Field Report: New Rohde & Schwarz Transmitter Powers Fox 7 KTBC

Fox-owned KTBC in Austin TX recently needed to replace an old TV transmitter that had outlived its manufacturer. In addition to better stability and reliability, KTBC’s new high-efficiency Rohde & Schwarz TV transmitter immediately shaved thousands of dollars off the station’s electrical bills.

Next to battery-powered wall clocks, many broadcast engineers would likely say that one of the most reliable electromechanical devices at a TV station is the transmitter. Engineers love to care for broadcast transmitters, and with good care they can modulate and transmit a good signal for decades. The problem with aging TV transmitters is that many have outlived their manufacturers and technology marches forward.

When Fox-owned KTBC in Austin TX signed on its first DTV transmitter, the station purchased an ample inventory of replacement parts for its new channel 7 Axcera. When Axcera stopped making DTV transmitters, the station’s transmitter spare parts inventory began to shrink. Over time, critical parts were becoming more difficult to find and buy. The station began to recognize the risks of not replacing its Axcera. According to KTBC Chief Engineer David Andrews, “The Axcera TPO was once 12 KW, but it could only make 10.5 KW, right at the edge, almost full out.”

KTBC transmits on channel 7 and is not affected by repack. It was simply time for a new transmitter. Research of candidate transmitters began last year. Part of the plan was to get a new transmitter installed ahead of repack. Rohde & Schwarz was selected for a variety of reasons. The company’s stable reputation in the broadcast market was a major factor.

A new Rohde & Schwarz liquid-cooled THV9 was ordered and it was installed and on-air a week before this year’s Super Bowl, broadcast on Fox.

Instant savings

The Axcera consists of 4 cabinets with a giant blower unit for each cabinet. The new R&S transmitter is all in a single cabinet. The 12 KW Axcera is air-cooled and needs 3 air conditioning units to support it. The new R&S THV9 is liquid-cooled with 16KW TPO, and the air conditioning only runs about half-time to maintain the building temperature.

The Rohde & Schwarz Liquid-cooled PA modules proved much less expensive to cool.

The Rohde & Schwarz Liquid-cooled PA modules proved much less expensive to cool.

According to Andrews, the new Rohde & Schwarz transmitter is on track to save the station approximately $29,000/year in power expenses. Because of its high efficiency, the THV9 dissipates so much less heat even its heat exchangers are smaller than usual. The only fans in the liquid-cooled THV9 cabinet are in its TCE900 exciter.

The R&S PHU901 power amplifiers at the core of the transmitter are built with state-of-the-art, LDMOS 50 V power transistors. A matching circuit ensures stable and efficient operation. The RF coupling network after the transistors is of discrete design and features minimal attenuation, which also contributes to increased efficiency. The power combiners and harmonics filters were also developed with the focus on reducing attenuation.

In addition to careful hardware optimization, new signal processing and system control concepts further boost efficiency. Specially developed power supplies allow the transmitter control unit to adapt the transistor supply voltage to requirements. This capability and the built-in, automatic, adaptive pre-correction all serve to further boost transmitter efficiency.

Being a VHF station, KTBC enjoys better transmitter efficiency than UHF stations, even when operating in the normal mode. “We did not purchase the Doherty option as the basic transmitter design is 20 to 30 percent more efficient than our current Axcera transmitter,” Andrews said.

Nothing but the input changed on KTBC's tower and antenna.

Nothing but the input changed on KTBC's tower and antenna.

Installation

The KTBC transmitter and tower is located on Mt. Larson, on the edge of hill country northwest of Austin, about ½ hour drive from KTBC studios. The RF system uses a circularly polarized ERI antenna with 1.75° beam tilt, broadcasting a VHF channel 7 signal from nearly 2000’ above sea level (ASL) atop a 1029’ tower.

KTBC used R&S services to install the transmitter. “The transmitter building is about 1000 square feet, and a radio station leases some space in it,” Andrews stated. Because space was not an issue, the new and old transmitters could sit side-by-side. “All they needed was power waiting,” Andrews said. From that point onward, “the installation went absolutely great.”

Installing a new RF patch panel required RF plumbing talent.

Installing a new RF patch panel required RF plumbing talent.

Installation and commissioning was completed in a matter of days. Much of that time was spent plumbing-in a new RF patch panel to keep the old transmitter available for back-up use.

The original 6” rigid transmission line and antenna remain in use.

The new RF patch panel was installed to remotely switch the antenna and dummy load between the old transmitter, now the standby transmitter, and the new R&S transmitter.

Operation

Switching the RF system is one of the many functions of KTBC’s Statmon transmitter monitoring and remote control system, which the station has used for several years. The main Statmon screen is in Master Control and it controls and reports the status of the transmitter, RF system and building. “There are alarms for everything,” Andrews explained. “It’s not something an operator would normally watch.” A R&S TCE900 transmitter control unit monitors the transmitter and establishes a connection to Master Control via a web interface. It also monitors and controls the transmitter’s central cooling system with pump unit and heat exchanger.

Rohde & Schwarz used field experience from numerous sources in the development of the operating concept for its new transmitter generation that includes the THV9. As a result, the TVE900 transmitter control unit displays the system and its status in a clear, easy to learn manner.

The only factors the new transmitter introduced to Master Control operators was the new TCE900 screen in Master Control and a new RF patch panel control function on the Statmon. Operators received hands-on remote patch panel training and participated in a pre-planned full live rehearsal on the weekend before the more permanent transmitter change-over took place.

ATSC 3.0?

Neither KTBC nor FOX TV has announced any ATSC 3.0 plans, and Andrews reported ATSC 3.0 was not a factor in the decision-making process. However, the station’s new THV9 is ATSC 3.0 ready. It has more TPO headroom than the Axcera it replaced, and its TCE900 exciter is field upgradable. Is that a coincidence or smart planning? Stay tuned to Fox 7 Austin and find out.

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