Not all antennas are required to generate radiation patterns reaching “to the moon.” For some applications, it is an antenna’s near-field (NF) radiation characteristics that are of most interest, such as for short-distance communications, or for biomedical engineering where a focused electromagnetic (EM) field may be needed to irradiate a tumor. To better understand the design considerations in creating near-field-focused (NFF) microwave antennas, a pair of researchers from Italy’s University of Pisa explored the different types of antennas and arrays that could be used for NF applications, along with various techniques that could help optimize antennas and arrays for NF applications.
NFF antennas are attractive for a number of short-range wireless applications, including for radio-frequency identification (RFID) and in antenna measurement facilities. The NFF antennas and arrays also support many non-communications applications, including for materials processing in industrial applications, wireless power transfer, and medical hyperthermia and imaging systems.
Focusing the EM field at a point in the antenna’s NF region results in an increase in the EM power density in a size-limited region close to the antenna’s aperture. This type of focus can be achieved by controlling the phases of the antenna aperture’s radiation sources in such a way that their EM field contributions add in phase at the target focal point. Antenna arrays provide the flexibility for achieving such in-phase focus, which can also be achieved with a conventional microstrip antenna array by making the proper adjustments to the microstrip antenna feed network.
NFF antenna parameters depend mainly on the antenna’s electrical size, L/λ, and the focal distance normalized to the antenna size, RF/L. For a given NFF antenna focused along the boresight direction, both the depth of focus (DoF) and the focus width increase when the focal point moves far from the array plane. An NFF array can increase the EM field amplitude in the antenna NF region while also reducing the antenna FF radiation. An antenna array optimized for NF radiation can achieve almost 20 dB higher field level than an unfocused array in its NF region.
A number of technologies are available for building NFF microwave antennas, including Fresnel zone plate lens antennas, transmitarrays, and reflectarrays. These approaches yield simpler NFF microwave antenna architectures than the feed network required for an electrically large NFF microstrip array. In transmitarrays and reflectarrays, the required phase shift is obtained by properly modifying one or more geometric parameters of the unit cell of the quasiperiodic transmitting or reflecting surface, respectively. Pyramidal and conical horns with a dielectric lens in front of the antenna aperture have also been used as NFF antennas. In addition, slotted waveguide antennas, radial line slot antennas, and arrays of open-ended waveguide have been used to form NFF microwave antennas.
Designing a NFF microwave antenna with specific features, such as dual focus areas, requires advanced synthesis techniques. One proposed technique involves representing an antenna’s amplitude and phase NF patterns in terms of the coefficients of spherical vector wave functions, and then solving a set of linear equations for them. Another synthesis technique is to reconstruct the NF amplitude and phase patterns with a least-squares method, using a set of field samples selected over a spherical surface. The researchers point out that because of the strong dependence of a NFF antenna on surrounding material, such as body tissue, each NFF antenna must be optimized for its intended application.
See “Near-Field-Focused Microwave Antennas,” IEEE Antennas & Propagation Magazine, Vol. 59, No. 3, June 2017, p. 42.