A Practical Guide To RF In Broadcast: Tuning And Monitoring Transmitters

How to tune for legal & standards compliance and performance, during installation and daily operations.

Amateur radio is one of the safest and most efficient hobbies to learn the critical fundamentals and characteristics of RF systems and signal propagation while having fun and not being responsible to anyone but the FCC.

At the time I learned about TV transmitters, I was a young amateur radio operator (ham) and knew the fundamentals of transmitters. Most TV stations signed off at midnight for transmitter maintenance. My early TV years were spent on a 7pm – 3am transmitter maintenance shift working with the long-time station transmitter engineer. In those days, we signed off at midnight and the union required two transmitter maintenance engineers, one to do the work and the other for safety when working with dangerously high voltages during the wee hours of the morning.

The first TV station that paid me to watch prime-time and late-night TV as an engineer was WDAF-TV. The station owned a RCA TT5-A analog TV transmitter, with 8 huge rack cabinets and dedicated RF Amplifier cabinets for the aural and visual signals feeding a combiner. The cabinets were filled with dangerously high voltages, big capacitors, heavy transformers, and glowing vacuum tubes. All the cabinets included a mechanical interlock to cut off rack power if the door was opened, which was regularly defeated by necessity during maintenance. For this reason, every cabinet also included a built-in, copper capacitor discharge stick, aka a ‘stick’ with several descriptive names preceding the word ‘stick.’

Dangerous Capacitors And Grounding

The first thing the long-time transmitter engineer taught me was the purpose and safe use of the capacitor discharge stick hook. His words, “You could save my life,” burned into my mind the moment he said them. The ‘discharge stick’ and hook was attached to the grounded rack chassis with 000-gauge insulated wire on one end with thick fiberglass insulation rated at 4KV covering up to a big bare copper hook on the other end. Its primary purpose was to discharge high-voltage capacitors. The hook feature was to pull fellow engineers away from electrocution. Muscles fully contract when electrocuted, and the hook was meant to pull a tight hand or hands away from an energized wire or connection. I never used the hook to save anyone or knew anyone who did, but every engineer I ever worked with knew what it was for. It was part of ‘the fun’ of working with high voltages capable of high current.

The transmitter racks were connected by a solid 4” copper ground strap running from rack to rack to station ground, which was a buried 4” copper strap surrounding the building perimeter. The station TV transmitter was fed by its own private electrical substation on a different service than the rest of the station. In terms of TV facilities and TV transmitters, grounding is everything. Sledgehammering a single 5-foot copper-coated steel spike from a box hardware store randomly into the ground is not a good RF ground. An efficient RF ground covers lots of territory and can easily discharge a direct lightning strike to the building.

There are two ways to discharge a capacitor. One is to use a screwdriver or an alligator clip jumper wire to short the capacitor’s terminals and possibly create sparks. The more professional method is to use a discharge stick. It’s not unusual to find a discharge stick at an electronics workbench because it’s safe and directly connected to the station ground like a transmitter discharge stick.

Always verify complete capacitor discharge with a voltmeter. Some engineers leave alligator jumpers connected to each end to ensure capacitors remain discharged during maintenance. Most transmitter power circuits contain “bleeder resistors” in parallel with the output of a high-voltage power supply to automatically discharge big capacitors when gear is turned off, but many transmitter maintenance engineers don’t necessarily trust them. Large capacitors can build up a residual charge without an external power source, which bleeder resistors are supposed to drain. However, bad bleeder resistors may or may not cause a transmitter warning indication. Most power supply circuits don’t depend on them to operate normally.

Fortunately, in 2023 vacuum tubes, the inductive output tube (IOT), and high voltage power supplies in broadcast transmitters have become virtually extinct. Today’s DTV transmitters have eliminated the dangers and risks associated with high voltage power supplies. Most modern solid-state DTV transmitters use off-the-shelf, commodity, 50VDC modular power supplies that plug into a mother chassis. There are no hard rules for what point a voltage becomes dangerous nor consistent definitions of the ‘low voltage’ dividing line, but the term ‘low voltage’ has clearly become electrical industry slang for 50 VDC or less.

TV transmitter power supply troubleshooting has been replaced by hot-swapping, and most modular 50 VDC power supplies are cheaper to replace than fix. Power supply modules are usually accessed in the front of the transmitter cabinet. Modern DTV power amplifiers are similarly modular, hot-swappable, and accessible from the front of the transmitter cabinet. A PA may be fixable on a workbench, but a spare PA can keep a station on the air at full power seamlessly as a faulty PA is repaired or replaced.

Transmitter Tuning

When analog TV transmitters were the only TV transmitters, the experienced engineer who trained me showed me some “tricks they don’t teach you at transmitter school.” One of those tricks was to increase the filament voltage of weak tubes to compensate for lower output. Other tricks were similar, and they were all meant to compensate for drifting circuitry and aging active components before necessary replacement.

The fact is that a TV transmitter is simply a series of tuned circuits and amplifiers optimized for the transmitter’s licensed frequency and power output. In the old days, TV transmitters were two separate transmitters. One was the visual transmitter (AM modulated) the other was the aural transmitter (FM modulated), and they were mixed in a combiner before the mask filter. Analog transmitter tube performance and component values drifted, and TV transmitters required constant monitoring and manual compensation adjustments as parts and circuits aged. Manual transmitter readings with clip boards and typewriters were logged hourly not only to meet FCC requirements, but for maintenance engineers to note and identify clues and trends prescribing the need for preventative maintenance intervention.

Digital transmitters are different to their analog predecessors in many respects other than simply being digital. One is the ubiquitous 50 VDC PA power supplies. Another is that the TV broadcast frequency band is significantly narrower than it once was. Transmitters are either VHF low-band (44-88MHz), VHF high-band (174-216 MHz), or UHF (470-614 MHz). Most importantly, solid state digital transmitters don’t drift because most are locked to GPS.

When the FCC adopted the NTSC’s recommendations of the original black & white TV legal standard in 1941, the technical details of the NTSC standard were defined by the National Television Standards Committee (NTSC). In 2011, when the FCC announced the DTV broadcast standard, it followed how the FCC adopted the NTSC’s recommendations of the original black & white TV legal standard in 1941. The technical details of the DTV broadcast standard were and primarily are defined by ATSC A/52, ATSC A/53, and ATSC A/54A, and in FCC rules 47 CFR § 73.682 - TV transmission standards.

Transmitter Monitoring

Monitoring requirements in 2023 are less about legal compliance and more about eliminating complaints. For example, the FCC does not specify a frequency tolerance for digital TV (ATSC 1 or 3.0) transmissions except in relation to lower adjacent analog TV stations. If required by the ATSC 3.0 transmitter and filter, the overall occupied bandwidth can be reduced from 5.832844 MHz to as little as 5.508844 MHz. Details are explained in the ATSC A/327 Physical Layer Recommended Practice section 4.2.2.

Also, according to FCC 47 CFR 73.1560 (h), “The power level of emissions on frequencies outside the authorized channel of operation must be attenuated no less than the following amounts below the average transmitted power within the authorized channel. In the first 500 kHz from the channel edge the emissions must be attenuated no less than 47 dB. More than 6 MHz from the channel edge, emissions must be attenuated no less than 110 dB. At any frequency between 0.5 and 6 MHz from the channel edge, emissions must be attenuated no less than the value determined by the following formula: Attenuation in dB = -11.5(Δf + 3.6); Where: Δf = frequency difference in MHz from the edge of the channel.” Such critical measurements require a professional spectrum analyzer.

In fact, monitoring in 2023 is crucial, but don’t depend on the FCC or your government RF regulatory agency to tell you when you are out of bounds. Most of the 276 MHz of lost TV spectrum is now used for Wi-Fi and paid Cellular customer connections. Those providers are managing and monitoring their spectrum more closely than the FCC ever could. The point is to not get a complaint, respond and act quickly if you do, and keep government regulators out of the loop. Things break. I once had a mask filter develop a mysterious, random issue that began interfering with the cell system on the same tower. They complained to us, we immediately returned the filter to the manufacturer for repair and retuning, and the problem was resolved without FCC intervention in about a week.

The Loss Of US TV Spectrum
The RF bandwidth allocated to US TV broadcasters is 38% of what it was before 1983, yet broadcasters remain restricted to 6 MHz channels when needing more bandwidth than ever for UHD and other services.

The RF bandwidth allocated to US TV broadcasters is 38% of what it was before 1983, yet broadcasters remain restricted to 6 MHz channels when needing more bandwidth than ever for UHD and other services.

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