What Are Multifrequency Antennas?

Multifrequency antennas are antenna structures engineered to radiate and receive electromagnetic energy at two or more distinct frequency bands while occupying a single physical element or compact assembly. Rather than deploying separate antennas for each service band, a multifrequency design achieves multiple resonances through geometric shaping, parasitic loading, or reconfigurable switching, reducing the mass, volume, and cost of the radio-frequency front end. The concept spans dual-band designs covering a pair of specific allocations to wideband and multiband structures that serve four or more frequency ranges simultaneously.

The need for multifrequency operation arose with the proliferation of mobile communication standards in the 1990s, when handsets were expected to operate across GSM 900 MHz and 1800 MHz bands, and then again as 3G, WLAN, and LTE services added further allocations from 700 MHz to 6 GHz. Modern devices may need to cover upward of ten frequency bands for 4G, 5G sub-6 GHz, Wi-Fi 2.4 GHz, Wi-Fi 5 GHz, Bluetooth, GPS, and millimeter-wave 5G simultaneously.

Design Techniques

Several structural approaches produce multiple resonant frequencies in a single antenna. The planar inverted-F antenna (PIFA) is widely used in handset applications because its geometry supports independent tuning of multiple resonances by adjusting the ground-plane slot positions, feed point, and shorting pin. Combining a PIFA with a parallel slot element lets each radiator contribute its own frequency bands independently, as demonstrated in multiband handset antennas that combine PIFA and slot radiators for compact integration. Fractal geometries, including Sierpinski carpets and Koch snowflakes, exploit self-similarity to produce multi-resonant behavior; a Sierpinski-based fractal PIFA can simultaneously cover GSM, UMTS, and HiperLAN bands from 1900 MHz to 5800 MHz in a 27 mm by 27 mm footprint. Frequency-reconfigurable antennas use PIN diodes or RF MEMS switches to alter the effective electrical length, selecting from a set of resonant frequencies under software control.

Impedance Matching and Isolation

A key challenge in multifrequency antenna design is presenting an acceptable impedance match across all target bands simultaneously, without high-impedance nulls between them that would disrupt broadband transceivers. Matching networks using lumped elements or transmission-line stubs can introduce losses unless carefully optimized. In multiple-antenna systems, such as those used in MIMO or diversity configurations, isolation between co-located multiband elements is equally important, because coupling at one band can degrade gain and radiation pattern at another. Defected ground structures (DGS), which introduce periodic or shaped slots into the ground plane, are one approach to improving isolation while maintaining multiband operation in compact antenna arrays.

Wideband and Multiband Convergence

For applications that require contiguous spectrum coverage from roughly 600 MHz through 6 GHz, the boundary between wideband and multiband design becomes significant. A wideband antenna maintains a near-constant impedance match over a continuous range, while a multiband antenna accepts significant mismatch in the gaps between its target bands. Ultra-wideband (UWB) antennas, defined by the FCC as occupying at least 500 MHz or 20 percent fractional bandwidth, represent an extreme case in which a single aperture spans a decade in frequency. Dielectric resonator antennas offer another route to multifrequency operation through multiple resonant modes within a low-loss ceramic element.

Applications

Multifrequency antennas have applications in a wide range of systems, including:

• Smartphones and tablets covering 4G LTE, 5G sub-6 GHz, Wi-Fi, and GPS bands
• Vehicle-mounted communication systems supporting cellular, satellite navigation, and V2X
• IoT devices that integrate LoRa, Wi-Fi, and Bluetooth in a single module
• Software-defined radios requiring a single aperture across HF, VHF, and UHF
• Radar systems using multiple frequency bands for target discrimination