A Practical Guide To RF In Broadcast: Transmitter & Receiver Fundamentals

The electromagnetic spectrum makes wireless communication and broadcasting possible. Mastering and managing the electromagnetic spectrum while complying with government technical regulations and industry standards is the responsibility of every broadcast engineer.

Naturally occurring radio waves are generated by lightning and some celestial objects like the sun. Artificial radio waves are generated by an electronic transmitter, connected to an antenna that radiates radio waves when excited by the RF alternating current from the transmitter. The radio waves are received by another antenna connected to a radio receiver, which amplifies and processes the received signal.

An unmodulated transmitter broadcasts radio waves on a specific frequency known as the carrier signal. The most basic technology to modulate a carrier signal is to turn it on and off, which is only useful for Morse code. Adding information such as sound, video or data to the carrier signal requires one or more of several types of signal modulation.

Amplitude modulation (AM) adds information to the carrier by varying its amplitude and is susceptible to atmospheric static. Frequency modulation (FM) adds information by slightly varying the carrier signal’s frequency and unaffected by atmospheric static. Analog TV used AM modulation for video and FM modulation for audio. DTV, including the latest generations of DVB and ATSC 3.0 are moving away from 8VSB modulation to orthogonal frequency-division multiplexing (OFDM) to encode (modulate) digital data on multiple carrier frequencies.

Transmitter Basics

The signal from every transmitter begins in an electronic oscillator and possibly frequency multiplying circuitry that generates the carrier wave on the desired transmitting frequency. The frequency stability of the carrier wave signal is crucial for compliance and reception. The oscillator can be locked to a resonant quartz crystal reference, GPS, or a synthesized frequency oscillator. Most transmitters, including TV broadcast transmitters, consist of three separate sections: The exciter, the power amplifier (PA) including high-current power supplies, and the output filter.

A broadcast transmitter exciter, also known as the modulator, is essentially a tiny TV transmitter with an RF output typically rated in milliwatts. It converts content from a source such as a control room or studio-transmitter-link (STL) into a modulation signal that excites the broadcast carrier with the preferred modulation type. The RF output of a TV exciter connects to one or many RF amplifiers in the transmitter cabinet depending on the transmitter design and total transmitter power output (TPO).

Modern TV transmitters use solid-state power amplifiers, each typically powered by an independent low voltage power supply. Until solid-state TV transmitters became available, UHF TV transmitters used an inductive output tube (IOT) for the PA that required a minus 35 KV power supply to operate.  TV transmitters have become significantly less dangerous and easier to repair. Each cabinet of old TV transmitters using high voltage PA tubes usually included a three-foot-long copper hook attached to the rack ground with 5’ of AWG 3/0 to discharge large capacitors or free a shocked companion from certain electrocution and a constant reminder of the danger inside.

Broadcast TV transmitters are built to order for a specific TPO on VHF or UHF TV bands. Each PA is typically plugs in a master chassis and is hot-swappable. Most solid-state PAs put out approximately 750 watts with some additional headroom, and they are usually powered by individual 50 V hot-swappable power supplies. Broadcast TV transmitters are also either air- or liquid-cooled often depending on the transmitter building cooling system and the heat load of the transmitter at full power.

Harmonic Filtering

The output of the RF amplifier(s) must be filtered with a low-pass filter and checked for spurious emissions such as harmonics with an RF spectrum analyzer. A RF harmonic is an unwanted signal often occurring at multiples of the original carrier frequency. Harmonics from the popular 500-600 MHz DTV band can interfere in bands used for other purposes, such as cell phones and Wi-Fi. You’ll hear from who your signal interferes with because they have spectrum analyzers, too. The output of the filter connects to the antenna transmission line.

Multiple PA TV transmitters provide individual monitoring of the power output of each PA module for preventative maintenance purposes. All TV transmitters are typically monitored for legal compliance of frequency, loudness, and modulation levels, TPO before and after the filter, and the antenna voltage standing rave ratio (VSWR).

VSWR

VSWR is the ratio of minimum to maximum voltage along a transmission line ½ wavelength or longer. A VSWR of 1:1 is the theoretical goal everyone wants, but many physical factors such as transmission line lengths and tiny impedance mismatches make a perfect 1:1 difficult to achieve.

Most optimal TV broadcast transmitters operate at under 1.25:1. VSWRs above about 2:1 can be toxic for transmission lines, filters, and transmitters. If a SWR mysteriously increases, the problem is in the transmission line or antenna. Something happened to cause the transmission line to arc, maybe a loose connection or a lightning strike, or a bad actor shooting at tower lights hit the transmission line. Things happen, and trying to pump full power into a high SWR load will create more problems. Reduce the transmitter power and identify the source of the VSWR problem ASAP.

Receiver Basics

A story about transmitter fundamentals wouldn’t be complete with mentioning receivers. The first were crystal radios used to receive Morse code from spark-gap transmitters operated by early amateur radio experimenters. Before modern semiconductor diodes, a crystal radio used a “cat whisker detector” for the diode.

The most basic AM radio crystal radio uses the capacitance of the antenna to make a tuned circuit with a coil, and uses a diode powered by the radio signal to demodulate AM radio to audio.

The most basic AM radio crystal radio uses the capacitance of the antenna to make a tuned circuit with a coil, and uses a diode powered by the radio signal to demodulate AM radio to audio.

Crystal radios need a long-wire antenna to receive a signal that can generate the microwatt or nanowatts necessary to make sound in a headset. Crystal radio selectivity is poor because it has only one tuned circuit and technology was limited to frequencies below about 500 kHz. Until Major E.H. Armstrong introduced the local oscillator and superheterodyne receivers in late 1919, everything above 500 kHz was considered shortwave and beyond the technology of early vacuum tube amplifiers.

The term heterodyne means ‘generated by a difference in frequency.’ A heterodyne created when two signals are mixed produces two new signals, one is the sum of the mixed signals frequencies, the other is the difference. The new signals are also known as ‘beat’ frequencies. Only the difference signal is typically used in heterodyne and superheterodyne radio receivers. In them, radio stations are tuned in by varying the frequency of the local oscillator to produce a difference signal on a fixed frequency known as the intermediate frequency (IF).

When commercial radio broadcasting began a century ago, the most popular domestic radios were of tuned radio frequency (TRF) design. They were cheap to build and easy to operate. A TRF receiver uses one or more tuned RF amplifier stages followed by a detector (demodulator) to extract the audio signal, and usually included an audio amplifier. The local oscillator tube and IF circuitry was cost prohibitive until improvements in vacuum tube technology and a growing number of broadcast stations created a demand for better and less-expensive home radio receivers.

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