A Brief Introduction to Slot Antennas
Learn about slot antennas, namely their history, characteristics, and electromagnetic (EM) behavior for low-voltage, small-form-factor electronic devices.
Though they date back to the middle of the twentieth century, slot antennas are the subject of much recent research and have become crucial elements in the design of compact, high-frequency wireless devices.
Most of us, I think, envision an antenna as a thing, be it a:
- Metal contraption on the roof of a house
- Massive dish that communicates with satellites
- Surface-mount chip antenna
- PCB trace
It turns out, though, that an antenna can also be the absence of a thing; a slot antenna consists of one or more apertures created by removing material from a conductive surface.
A (Very) Brief History of Slot Antennas
Browsing the research literature gives the impression that the slot antenna is a recent innovation associated with the proliferation of compact, high-performance RF circuitry; however, slot antenna R&D actually began before World War II. As with many other technologies, military hostilities favored advancement, in this case, because slot antennas had the potential to improve the performance of radar systems.
Slot antenna technology has long been associated with relatively high frequencies, but in the early days, “relatively high” could mean hundreds of megahertz, and antennas optimized for frequencies in that range were rather large. These types of antennas are more specifically known as slotted waveguide antennas (SWAs), which is when a larger conductive object is the waveguide, and the apertures in this object, sized according to wavelength, powerfully influence how the overall structure will radiate (Figure 1).
Figure 1. This is an aluminum prototype version of an SWA that is intended for use as a wearable made of conductive fabric. Image used courtesy of Mikulić et al
SWAs are still used in maritime and airborne radar systems, offering good performance relative to cost and complexity.
As the 20th century continued, scientists and engineers gradually accumulated a large body of knowledge on the design, analysis, and implementation of slot antennas. In this article, we’re more interested in slot antenna technology as it is used in low-voltage, small-form-factor electronic devices.
Slot Antenna Key Characteristics
Slot antennas are common in high-frequency applications. Early SWAs were incorporated into radar systems operating at low microwave frequencies, and recent research involving slot antennas is exploring applications beyond 100 GHz. The wearable antenna mentioned above is designed for IoT-style wireless communication in the 5.8 GHz ISM (industrial, scientific, and medical) band, and there seems to be significant interest in slot antennas for use in millimeter-wave 5G designs.
The performance of a slot antenna is dependent on various factors, such as geometry and whether the slot has a back-cavity. In general, though, slot antennas are attractive for advanced RF devices because they can typically offer:
- Wide bandwidth
- Good efficiency
- Versatility
- Low cost
- Ease of manufacturing
- Low-profile form factor
The Electromagnetic Behavior of Slot Antennas
A theoretical, rudimentary slot antenna is simply a rectangular aperture in a conductive plane. If an RF (radio frequency) voltage signal is applied to opposite sides of the aperture, current will flow around the perimeter, and the structure will radiate.
The idea of empty space functioning as a high-performance antenna is counterintuitive, and you may find it helpful to invoke Babinet’s principle and then imagine a slot antenna as the “dual” of a foundational antenna configuration known as a dipole. Note, if you’re not familiar with dipole antennas, you can read about them in this excellent article by Mark Hughes.
Babinet’s principle is actually taken from optics, and it states the following:
“The sum of the wave transmitted through a screen . . . plus the wave transmitted through the complementary screen, is the same as if no screen were present.”
This concept was extended into the electromagnetic realm by Henry Booker, and when the Babinet–Booker principle is applied to a slot antenna, it suggests that we can create a complementary conductive pattern and expect similar radiation behavior. Figure 2 shows an example of this.
Figure 2. Slot in the middle of the conductive plane (a) and an example signal fed into the dipole antenna (b). Image used courtesy of John Borchardt
The above figure is taken from a paper written by John Borchardt, a researcher at Sandia National Laboratories. Figure 2(a) shows the slot in the middle of a conductive plane, and in Figure 2(b), a signal is fed into a dipole antenna whose conductive sections correspond to the geometry of the aperture. Borchardt used the dipole version to calculate the impedance of the slot version.
Adding to the concept of slot antenna behavior, Figure 3 provides another example, this time from the Evolution of Compact Slot Antennas by Alan Sangster.
Figure 3. Example slot antenna behavior diagram. The image was redrawn with reference to and courtesy of Alan Sangster
In this case, the slot antenna is implemented as an aperture in a microstrip transmission line. Note the orientation, where the slot, directly above and perpendicular to the microstrip, is oriented so as to disrupt the current flow. This disruption leads to both capacitive and inductive effects, and when the geometry of the slot (relative to signal wavelength) favors inductive-capacitive resonance, the transmission line functions as an effective radiator.
Slot Antennas—Old Tech for New Designs
Slot antennas represent fairly old technology that has acquired new relevance and new design techniques in the age of 5G and IoT. Though there is much more that could be said about slot antennas, I hope that this article has given you a solid foundation for further study.
