A Practical Guide To RF In Broadcast: Transmitter Plant Planning

Broadcast transmitter facility planning, design and construction… and what an engineering consultant can help with.

Of all the systems in a TV station, the transmitter RF system provides the most opportunities for hardware mishaps that can put a station’s reputation and license at risk in the blink of an eye. The tower could fall. The transmitter could catch fire or drift off-frequency. A RF filter could fail and cause interference with another station or a data carrier. Broadcasting is a trouble magnet, attracting the most unlikely random technical issues occurring at the worst possible moments.

TV transmitter planning, upgrades, and operations usually begin and end with the designated chief operator and acting chief operator because if anything breaks, they must identify the problem and fix it. Significant changes usually involve engineering consultants and contractors and must be approved by a corporate committee and sometimes the FCC, before work can begin. Nearly all RF system changes are initiated by need, by FCC direction, or new technology like ATSC 3.0.

FCC Rule 47 CFR § 73.1580 Transmission System Inspections states: “Each AM, FM, TV and Class A TV station licensee or permittee must conduct periodic complete inspections of the transmitting system and all required monitors to ensure proper station operation.” TV transmitter engineers, chief operators, and chief engineers are expected to be skilled RF technicians and technical FCC rules experts while maintaining 24/7 compliance with federal rules and regulations.

Each TV station is different, and every station RF system is a unique combination of brands, transmission line lengths and sizes, towers, guy wires, antennas, transmitter buildings, HVAC systems, installations, and terrains. This chapter discusses the ubiquitous active and passive hardware devices in the signal flow from the studio to the signal radiated by the antenna on the tower. More specific details as to what exactly will work best in your station’s scenario are best determined by a professional broadcast engineering consultant and manufacturers of items you may need. You may also want a communications attorney to assist with FCC filings. Engineering design and construction of RF facilities is necessarily detail oriented.

Upgrade or New?

Are you building a new facility or upgrading an existing facility? If you are building a new RF facility literally from the ground up, you will need to identify a practical location, negotiate a property lease, design and build a building or buy a pre-fab unit with redundant HVAC systems, add electrical service and a backup generator with a transfer switch, add internet service, and install the RF system from the Studio Transmitter Link (STL) to the tower and transmitting antenna.

Figure 1 – The seven elements of DTV transmission are the STL, exciter, transmitter, filter, transmission line, tower structure and antenna.

Figure 1 – The seven elements of DTV transmission are the STL, exciter, transmitter, filter, transmission line, tower structure and antenna.

Coordination of delivery and order lead times is critical. You’ll need a tower first, then the antenna, transmission line, power, emergency power and a building before you are ready for a transmitter. If you’re upgrading or remodeling, control of the dust and the mess of demolition can be crucial, particularly when keeping a transmitter on the air in the same space. Every TV station RF project faces similar challenges, and most stations handle them differently due to individual circumstances.

If you are upgrading an existing facility, can you do the work without disturbing on-air operations, or will you need a standby RF system available elsewhere while the upgrade is in progress? Every project begins with a unique set of circumstances and specifications. In the case of TV transmission, many of the key specifications such as location, antenna height above average terrain, and effective radiated power (ERP) are enumerated in each station’s license. The project plan should identify every action, anticipated expense, and source related to the project, along with an estimated timeline.

Next Step: ATSC 3.0

In 2020, nearly 1000 US TV stations completed repacking to new channels. The smart ones built in as many ATSC 3.0 upgrade abilities as possible into their new RF systems and paid for the upgrades during implementation. Today, in the USA the TV RF focus is on ATSC 3.0 and the transition from ATSC 1.0. ATSC 3.0 uses software-based encoding and has peak power levels 10 dB above the average power of the transmitter output. ATSC 3.0 also has a wider bandwidth than ATSC 1.0. It uses 97% of the allocated spectrum compared to 90% for ATSC 1.0.

The STL connects the output of the studio or master control to the transmitter exciter via IP-based WANs or microwave. The Broadcast Auxiliary Microwave Service (BAS) 2 GHz band was created by the FCC specifically for broadcast STLs and point-to-point transmission. Newer STLs can use either the BAS microwave band or a UDP-based data transfer protocol (UDT) for IP-based content data and metadata over wire, microwave, or fiber, typically using Secure Reliable Transport (SRT) protocol. With a good connection, there’s little reason not to go with the UDT solution. However, not all internet service is perfect and there will always be a few remote locations where UDT will not function properly. Otherwise, UDT is a proven, stable component of STLs and exciter inputs.

ATSC 1.0 and 3.0 exciters need an Asynchronous Serial Interface (ASI) input to simultaneously carry all the channels, subchannels and metadata. ASI carries MPEG data serially as a continuous stream with a constant rate at or less than the SDI rate of 270 megabits per second. The only purpose of ASI is the transmission of an MPEG Transport Stream (MPEG-TS). MPEG-TS is the universal standard protocol universally used for real-time transport of broadcast audio and video media. When a composite data transmission signal of asynchronous but formatted data is transmitted as RF, it is typically called DVB-S, DVB-T, or ATSC. When carried unmodulated on coaxial cable it’s called ASI.

Secret For Better Signals

FCC ERP measurement is based on horizontally polarized (H-Pol) radiated power. Double the transmitter power output (TPO) can be used to feed a Circular Polarized (CP) antenna. Adding a vertical component adds to the H-Pol signal and can increase overall signal strength compared to H-Pol only, depending on the position of the receiving antenna. CP also minimizes signal dead spots.

To take full advantage of ATSC 3.0, stations should investigate antennas that use either CP (50% H-Pol/50% V-Pol) or Elliptical Polarization (70% H-Pol/30% V-Pol or similar). Adding a vertical component to the transmitting antenna aids mobile service and improves deep indoor signal penetration. Most FM stations use circular polarization to minimize multipath reception issues in autos and homes.

SFNs Ahead?

I built and operated one of the first commercial single frequency networks (SFNs) in the US, KRBK-TV in Springfield MO. We had five identical but independent transmitters operating on Channel 49 across a geographically large DMA consisting primarily of fields, woods, and cows. The ATSC 1.0 SFN only worked in the narrow geographic area it was tuned work in. The transmitter installer tuned all the delays to ‘exactly between SFN transmitters.’ That didn’t work if the town you want to cover is near one of the transmitters. We learned how to tune SFN delays for the best signals where viewers were. The station was sold and changed channels during repack, moved to a 2000’ tower, and abandoned the Channel 49 SFN.

ATSC 3.0 SFNs are fully automatic by design. NextGen TV tuners sense multiple signals and build the best display signal from all of them. In the world of SFNs, ATSC 3.0 SFNs are quite powerful. In addition, extreme local transmitters can offer extreme local news, alerts, promotions, and advertising, as well as neighborhood school closings and other important public information targeted by zip code.

SFNs follow the highly localized cellular data RF model: Low power from a network of short towers. ATSC 3.0 SFNs similarly covering rural interstate highways with repeaters may be the future of ATSC 3.0 Broadcast Internet datacasting.

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