AVENT HORIZON
The Incredible Vanishing Wireless Spectrum And what it means to you. By Pete Putman, CTS, KT2B ROAM Consulting LLC If you haven’t been paying attention, the Federal Communications Commission (FCC) concluded a big spectrum auction two years ago. (Yeah…I know… it was kind of a yawner.) Unlike the era almost five decades ago when I first got my ham radio license, when the FCC “regulated the radio and TV spectrum in the public interest,” it’s now clearly all about the Benjamins. If you have enough of them, you can buy megahertz and gigahertz for whatever purpose you have in mind. Back then, TV stations wanted to be on channels 2 through 6 (54MHz to 88MHz) and channels 7 through 13 (174MHz to 216MHz). That’s where the eyeballs were, and those channels were easy to tune in with (mostly) reliable reception. By contrast, the UHF band of channels 14 through 83 (470MHz to 890MHz), which started in 1952 with just 15 TV stations, was largely considered an RF “desert,” populated by independent and public stations, along with TV repeaters.
100 Sound & Communications September 2019
The reason? It just wasn’t easy to make a stable UHF TV receiver with a low-noise figure. On the transmitter side, it cost more money to operate on UHF channels because of the power required to overcome the problems with receivers. Fewer viewers meant reduced advertising revenue. In fact, way back in 1962, the FCC had to issue a mandate that all new televisions include a tuner for UHF—other wise, no one would watch! UHF reception on a TV was still a novelty in 1970. By 1990, it was just as easy to tune in to as any VHF station was. Why? Over time, as electronics transitioned from vacuum tubes to transistors and then to integrated circuits, the phase-locked loop (PLL) came into existence. The PLL and its descendants have made not only UHF television practical, but also frequency synthesizers from audio to RF, mobile phones, satellite broadcasts and super-stable oscillators. Even so, there still weren’t ver y many UHF stations in the US three decades after the first broadcasts, so channels 70 through 83 were removed from TV use and reallocated for early cell-phone communications. (Allegedly, you could pick up many of those calls simply by using the UHF tuner on your TV!) There just weren’t enough stations in that part of the band to warrant continued operation. Separately, a new categor y of audio products had come to market in the 1970s. These “wireless” microphones were low-power transmitters, typically using less than 100mW per FCC rules, operating on unused television channels. These products started out on the so-called low-band VHF spectrum, but operation there proved to be unreliable due to poor selectivity of the receivers, impulse noise from appliances and motors, and interference from distant “skip” signals. So, wireless mic manufacturers moved up to the high-band VHF spectrum in the 1980s, dodging and weaving around TV stations from channels 7 through 13. Operation was with frequency modulation (FM) and instances of interference were considerably reduced—that is, as long as the receivers had decent selectivity, and as long as there was no noise or tropospheric signal ducting taking place that would wipe out your receivers. With the coming of the 1990s, wireless mic manufacturers tried different bands, including the UHF television band and even the 902MHz to 930MHz industrial, scientific and medical (ISM) band. Digital TV was on its way, and some more advanced brands were building expensive digital wireless mics with ver y selective receivers, immune to just about all interference. By the mid-’90s, it was generally conceded that the UHF spectrum was the best place for wireless-microphone operation, and VHF was to be left behind with dial phones and iceboxes. (continued on page 98)