This work presents a review on the concept of harmonic or secondary radar, where a tag or transponder is used to respond at a harmonic multiple of the incoming interrogation signal. In harmonic radar, the tag is called a harmonic transponder and the necessary frequency multiplication is implemented using a nonlinear element, such as a Schottky diode. Different applications and operating frequencies of harmonic transponders are presented, along with various tag design aspects. The designer may have to deal with certain tradeoffs during the design with respect to a number of transponder properties, and the role of these tradeoffs is also considered. Additionally, techniques usable for characterization of harmonic transponders are discussed.
In recent years, the use of different wireless and/or contactless techniques for identifying and tracking various objects has increased significantly. Different technologies that are used range from conventional bar code systems and radio frequency identification (RFID) tags to sensors that potentially allow carrying additional information on the properties and state of an individual object or of the environment. Even though the wireless sensing scheme has become more and more ubiquitous, planned machine-to-machine communication, Internet of Things (IoT), and related technologies bring forward a need for ever increasing wireless sensing [
The general concept of wireless sensing involves a sensor and a reader device used to interrogate it. In many implementations, the reader contains both the transmitter (Tx) and receiver (Rx), but in principle they can be separate devices as well. Depending on the sensing scheme, the forward and backward communication from Tx to sensor and from sensor to Rx, respectively, can occur at the same or different frequency bands.
An example of a sensor implementation in which two distinct frequency bands are used for communication is the so-called harmonic or secondary radar, which is one example of nonlinear radar. In the harmonic radar approach, the sensor or tag is called a harmonic transponder. It receives the interrogation signal at a certain fundamental frequency
Illustration of the harmonic radar concept. Here, the second harmonic frequency
Harmonic radar is a special case of the more general nonlinear radar concept, in which the response signal is also created through nonlinearities in the transponder. Examples of this can be found in some automotive radar and intermodulation sensor applications, where two closely spaced interrogation signals (
One of the benefits of using harmonic radar is the possibility to obtain an improved performance in the presence of strong environmental clutter. If the tag would simply respond back at the interrogation frequency in an environment with strong clutter, its response could be obscured by reflections from surrounding objects or by interference from other radio systems operating at the same frequencies. By using a harmonic response signal, it is easier to conclude that the observed response is caused by the tag rather than the surroundings. This is due to the fact that most “natural” objects do not display nonlinear properties at typical power levels used in wireless sensing and therefore are not able to reflect back at other frequencies than the incoming frequency. In the harmonic radar concept, particular attention must be paid to the transmitter power level and nonlinearities, so that the weak received signal can be distinguished from transmitter-based harmonics [
This review article provides an overview on different transponder implementations that have been applied in harmonic radar. Section
Primary components of a harmonic transponder are a nonlinear element and an antenna. The nonlinear element can be, for example, a Schottky diode, and it performs the required frequency multiplication from the fundamental frequency to the desired harmonic frequency (typically from
The requirement that the transponder has to operate simultaneously on two different frequency bands (which additionally need to be harmonically spaced) provides a further design challenge. This complicates the transponder design compared to that of regular RFID tags (either single- or dual-band). Antenna designs for conventional RFID tags operating at the UHF range have been previously reviewed in, for example, [
In some cases, it is desired that the transponder is small in size. The size limitation may pose some challenges regarding the design. A well-known property of designing (electrically) small antennas, for example, for handset applications, is that it is fundamentally impossible to obtain an antenna that would simultaneously have small size, good efficiency, and broad bandwidth. Rather, two out of these three properties or requirements can be met simultaneously. In the case of harmonic transponders, similar constraints are set both by the antenna and the nonlinear element. The impedance characteristics of the diode affect the complexity of the required matching scheme. Increasing the complexity of the matching scheme may necessitate a larger antenna, which can, on the other hand, improve the bandwidth and efficiency properties.
An expression for the power generated by transponder at the second harmonic frequency (
The task of the transponder designer is to weigh the different design and performance aspects for the purpose of the targeted application. In some cases, it may be, for example, sensible to allow a slightly worse matching level in order to have a more compact-sized design. Other considerations can also be valid.
The concept of harmonic radar was first introduced in the 1960s [
Recently, the use of harmonic radar and transponders has been suggested for remote sensing and detection of vital signs in biomedical applications [
In certain applications, such as insect and amphibian tracking, particular attention has to be paid to the size of the transponder. Obviously, these transponders need to be tracked, but at the same time they have to be sufficiently small and lightweight so as not to prevent the natural movement of the animal. It is also good to keep in mind that, compared to RFID tags that can provide information (or ID) of the object onto which they are attached, typical harmonic transponders do not in most cases enable such a possibility. The transponder mainly provides information on whether it is detected or not; that is, does the receiver pick up a signal at the correct (harmonic) frequency?
Depending on the targeted application and also in part on existing frequency allocations transponders operating at different frequency bands have been presented. Table
Summary on the features of some harmonic radar tags found in the literature.
Reference |
|
Transponder dimensions | Mixing element | Type of matching |
---|---|---|---|---|
[ |
1 & 2 |
|
Schottky | Direct |
[ |
9.41 & 18.82 |
|
Schottky | Direct |
[ |
0.917 & 1.834 |
|
Schottky | Direct |
[ |
1.59 & 3.18 |
|
Nonspecified diode | Stepped-impedance microstrip lines |
[ |
2.45 & 5.9 |
|
Schottky | Direct |
[ |
5.9–6 & 11.8–12 |
|
Schottky | Direct |
[ |
38.5 & 77 |
|
Schottky | Direct |
[ |
0.914 & 1.828 |
|
Schottky | Direct |
[ |
2.4 & 4.8 | 30 mm (diagonal) | Schottky | Impedance transformer based on microstrip lines |
[ |
1.3 & 2.6 |
|
Schottky | Direct and external matching |
The unlicensed Industrial, Scientific and Medical (ISM) band has also found use in transponder applications and here fundamental frequencies of 2.45, 5.8, or 5.9 GHz have been applied [
On a principal level, the required operating frequencies can be obtained by matching the diode directly to the antenna by suitably modifying the antenna geometry or by applying a dedicated matching circuitry. An illustration of the two approaches is shown in Figure
Schematic illustration of the two principal matching techniques applicable to harmonic transponders: (a) direct matching and more complex antenna geometry and (b) external matching circuit and simpler antenna geometry. The figures are only indicative and not to scale.
In the following, different harmonic transponder implementations based on the direct matching scheme presented in the literature are investigated, and the potential, benefits, and drawbacks of using an approach based on separate matching circuits are considered. In both of these approaches, the difference between the impedance characteristics of the antenna and those of the diode (exact impedance level and how steeply it changes as a function of frequency) determines how complex the matching scheme needs to be.
Considering actual physical implementations, the particular application and intended transponder shape affect to some extent the manufacturing techniques that are suitable. From the literature, different antenna types used in harmonic radar applications range from relatively simple wire antennas (e.g., [
Generally, many of the techniques used to manufacture small antennas can be used, including microstrip technology, but the use of modern or more advanced technologies such as 3D printing has also been suggested. Antennas have been manufactured using 3D printing both for conventional RFID tags [
The first design strategy considered here is to match the diode impedance directly to the load at the desired fundamental and harmonic frequencies (in practice,
Depending on the chosen implementation, the antenna can consist of one or more dedicated parts, each having their own effect on the overall transponder operation. One such design path has been considered in [
(a) Example transponder antenna and (b) obtained theoretical and measured response at the second harmonic frequency
The antenna shown in Figure
Regarding the transponder performance at different frequencies, it was observed in [
The performance of harmonic transponders can be characterized and optimized using various analytical, numerical, and empirical techniques. Depending on the requirements of particular implementations, one or more of these approaches may need to be used simultaneously. For instance, in [
A somewhat similar approach has been considered in [
Direct matching based designs have also found many other implementations for harmonic transponders, and some of these are illustrated in Figure
Examples of different transponders found in the literature: (a) dipole with inductive loop [
The design of [
The previously described implementation technique is in principle straightforward, but it can in practice be quite time-consuming. This can be the case, especially if the nonlinear element has a challenging impedance level or if the designer wants to switch to a different diode whose impedance characteristics considerably differ from those of the previously used diode. The effect of using different diodes with a fixed antenna geometry has been investigated in [
For this reason, one possible alternative is to use a separate, dedicated matching circuit that is responsible for creating the two resonances. The main difference to the previous approach is that the overall performance is considerably less dependent on the properties of the antenna geometry, meaning that the antenna design becomes easier. With a separate matching circuit, the design challenge is partially transferred from an antenna design problem to a circuit design problem, in particular one for finding a sufficiently well-performing circuit topology.
In addition to matching circuits based on discrete, lumped components, it is also possible to consider circuits with distributed components, such as the stepped-impedance microstrip lines in [
As discussed in, for example, [
A key drawback or issue using especially the approach based on lumped components is the potentially high losses caused by the circuit components. This aspect needs to be properly accounted for during the design process. Another matter which has been discussed in the framework of conventional RFID tags is that using separate, surface-mounted components is typically challenging due to issues related to cost and fabrication [
In addition to the types of nonlinear element and matching scheme used, it is possible to classify harmonic transponders based on the number of antennas that are implemented. One approach is to use separate antenna elements (and possible matching circuits) at the fundamental and harmonic frequencies, and these are then connected to the nonlinear element. In this case, each antenna element is applied only for one resonance frequency.
Another possibility is to use a single radiating element to which two different resonances are generated. This can be achieved by using an antenna element having separate resonant paths for different resonant modes or through taking advantage of several resonant modes of a single resonant element for dual-frequency operation [
The number of antennas affects in part the overall complexity of the design. Having more than one antenna typically increases the space occupied by the transponder, but on the other hand this may make the design easier (e.g., through having to match the nonlinear element to the antenna only at one frequency). In some applications, the use of antenna arrays with more than two elements has been suggested ([
A slightly different approach is taken in [
One issue related to improving the design and investigating the performance of harmonic transponders is their characterization. This is due to the fact that the transponder antennas may not necessarily have place for connecting a measurement cable, which can also affect the performance of the device under study. The cable can even begin to act as a significant part of the radiating structure. Furthermore, many conventional measurement techniques are used in a 50
Harmonic transponders can be characterized using different techniques, depending in part on the properties that are of interest. The harmonic response of the transponder of Figure
An alternative technique that has been proposed to design, investigate, and characterize both conventional RFID tags and harmonic transponders is based on intermodulation response [
The intermodulation approach enables obtaining valuable information regarding the operation of the transponder, such as radiation pattern and matching level. These can be beneficial both for analyzing the performance of the transponder and for improving it, if necessary. The intermodulation measurement is contactless, which means that the transponder can be measured with the actual load (diode) connected to it, even in the proximity of some object the transponder may be attached to. In the light of current knowledge, this is the best possible approach to characterize harmonic transponders and one that can most accurately replicate real usage conditions.
One aspect related to the different measurement techniques is the amount of equipment needed. Compared to harmonic measurements, the intermodulation-based measurements are more hardware-intensive. This is due to the fact that separate measurement equipment is required for the two closely spaced frequencies, compared to the case of transmitting just one frequency at a time in the harmonic measurements.
During the measurements (both in the harmonic and intermodulation case), it is important to make sure that the wanted, possibly small, response is not suppressed by harmonic or intermodulation responses generated by the measurement equipment itself. This can be obtained, for example, through choosing a suitable equipment placing or measurement geometry. Additional information on these aspects can be found in [
This paper has presented an overview on harmonic radar and transponders. The concept provides a means of obtaining improved object detection in areas with strong environmental clutter. Different ways of implementing harmonic transponders have been investigated and compared. Transponders of various shapes and sizes can be found from the literature for various frequencies and applications. Harmonic frequency generation is obtained using a nonlinear element, which in most cases is a Schottky diode. Potential techniques for characterizing the properties of harmonic transponders have also been discussed, and their possibilities and limitations are considered.
The future outlook of harmonic transponders seems promising, as new applications for the concept have been proposed in recent years beyond conventional animal tracking and locating of avalanche victims. Most of the implementations that have been presented so far use transponders with an antenna directly matched to the diode. Future and ongoing work in the field includes investigating further alternative design strategies including wider use of transponders with external matching circuits, as well as considering possible novel applications for the harmonic radar concept.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported in part by the Academy of Finland under Decision 289320. Kimmo Rasilainen wishes to thank the Aalto ELEC Doctoral School and the Nokia Foundation for financial support.