Designing lightweight antennas

By John Keller, Editor

Radio communications experts and antenna manufacturers wrestle with the sometimes-conflicting requirements of size, efficiency, bandwidth, and cost as they design communications systems of the future.

The RF antenna for aerospace and defense applications is one of the least glamorous yet most essential subsystems in all of aerospace systems design. Without them, wireless communications capability is impossible, yet users are demanding ever-smaller, lighter, and cheaper antennas to support an increasing appetite for broadband voice and data communications.

The General Dynamics full-motion SATCOM-on-the-Move (SOTM) terminal, shown above, provide uninterrupted, video, voice, and data communications from moving combat vehicles operating in tough terrain.

The problem, however, is the inability of antenna designers to shrink size and weight as quickly as integrated circuit designers have been able to do. Radio waves-particularly in the lower frequencies-are just so long; nothing can change that. They require antennas of a certain size to transmit and receive RF energy with acceptable efficiency.

"There are basic physics involved with most antennas," explains Lewis Johnston, vice president of advanced programs at Thales Communications Inc. in Clarksburg, Md. "The lower the frequency, the larger the antenna needs to be to be efficient."

Multiband antennas

One way antenna designers save on space and weight is to devise multiband antennas that work reasonably well on several different RF frequency bands. "We work on integrating more than one antenna element into one antenna, says Ross Mizzola, product manager for Falcon radio accessories at Harris RF. "In the past you would have had vehicles with dedicated UHF, L-band, and VHF antennas. There is not space for all of this. Our solution is at least two different bands to one antenna."

Designing one antenna to operate on two or more frequency bands can be daunting, and often involves proprietary information that antenna and radio communications manufacturers prefer to keep secret from the competition. "This is extremely challenging," Mizzola says. "There are lots of things that need to happen in the same place, and we need to optimize performance and reduce interference."

Antenna designers also are experimenting with placing electronics close to the antenna elements themselves that fine-tune the antenna for peak performance in different bands. In the modern era of tight defense budgets, however, this is not always feasible. "You can achieve some capability there," Johnston says. "The limitation there is implementation. How do you do this and end up with something that is rugged and still won't cost more than the radio itself?"

Innovative designs

Not all radio communications have the same size limitations as handheld radios, and can afford some unique design innovations. The ITT Exelis Antenna Products and Technologies group in Bohemia, N.Y., specializes in RF antennas for aircraft. Sometimes ITT antenna designers use the structure of the aircraft itself as part of the antenna design.

"On the platform, we need to use the laws of physics to get an antenna of sufficient size to radiate properly," explains Ken Plate, director of the business at ITT Antenna Products. "Over the narrow bands there are tricks you can play in making the antenna seem larger than it is by enhancing matching rather than incorporating the antenna's physical size. You might be able to use some of the structure around you."

ITT is in charge of designing the communications antennas on the Raytheon Miniature Air Launched Decoy (MALD), which acts as a distraction and help defend aircraft from radar-guided missiles. As it turns out, though, ITT does not merely design MALD's antennas, but the decoy's wings, as well.

"The MALD decoy itself is very small, and the frequency was low enough so it wouldn't fit in a traditional antenna, so we put the antenna in the wings," Plate says. "The skeleton, or frame, of the wing gives it its strength, and we concurrently designed the wings to act as the radiating structure of the antenna; the wing is the antenna."

This approach will become commonplace on unmanned aerial vehicles (UAVs) as the aircraft become smaller and require ever-more communications bandwidth to relay reconnaissance and surveillance data, Plate says.

Satellite communications antenna designers contend with needs for low size and weight, as well as affordable costs

Satellite communications (SATCOM) on the battlefield is becoming increasingly important for front-line warfighters who need real-time information on enemy positions, the locations of their comrades, for calling in air strikes and close-air support, and for calling in medical help when someone gets hurt.

The ideal SATCOM antenna for forward-deployed infantrymen might be something resembling a one-cubic-inch dome that sits on top of the warfighter's helmet, out of the way and connected to satellite data channels all the time. That dream, however, remains elusive as SATCOM antenna designers face the same challenges that all RF antenna makers confront-barriers thrown up by the laws of physics.

The design tradeoffs essentially boil down to size of the antenna, transmission power, and communications bandwidth. Typically antenna designers can build systems with two of these characteristics, but not all three.

Antenna experts at the SATCOM Technologies segment of General Dynamics C4 Systems in San Jose, Calif., confront these design tradeoffs daily, explains Tim Shroyer, chief technology officer at General Dynamics SATCOM, which specializes in small-aperture SATCOM antennas for mainstream military backhaul communications, as well as small SATCOM antennas for gas stations and convenience stores for customer credit card verification.

"For broadband satellite communications you are limited by physics, which constrains the size of the antennas," Shroyer explains. "Users would always like the antennas to be smaller, but you can't support the data rates that broadband communications users need with antennas that are less than 30 centimeters in diameter. They just won't get the desired data rate."

In fact, some of the latest broadband military SATCOM dish antennas typically measure 30 to 50 centimeters in diameter, which is about 20 to 25 inches. A SATCOM antenna two feet in diameter cannot fit on a helmet or on any other equipment of a foot soldier on the move. These antennas must be set up and taken down as needed, or else be fitted to ground vehicles.

"We have users who ask about making them 10 centimeters in diameter, but it wouldn't get them much throughput at all, so they wouldn't be very useful," Shroyer says. There are alternatives to antenna size, such as increasing transmission power, but they are not attractive options for today's size-and-weight-constrained military.

Increasing power at the satellite introduces difficult design choices involving batteries and other power-management subsystems, as well as increasing satellite payload size, which increases launch costs. Increasing power at the satellite also can cause RF interference to nearby satellites.

Other possibilities on the satellite end involve spreading power over wider communications bandwidths, but this approach reduces the efficiency of the satellite, Shroyer says.

Increasing power at the ground transceiver, meanwhile, also introduces power and weight issues that are not acceptable to deployed warfighters. Suffice it to say that increasing power is rarely a viable option in satellite communications.

"The only way to make broadband satellite communications antennas much smaller would be to change the amount of power," Shroyer says. "Communications capacity is always limited by signal power compared to the noise. There's nothing we can do with the antennas themselves to help make them work smaller."

One potential way to shrink deployed SATCOM antennas works by spreading energy transmitted from the satellite to a relatively small number of users on the ground, Shroyer explains. This approach, however, must limit data rates and may not be practical for the warfighter.

"It's an interesting situation," Shroyer says. "Electro magnetics for terrestrial antennas are very well understood, and are driven by the size of the antenna and the wavelength; the larger the antenna, the more gain you have. There is nothing that will change this overnight."