The Rotman lens antenna system harvests 28 GHz 5G power

What you will learn:

  • How did a Georgia Tech team overcome certain limitations to develop an antenna that extracts power in the 28 GHz band?
  • The key role that the Rotman lens plays in the implementation.
  • Test results of the lens-based rectenna.

With all of the ongoing 5G activity, it makes sense that non-5G applications also look for ways to harness and harness some of the technological advances and energy associated with it (figuratively and literally). This was achieved by a team from the Georgia Institute of Technology (Georgia Tech) with an innovative combination of antenna and power harvester that is intended to effectively capture and correct ambient energy in the 28 GHz 5G band. This goes well with many smaller applications such as IoT devices as the need for batteries is reduced or possibly even eliminated.

The idea of ​​such an antenna (called a “rectenna”) is not new. However, it is challenging to develop one that works and works well at 28 GHz due to some difficult technical realities and tradeoffs. On the one hand, it is desirable to have a larger, high gain antenna in order to capture a significant amount of RF energy despite the high path loss at these higher frequencies, but such antennas have a narrow angular field of view.

While it is possible to use a basic array of small antenna elements, each acting as a rectenna and then combining their DC outputs, this obvious approach does not increase the turn-on sensitivity (lowest turn-on power) of the entire rectenna system, and thus its threshold for useful functioning.

To overcome the limiting trade-off between the Rectenna’s angular coverage and switch-on sensitivity, as well as other limitations, the team installed a unique version of a Rotman lens between the antennas and rectifiers. This eponymous lens itself isn’t new: it was introduced by Walter Rotman in the 1960s and is one of the most common and inexpensive designs for beamforming networks (see references below).

The lens enables multiple antenna beams to be formed without the need for switches or phase shifters. In highly simplified terms, the beam openings are positioned in such a way that constant phase shifts are achieved at the antenna openings, the antenna elements being fed on phases which change linearly over a row. As a result, it behaves like a phased array. (An important feature of this lens is that while it has many 50 Ω connections attached, these are isolated and therefore do not affect the loss or noise figure of adjacent beams.)

“We solved the problem of being able to only look in one direction with a system with a wide angle of coverage,” said lead researcher Aline Eid in the ATHENA laboratory at the Georgia Tech School of Electrical and Computer Engineering, set up for the advancement and developing novel technologies for electromagnetic, wireless, RF, millimeter wave and sub-terahertz applications.

Start with a subarray

The project team started the design with an antenna sub-array. Several such sub-arrays, rectifiers, and DC combiners were then used to demonstrate the wide angle coverage and turn-on sensitivity. Among the many data points, in their current configuration they showed a harvesting ability up to a distance of 2.83 meters, which should achieve a DC power of about 6 μW at 75 meters at 75 dBm effectively isotropically blasted power ((EIRP). In addition, the system has a very low profile to conform to surfaces such as walls, bodies, vehicles and other devices.

Based on their detailed analysis of the number and size of the subarray elements and the available materials, they came to the conclusion that a subarray combination of eight antennas and six beam ports was an almost optimal compromise between a high array factor of 5.95 dB and a total of 120 ° provides angular coverage while maintaining a reasonable number of antennas and beam ports. The design was printed on a flexible copper-clad liquid crystal polymer (LCP) substrate (εr = 3.02) using an inkjet printed mask technique followed by etching (Fig. 1).

% {[ data-embed-type=”image” data-embed-id=”60a2c5e82f5c1326598b47e2″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”1. The antenna design was printed on a flexible copper-clad liquid-crystal-polymer substrate using an inkjet-printed masking technique followed by etching.” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/05/ED_28_GHz_5G_Harvesting_Fig1.60a2c5e7b56cb.png?auto=format&fit=max&w=1440″ data-embed-caption=”1. The antenna design was printed on a flexible copper-clad liquid-crystal-polymer substrate using an inkjet-printed masking technique followed by etching.” ]}%

The entire lens-based system

The overall system architecture (Fig. 2) has eight antenna sub-arrays attached to the Rotman lens from one side and six rectifiers facing on the opposite side, where a serial DC combination is implemented using Schottky diodes MACOM MA4E2038.

% {[ data-embed-type=”image” data-embed-id=”60a2c5ff2f5c13cf798b4641″ data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”2. The Rotman lens between the antennas and the rectifiers is a key element of the harvesting topology (a); a plot of the simulated maximum array factors and angular coverages for different-size Rotman lenses (b); photo of the fabricated Rotman-lens structure (c).” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/05/ED_28_GHz_5G_Harvesting_Fig2.60a2c5fe764e6.png?auto=format&fit=max&w=1440″ data-embed-caption=”2. The Rotman lens between the antennas and the rectifiers is a key element of the harvesting topology (a); a plot of the simulated maximum array factors and angular coverages for different-size Rotman lenses (b); photo of the fabricated Rotman-lens structure (c).” ]}%

The Rotman-based Rectenna turns on well below –6 dBm / cm2, which the team says is quite cheap compared to other devices in the published literature. The rectenna output voltage was also measured over its operating frequency range with the system positioned at the same harvesting angle, within 10 inches of the source horn antenna, and from 27.8 to 29.6 GHz.

Toshiba 1SS384TE85LF bypass diodes were used in the DC combiner design to create a low impedance current path around all other rectifiers that were receiving very low or near zero RF power (Fig. 3). This topology is optimal when only one diode is switched on. This can be assumed if a single dominant power source irradiates that particular design from a certain direction.

% {[ data-embed-type=”image” data-embed-id=”60a2c61466c903eb428b489b” data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”3. The Rotman-based rectenna power summation network (a) and picture of the setup used to measure the angular response of the rectenna (b).” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/05/ED_28_GHz_5G_Harvesting_Fig3.60a2c6135bda3.png?auto=format&fit=max&w=1440″ data-embed-caption=”3. The Rotman-based rectenna power summation network (a) and picture of the setup used to measure the angular response of the rectenna (b).” ]}%

This simplified scheme has only four diodes. Different colors mark the paths that the current takes for each case in which an RF diode is switched on while the diodes connected in series are switched off. This dc combiner was made on a flexible one 125-μm-thin polyimide Kapton substrate connected to the Rotman lens base via a series of individual connectors to make the whole system completely flexible and pliable. The DC combiner uses a reduced number of bypass diodes and increases the angular coverage of the system by more than 30% compared to existing designs.

To test the performance under convex and concave bending conditions, the lens-based rectenna was placed on cylinders with different curvatures at a distance of 70 cm from the transmitter, with a power of 25 dBm at 28.5 GHz being transmitted (Fig. 4). The tension was collected using a 1kΩ Loading for the planar and three curved conditions related to the angle of incidence of the source.

% {[ data-embed-type=”image” data-embed-id=”60a2c62ca314ee93398b487f” data-embed-element=”span” data-embed-size=”640w” data-embed-alt=”4. The flexible Rotman lens-based rectenna placed on a 1.5-in. radius cylinder (a) and measured harvested powers versus incidence angles for different curvatures (b); long-range harvesting testing setup (c).” data-embed-src=”https://img.electronicdesign.com/files/base/ebm/electronicdesign/image/2021/05/ED_28_GHz_5G_Harvesting_Fig4.60a2c62c6761e.png?auto=format&fit=max&w=1440″ data-embed-caption=”4. The flexible Rotman lens-based rectenna placed on a 1.5-in. radius cylinder (a) and measured harvested powers versus incidence angles for different curvatures (b); long-range harvesting testing setup (c).” ]}%

The graph shows excellent consistency and stability in the system’s purge and rectification capabilities, even though several subsystems are subject to warping and bending pressures: the antenna subarrays, the Rotman lens, and the rectifiers. A slight damping can be seen at the edges, but otherwise the system works unhindered by the bend. This property is well suited for use with wearables, smartphones, and compliant 5G energy harvesters for IoT nodes.

A high-performance antenna system with a conical horn antenna with 19 dBi and a dielectric PTFE lens with 300 mm diameter (for high directivity), which offers an additional gain of 10 dB, was used to demonstrate the lens-based rectenna for harvesting over longer distances. With a transmit power of 25 dBm (and an associated EIRP of around 54 dBm), which corresponds to an incident power density of around -6 dBm / cm², the lens-based rectenna showed an extended range of 2.83 meters with an output voltage of 10 under open load conditions mV. The researchers believe this is the most distant demonstration of rectenna at millimeter-wave frequencies to date.

The project was supported by the Air Force Research Laboratory and the Emerging Frontiers in Research and Innovation (EFRI) program of the National Science Foundation. You can find detailed information in the work with three authors with the deceptively inconspicuous title “5G as a wireless power network”Published in Nature Scientific Reports. (Personal perspective: while the paper has all the details, dates, and reference citations I wanted, I found it a little difficult to follow. It requires better organization, layout, clarity of sentences and story flow, and much shorter paragraphs.)

References

Microwaves & RF, “Rotman Lens electronic beam steering targets 5G signals”

W, Rotman and RF Turner, “Wide Angle Microwave Lens for Line Source Applications”, IEEE transactions on antennas and propagation11 (6), pp. 623-632, 1963.

W. Rotman, US Patent 3170158, “Multiple Beam Radar Antenna System”

Microwaves 101, “Rotman Lens”

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