mmWave equipment on connected vehicles may have dielectric resonator antennas that do not resemble conventional antennas.
Automotive designers have always paid attention to technologies that are light and compact. The preference has not changed for connected vehicles. As connected functions increasingly rely on mmWave frequencies, designers are exploring ways to reduce the size of radio frequency (RF) electronics. In this regard, various antennas relying on monopoles, dipoles and patch antennas have been proposed for millimeter wave use. However, these antennas typically have issues with radiation efficiency and a narrow impedance bandwidth due to lossy silicon substrates.
Enter the Dielectric Resonator Antenna (DRA). It usually has a cylindrical, puck-shaped shape, rather than the radiating elements of a typical antenna. DRAs do not experience conduction losses and can be highly efficient heatsinks. The first DRAs used ceramic materials characterized by high permittivity and high Q (between 20 and 2000). Newer DRAs are the plastic polyvinyl chloride, PVC.
DRAs are specialized versions of dielectric resonators. The dielectric material has a large dielectric constant and a low dissipation factor. (Recall that the dissipation factor is the reciprocal of the ratio of the material’s capacitive resistance to its resistance at a given frequency.) The resonant frequency is determined by the overall physical dimensions of the resonator and its dielectric constant. In a dielectric resonator, RF is confined inside the resonator material by the abrupt change in surface permittivity, so RF waves bounce back and forth between the sides. At resonant frequencies, standing waves are formed in the resonator, which oscillate with large amplitudes.
Thus, dielectric resonators resemble cavity resonators in their behavior. Cavity resonators are hollow metal boxes used as microwave resonators. Here the RF reflections are due to the large change in permittivity rather than the conductivity of the metal. Metal cavity resonators do not work at millimeter wave frequencies because in this frequency range their metal surfaces become lossy reflectors.
It should also be noted that while the electric and magnetic fields are zero outside the walls of a metallic cavity, these fields are not zero outside the dielectric walls of the resonator. However, the electric and magnetic fields decrease significantly from their maximum values ​​away from the resonator walls. Most of the energy is stored in the resonator at a given resonant frequency.
There are three types of resonant modes in dielectric resonators: transverse electric (TE, magnetic field only along the direction of propagation), transverse magnetic (TM, magnetic field is transverse to the direction of propagation while electric field is normal to the direction of propagation). direction of propagation), or hybrid electromagnetic (HEM, both electric and magnetic fields have a component in the direction of propagation) modes. There are theoretically an infinite number of modes in each of the three groups, and the desired mode is usually chosen based on the needs of the application.
To understand why DRAs can be smaller than conventional antennas, consider that the size of a metal antenna is proportional to the wavelength of its resonant frequency in free space. Conversely, the size of the DRA is proportional to the free-space wavelength divided by √ϵr, where ϵr is the relative permittivity of the DRA material. Thus, to shrink the DRA, use a high ϵr material. The use of a low-loss dielectric material also gives DRAs their high radiation efficiency. And a DRA antenna can be made with a relatively large bandwidth by adjusting the device dimensions and the dielectric constant of the material.
The resonant frequency of the modes that a given cylindrical DRA supports is a function of the resonator height, radius, and dielectric constant. The equations for specific resonance frequencies include roots of Bessel functions and their derivatives. Bessel functions are standard solutions of a differential equation known as the Bessel differential equation. Bessel functions are often described as a way to describe vibrations in a medium with variable properties, which is why they appear when describing DRA.
Although cylindrical DRAs are probably the most widely used, rectangular DRAs are also common. The main advantage of the rectangular shape is that it offers more design flexibility than cylindrical types by adjusting the height, length and width dimensions. Rectangular DRAs also have less cross-polarization than cylindrical types. (Cross polarization is
polarization orthogonal to the desired direction. If the antenna field is intended to be horizontally polarized, the cross-polarization direction is vertical. (If the antenna waves are right-hand circularly polarized, cross-polarized is left-hand circularly polarized, etc.) The DRA resonant frequency in this case is a function of the square root of the sum of the length, width, and height dimensions.
It is also possible to find hemispherical and cruciform DRA and DRA arrays. Cross-shaped DRAs tend to be used to produce circularly polarized waves. Arrays of crossed DRAs can be used to obtain greater antenna bandwidth.
Similarly, so-called super-shaped DRAs can be found. These are DRAs with a more complex 3D shape, usually made as a way to increase the antenna’s bandwidth. An example of a superformed DRA would be one where the basic shape is cylindrical, but instead of a curved side, it has eight flat sides.
One aspect of DRA technology is that specific emission modes can be excited depending on where the RF is fed into the DRA device. (Probe-fed DRAs are not practical when the DRA is made on a printed circuit board or integrated into an IC.) Probably the most common configuration for discrete DRAs is one in which the DRA is on a ground plane and driven by a coaxial cable fed through the substrate. Interestingly, the coaxial probe can penetrate the DR but can also be placed directly adjacent to it. Adjusting the length and position of the feed probe tunes both the input impedance of the DRA and its resonant frequency.
The main advantage of using probe power is that it couples a large amount of signal into the DRA, which in turn results in high radiation efficiency. The disadvantage of this approach is that a hole must be drilled in the DRA material that exactly matches the length and radius of the probe. Slight mismatches can change the dielectric constant of the resonator and shift the resonant frequency of the antenna. Placing the probe next to the DRA rather than inside it is less sensitive to size discrepancies but couples less signal to the antenna.
Another way to power the DRA is through printed transmission lines. In conventional line-fed microstrip DRAs, the dielectric resonator resides directly on the transmission line printed on the PCB substrate. The overlap distance on the printed wire determines the coupling strength and the specific transmission mode that is excited. The strongest coupling is when the overlap distance is slightly shorter than a quarter wavelength from the dielectric resonance frequency.
The main disadvantage of microstrip transmission line power is that the transmission line is not isolated from the DRA, which can affect the radiation efficiency of the DRA. Also, when the dielectric resonator sits on top of a transmission line, there is an air gap between the resonator and the PCB, which can degrade antenna performance.
The other way to create a microstrip transmission line feed is to place the DRA on the PCB and extend the PCB transmission line trace from the board to one of the outer sides of the DRA. This approach eliminates the air gap that occurs when the DRA is located on the microstrip transmission line. The shape of the transmission line can be optimized to improve the performance of the DRA, usually in terms of the antenna bandwidth.
A rectangular DRA powered by a CPW circuit with examples of capacitive and inductive slot geometries.
Another approach is the coplanar waveguide (CPW) fed DRA. A coplanar waveguide can take the form of a circular circuit or a capacitive or inductive feed channel. The main advantage is that setting the connector slot under the DRA can optimize the performance of the DRA. A capacitive slot, for example, can add a resonance so that one resonance is coupled to the DRA and another to the power slot itself. CPW feed structures are widely used for mmWave applications because the PCB ground plane separates the dielectric resonator from the lossy silicon substrate to obtain high antenna efficiency.
The most widely used technique for feeding DRA is through a slot in the ground plane, a method known as aperture coupling. Here, the DRA is located on the microstrip transmission line of the PCB and has a slot in the ground plane of the PCB, which is also located below the DRA and is oriented at 90° to the transmission line. The energy from the transmission lines passes through the slot to the resonant modes of the DR.
The best resonance (impedance match) occurs with the DRA perfectly centered at the top of the slot. The main advantage of this method is that direct electromagnetic interaction between the feed line and the DRA is avoided, which reduces spurious radiation from the feed network and increases the purity of the DRA polarization. The disadvantage of this approach is that it becomes impractical at lower frequencies; the slot length must be about half a wavelength at the resonant frequency, which is a challenge at lower frequencies when the DRA must remain compact.
In general, the size of the DRA is inversely proportional to the dielectric constant ( ) of the resonator material. But using high also limits the antenna’s bandwidth. Multi-layer DRA topologies are sometimes used to mitigate the bandwidth problem by optimizing the pitch and height of each layer.
Some DRAs contain a conductive plate strategically placed as a means of reducing the size of the antenna. On a rectangular DRA, for example, a conductive plate may pass to one of the outer surfaces of the antenna. The price paid for the size reduction is usually reduced antenna bandwidth.
Another way to get more gain from a given DRA is to excite it into a higher order resonant mode. This makes the DRA electrically larger with respect to its fundamental resonant frequency.
References
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