By John Keller, chief editor
Military & Aerospace Electronics
One of the most difficult ways to communicate by radio is over the high-frequency (HF) bands. These channels, which lie between 2 MHz and 30 MHz, are touchy and unpredictable, prone to noise, fading, jamming, and interference. In thunderstorms or times of intense sunspot activity, they can become virtually unusable on conventional transceiver gear. In sum, HF radio is widely considered to be a big source of frustration for radio operators and field commanders alike.Still, the HF bands offer profound advantages to those who can deal effectively with the problematic nature of this medium. To begin with, HF communications have the potential to move voice and data communications around the world by bouncing signals off the ionosphere, and at a fraction of the cost of heavily burdened satellite communications frequencies. In addition, HF transmissions often can circumvent geographic barriers to line-of-sight communications such as mountains or buildings by skipping signals off the ionosphere at steep trajectories. The benefits of HF radio communications are too good to ignore.
Technological and economic conditions today are converging to make HF radio perhaps more appealing and useful than it was during the heyday of HF radio between about 1920 and 1960. Not only are satellite communications channels rapidly filling beyond capacity, but the costs of launching satellites and building satellite receivers also continues to increase. Competition for satellite-communications RF spectrum also is fierce from commercial television, music, and navigation service providers.
Yet at the same time new adv-ances in software-defined radios, digital communications technology, new integrated circuit materials, and embedded encryption are working in HF radio's favor. We may be on the cusp of a substantial HF radio renaissance.
Among the prime movers in advancing the software and component technologies for HF radio is the future Joint Tactical Radio System, otherwise known as JTRS. This project seeks to build a tactical secure radio that can operate continuously in frequencies from 2 MHz to 2 GHz, and beyond in handheld, manpack, vehicular-mount, and airborne versions. The push cram so much functionality into one radio platform is forcing designers to face issues that almost certainly will pay big dividends to HF communications.
The decline of HF
Interest in, and knowledge of, the HF, or so-called "shortwave" radio bands has been in decline for years, if not for decades. For the military, the medium has been considered to be less than reliable beside competing modes such as satellite communications, VHF and UHF radio repeaters, proposed radio repeaters aboard future blimps and unmanned aerial vehicles, and even commercial telecommunications services. Let's face it, few radio operators and tactical commanders want to deal with what they perceive as static-ridden, fading, full of interference, and subject to the vagaries of sunspot activity and condition of the ionosphere.
It wasn't always this way, far from it. Historically, HF was the world's mainstay of long-haul communications until satellite communications started coming into their own in the latter half of the 20th century. Maritime communications relied almost exclusively on HF virtually since the dawn of the electronic age. During World War II, for example, ships, submarines, and long-range patrol aircraft communicated with the bases by HF radio. The reason, primarily, is the utility of HF at transmitting Morse code over vast distances. Not only can HF signals bounce off the ionosphere, but they also have the ability to hug the ground or the ocean's surface and literally follow a curved path.
On the commercial side, the HF bands are the homes of the world's shortwave radio stations, which bring news and information to many of the world's people. Satellite communications were not the only reason for the slide in popularity of HF. Where decades ago many people received vast amounts of news and information over HF radio, today they turn to the Internet. For a while now, military HF frequencies have been largely quiet, and commercial HF use has become an arcane hobby. Even maritime authorities have phased out the mandated use of HF and Morse code in favor of satellite communications.
Yet those irritating traits of HF radio appear to be disappearing as HF radio designers at companies such as Harris Corp. RF Communications Group in Rochester, N.Y., and Rockwell Collins Inc. of Cedar Rapids, Iowa, apply the latest software and component technology to integrate tactical HF radios that are nearly as easy and seamless for battlefield commanders to use as are cellular telephones.
Not only does a resurgence in the use of HF radio for voice and data communications offer cost-effective benefits to military and aerospace users, but these bands also offer yet-untapped potential to new efforts in homeland security and counter-terrorism.
Radio engineers at companies such as these are turning to advances in radio frequency application-specific integrated circuits - better known as RF ASICs - as well as digital signal processors, fast analog-digital converters, and automatic gain control, to help them filter out HF background noise and interference to help HF communications sound as clear as VHF and satcom communications.
Other technological improvements leading to improvements in HF radio include silicon carbide (SiC) and gallium nitride (GaN) integrated circuit materials, which not only can handle more power in smaller packages than can today's RF circuitry, but also are easier for engineers to match over a wide frequency range, says Richard J. Buckley, senior member of the technical staff at Harris RF.
What types of enabling technologies might speed the improvement of HF radio, as well as other broad-spectrum radios such as the JTRS? Buckley says several new technologies are on the horizon. Among the technological breakthroughs that Buckley says are necessary are fast 24-bit A-D converters, which he says are three to five years in the future. Farther in the future, he speculates, may be new generations of programmable RF circuitry, which might do for analog electronics what today's field programmable gate arrays do for digital electronics.
Also on the horizon, Buckley says, are improvements in the use of digital signal processors to create digital intermediate frequency (IF) use, which amplifies RF energy received from the antenna to levels where the radio's electronics can do substantial filtering. "In five to 10 years our radios will be digital almost to the antenna," he says.
