consulting

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