Designing the perfect lightweight antenna

Radio communications experts and antenna manufacturers wrestle with the sometimes-conflicting requirements of size, efficiency, bandwidth, and cost as they design the voice and data battlefield 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 military and 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, even on the forward edge of the battlefield.

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."

Warfighters in the field, who are under intense pressure to carry heavy loads on foot over rugged terrain, don't want big, heavy antennas. Instead, they want the smallest, lightest antennas possible, and sometimes they make shortcuts to get antenna sizes down to a manageable level.

The Thales AN/PRC-154 Rifleman radio operates on two separate bands-225 to 450 MHZ, and 1,200 to 1,800 MHz-and requires one antenna that can handle both bands. The Thales AN/PRC-154 Rifleman radio operates on two separate bands-225 to 450 MHZ, and 1,200 to 1,800 MHz-and requires one antenna that can handle both bands.

The military Single Channel Ground and Airborne Radio System, better-known as SINCGARS, for example, operates in the 30 to 88 MHz frequency spectrum-a relatively low band with long radio waves, which typically requires a whip antenna longer than a man is tall to provide reliable long-range communications.

"Users will fold that long blade antenna back, tape or Velcro it down, and it hurts the operational efficiency and transmission range when they do that," Johnston says. "Users want antennas that are as small as possible, but they also want to be able to communicate at as long a range as they can. There is a physics tradeoff between the size and efficiency of the antenna, and that is a battle we are always fighting."

Demand from users for small, lightweight antennas for terrestrial line-of-sight radios is an everyday occurrence at the Harris Corp. RF Communications Division in Rochester, N.Y.-a Thales competitor for military radio communications business throughout the world. Nevertheless, radio and antenna systems designers only can do what is possible given the physics of RF and microwave communications.

"The challenges we generally have with antennas is higher, taller, bigger, longer; these are keys to getting good performance out of an antenna," explains Ross Mizzola, product manager for Falcon radio accessories at Harris RF. "Wearable antennas are something soldiers want in order to increase their mobility, and to give them a smaller, lighter package."

The Thales AN/PRC-148 handheld software-defined radio, shown above, uses a multiband antenna design to cover frequencies from 30 to 512 MHz.
The Thales AN/PRC-148 handheld software-defined radio, shown above, uses a multiband antenna design to cover frequencies from 30 to 512 MHz.

The cell phone model

Shouldn't it be easy to design small, wearable RF antennas for broadband voice and data communications on the battlefield? Just look at the ubiquitous cell phone. These devices that nearly everyone carries are smaller than a deck of cards, and offer users voice communications, real-time access to e-mail, instant text chat, and speedy Internet connections on the go, yet they don't require large and bulky antennas. In fact, they have small patch antennas that are virtually invisible to the user.

All that is true, but cell phones require an extensive network of land-based stations and repeaters to make the most of their tiny antennas. This kind of infrastructure simply is not available to warfighters on the front lines of battle, where little, if any, fixed communications infrastructure exists, conditions change constantly, and often battlefield communications must content with RF jamming and other kinds of tough interference.

"These small [cell phone] antennas don't have to go very far, and don't have to optimized, because of the infrastructure around us," Mizzola points out. Commercial cell phones could not contend with this environment, so rugged, long-range RF communications are essential for military operations, and this often requires big, beefy antennas.

"When you try to make antennas smaller, lighter, and shorter, you push the envelope, and there are lots of other factors that come into play," Mizzola says.

More with less

Facing the physics of RF communications, coupled with customer demand for smaller, lighter, cheaper antennas, military communications experts must do more with less. "We've been asked for line-of-sight ground SINCGARS VHF antennas to replace a 40-foot, 40-piece antenna mast called the OE-254," says Justin Genest, sales application engineer at Cobham Electronic Systems Inc. in Lowell, Mass.

"The OE-254 takes two or three guys about 40 minutes to set up. We are replacing it with a five-piece antenna called the COM201B that weighs 10 pounds, can be operated on the ground, or can be elevated on a mast."

The COM201B replaces the 40-foot OE-254 antenna with a six-foot assembly when placed on the ground. "It has a flatter radiator, so the length-to-diameter ratio gives it unique propagation," Genest explains. "It transmits a traveling ground wave as well as traditional line of sight. The thick radiating element gives unique properties, even in urban terrain."

The small size of the COM201B, which gives equivalent performance to the much-taller OE-254 gives it much better portability on combat zones. "Guys can jump with it, and Special Forces love it," Genest says. Unlike the larger OE-254, the COM201B does not have the same UHF functionality because the shorter antenna has no room for an upper radome housing a UHF antenna.

Different VHF antennas from Cobham include the COM237 30-to-512-MHz vehicle-mount antenna, and the COM23701 ground- or mast-mount antenna, as well as the COM23703 reduced-height version for vehicle-mount applications where tight clearances are crucial.

On the COM23703 "we reduced the height by about a foot, and this was a breakthrough because we can tune it and reduce the height by 11 inches and still have the same performance," Genest says. The reduced size resulted from a U.S. Marine Corps requirement for communications on the Joint Assault Bridge vehicle, he says.

The Cobham COM201B low-band VHF antenna, shown above, uses a thick radiating element to achieve efficiency sufficient to replace a 40-foot fixed-site antenna.
The Cobham COM201B low-band VHF antenna, shown above, uses a thick radiating element to achieve efficiency sufficient to replace a 40-foot fixed-site antenna.

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 Harris' Mizzola. "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."

Thales manufactures a popular military handheld radio called the AN/PRC 148 that covers frequency bands from 30 to 512 MHz, which covers the VHF low and high bands, as well as the UHF low bands. The company's Liberty multiband land-mobile radio for civil first-responders, meanwhile, covers frequency bands of 136 to 174 MHz, 380 to 520 MHz, and 763 to 805 MHz.

Cobham uses antenna-engineering techniques to shrink low-band antennas to fit on armored combat vehicles.
Cobham uses antenna-engineering techniques to shrink low-band antennas to fit on armored combat vehicles.

This is a lot of frequency bandwidth for one radio antenna to cover, but in today's radio communications market there are few alternatives to designing one antenna to handle it all. "The users don't want to change antennas," says Thales' Johnston.

Although Thales has "a really good solution" for such multiband antenna needs, there still are drawbacks. "On the PRC-148 family we have one antenna that will work over the whole band, but on the low end it is not an efficient radiator," Johnston says. "We recommend they use a different antenna for that."

For the Thales PRC-154 Rifleman radio, which is part of the military's Joint Tactical Radio System (JTRS), company engineers address the wideband antenna problem by using relatively wideband communications channels on the radio's upper frequencies. The radio operates on VHF-high and UHF-low frequencies of 225 to 450 MHz, as well as on L-band frequencies of 1,200 to 1,800 MHz.

On L-band frequencies, each channel in the radio is three megahertz wide to accommodate antenna design issues, and to accommodate high-speed digital data transmissions. With wider frequency channels "we are still limited to line-of-sight, but the physical size of the antenna can be smaller and still get effective radiation," Johnston says.

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.

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 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.

"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."

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Mil & Aero Magazine

April 2015
Volume 26, Issue 4

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