Introduction
In microwave systems that use a single antenna opening for the reception and transmission of signals, such as radar systems, nonreciprocal components remain essential. For instance, circulators are used to route incoming and outgoing signal flows to the receiver and transmitter, respectively [1]. Likewise, in the signal acquisition chain, nonreciprocal components, especially isolators, allow for controlling unwanted reflections and ensuring source protection.
In particular, isolators are mainly used to combine transmitters, decouple amplifiers, and decouple generators from their load [2–4]. However, compactness, weight, and power efficiency remain major challenges in the manufacture of these components.
Tunable devices are key elements in cognitive radio communication. For instance, an intelligent system that can change its operating frequency will need a tunable isolator to help control the number of active elements.
Previous studies have developed several kinds of isolators, such as those based on ferromagnetic resonance, Faraday rotation, and/or field displacement, for use in waveguide and planar technologies [5–12]. However, the miniaturization and integration of these isolators remains a major problem for the microwave telecommunications industry.
To increase the operating frequency and ensure the compactness of nonreciprocal components, researchers have proposed using new structures that combine ferrites and metamaterials, such as high-impedance wires, as well as magnetic nanowires in an alumina matrix [13–17]. Notably, the operating frequencies of these structures were magnetically tunable. However, for microwave applications, the required DC magnetic bias field is so high that it cannot be provided by a small, lightweight permanent magnet. As a result, these isolators cannot be easily used in integrated systems.
The tunable isolator presented in this article is based on the structure presented in [18, 19]. The proposed isolator features a coplanar waveguide (CPW) on a ferrite substrate (yttrium iron garnet [YIG]). Its operating frequency can be tuned either magnetically by applying a variable magnetic field or mechanically. Notably, since mechanical frequency tuning is more practical for implementation in integrated devices, the proposed isolator aims to achieve exactly that. Even when considering a weak magnetic bias field, the experimental results show that the operating frequency of the proposed isolator can be tuned from 13.6 GHz to 15.15 GHz without any change in the applied magnetic field.
Theory and Design
The basic structure of the proposed design involves a coplanar high-impedance wire (HIW) realized on a ferrite substrate, described in detail in a previous publication [18]. The structure (Fig. 1) is based on a CPW constructed on YIG substrate (ηr = 15.3 and tanδ = 0.002), with dimensions of 8 mm × 11 mm × 1 mm. The saturation magnetization MS of the YIG was around 1,750 Oe, with a ferromagnetic linewidth ΔH = 20 Oe at the X-band. Unopened slots, considered short-circuited stubs, are periodically etched on a ground plane. The dimensions were fixed in compliance with the design of an HIW [20]—the unit cell period p was kept considerably smaller than the slot length d. Furthermore, the resonant frequency was mainly determined by slot length d and the effective permittivity and permeability of the medium.
In [17], the following equation was devised to calculate an approximation of the cut-off frequencies:
where n = 0, 1, 2, 3 ..., c indicates the light celerity, while ηeff and μeef are the effective permittivity and permeability of the slotline, respectively. The theoretical explanation for this equation is detailed in [18].
When an in-plane DC magnetic bias is applied along the x-axis (see Fig. 1), the magnetic field around the coupled slots is characterized by right-handed circular polarization (RHCP) for forward input and left-handed circular polarization (LHCP) for backward input. Effectively, the propagation constant in the stubs takes two different values, depending on the direction of propagation. Using Eq. (1), for a fixed slot length, it is possible to achieve the tunability of the resonance frequency by adjusting either the effective permeability or the effective permittivity of the medium. Moreover, variations in the DC magnetic bias field can also change the effective permeability of the slot. Consequently, the resonance frequencies of a coplanar HIW can be tuned using Kittel's equation [21]. Another way to achieve isolator tunability is to keep the bias magnetic field constant and change the effective permittivity of the medium by adding a metal plane close to the conventional CPW [22].
Fig. 2 presents the modified structure of the coplanar HIW, with a copper layer placed on it at a variable distance of hd. When the distance between the copper plate and the structure was modified, the configuration of the electromagnetic field and the effective permittivity changed, suggesting that the operating frequency varies as a function of the distance hd (see Eq. 1).
Notably, an investigation into the effective dielectric constant ηeff and characteristic impedance Z0 of a conventional CPW fabricated on a finite-thickness dielectric substrate with a top metal cover has been presented in [22].
The design of the proposed isolator involved several steps: the slot length d was fixed according to the working frequency, while the slot width g and the period p was chosen so as to uphold the conceptual validity of high impedance wire valid (p << signal period). The length of the structure was fixed by limiting the number of slots (usually, nine slots are sufficient). The permittivity and permeability of the ferrite substrate are known. Notably, a substrate with low dielectric and magnetic losses was preferable for the purpose of this study. Furthermore, the operating frequency was adjusted by adding the copper plate on top.
Experimental Results
The prototype was built on a YIG substrate with a relative dielectric constant of 15.3 and a thickness of 1 mm. The substrate was covered by a 3 μm-thick copper layer. The CPW used for the prototype had a characteristic impedance of 44 Ω corresponding to W/S/W of 0.23/0.372/0.23 mm. Furthermore, the stub network etched on the ground plane had the following features: width of stub (g = 16 μm), length of slot (d = 4.7 μm), period of network (p = 400 μm), and number of stubs (N = 19).
The total area of the structure was 8 mm × 11 mm. The dimensions of the top copper plate, which enabled mechanical tunability of the operating frequency, were 10 mm × 6 mm. The distance hd between the copper plate and the coplanar HIW varied from 200 μm to 1,600 μm. Moreover, to ensure accuracy and uniform variation of this distance, several 200 μm-thick layers of polyfluor (PTFE) were interspersed between the coplanar HIW and the top copper plate, so that a variation in the number of PTFE layers also led to a variation in the distance hd.
The S-parameters of the prototype of the proposed structure are presented in Fig. 3. A vector network analyzer (VNA) was employed to measure the nonreciprocal propagation using a probe station. The bias field was created by an electromagnet (Fig. 4). Furthermore, OSTL (open short thru line) calibration was used to calibrate the network analyzer. Notably, the transmission parameters S12 and S21 were different at the operating frequency.
Notably, the magnetic bias field can be applied along the stub direction (in the plane perpendicular to the CPW), either by an electromagnet or a permanent magnet.
In the bias-field configuration (transverse polarization), the demagnetization field was negligible, while the magnetic material was saturated using a low magnetic bias field (≈70 Oe). Considering the objective of miniaturization, a weak bias field should be more convenient for small magnets to create. The experimental results obtained using a low-bias field of 70 Oe are depicted in Fig. 5. At 13.6 GHz, the isolation is around 40 dB, the insertion loss is about 2 dB, and the return losses are around 10 dB.
Furthermore, the operating frequency of the substrate was outside the gyromagnetic resonance band, leading to low magnetic and dielectric losses. Notably, owing to the thickness of copper, metal losses can be neglected in this analysis. The high isolation observed at the resonance frequency of the HIW can be attributed to the radiation loss phenomenon, similar to that observed in leaky-wave antennas [23–27]. The structure of the surface adopted in this study can also be compared to a circular split resonator for antennas [28], filters using surface plasmons [29], wideband circular polarization metasurface antennas [30], and wideband bandpass filters based on a substrate integrated waveguide and microstrip line [31].
As mentioned in Section II, the operating frequency can be tuned by changing the DC bias field—a classic method used to achieve the tunability of the resonant frequencies of ferrite devices. Fig. 6 illustrates the variation in the operating frequency of the isolator with regard to different values of the DC magnetic bias field. It is observed that when the magnetic bias field increases from 70 Oe to 2,300 Oe, the operating frequency increases from 13.6 GHz to 14.3 GHz. Notably, as a result of the high applied bias field, this method failed to achieve tunability of the operating frequency.
A more practical way of tuning the operating frequency is to fix the bias magnetic field and place the copper plate at an adjustable distance above the insulator. Notably, the copper plate used for this experimental demonstration was 8 mm long and 3 mm wide. Several spacers of 200 μm thick PTFE were placed on the structure to separate the HIW structure from the copper plate. Fig. 7 depicts the S-parameters measured for three values of hd, with the DC magnetic field Ha = 70 Oe kept constant. It is observed that the operating frequency decreases when the distance between the isolator and the copper plate varies from a few hundred micrometers to about 1.2 mm. Beyond this distance, the copper plate has no influence, and the isolator operates at its initial frequency. Fig. 8 traces operating frequency variations as a function of distance hd when a constant bias field of 70 Oe is applied to the structure.
The variations in isolation and insertion losses are illustrated in Fig. 9.
When the plate was placed at a height of 1,600 μm or more, the operating frequency achieved was 13.6 GHz, consistent with the initial structure (without the copper plate). When the distance from the copper plate to the line decreased, the operating frequency increased progressively until it reached 15.15 GHz for a minimum distance of 200 μm. Furthermore, the isolation remained more than 20 dB, and the insertion loss was less than 2 dB. Hence, without changing the value of the magnetic bias field from 70 Oe, the operating frequency could be tuned from 13.6 GHz to 15.15 GHz while maintaining good isolator performance.
Moreover, the copper plate tended to change the configuration of the magnetic and electric fields above the coplanar HIW, confining the radiation inside the shielded component. As a result, the effective permittivity ηeff and characteristic impedance Z0 changed, leading to variations in the resonance frequency. This allowed for the frequency of the structure to be mechanically tuned for different values of the magnetic applied field. Fig. 10 summarizes the results of the measurements carried out in the magnetic bias field ranging from 70 to 2,080 Oe, considering several values of hd. It indicates that mechanical tuning can be added to the magnetic one to tune the operating frequency by about 2 GHz.
Table 1 summarizes the performance of the proposed tunable isolator and compares it with other recently proposed isolators in the literature.
Conclusion
In this study, a nonreciprocal microwave device with a coplanar defected ground transmission line realized on a ferrite substrate is designed, fabricated, and measured. The proposed device can operate in the Ku-band with a low magnetic bias field, which can easily be generated by a small magnet or a self-biased ferrite. Compared to other structures proposed in the literature (see Table 1), the simplicity, small footprint, and low-bias magnetic field of the proposed isolator are its main advantages. At only a 70 Oe magnetic bias field, the prototypes showed good performance, successfully implementing a narrow-band tunable isolator characterized by isolation of over 20 dB and insertion loss of about 2 dB at 13.6 GHz. Furthermore, new developments are already under consideration for further reducing the insertion loss and enlarging the bandwidth.
In the prototype proposed in this study, satellite connectivity was specifically targeted. The Ku-band remains the band that is most commonly used by satellites, especially for the transmission of television services. Using the proposed CPW access line, our structure can easily be connected with MIMO antenna in the Ku-band, such as the one described in [28]. Consequently, this shield can be used in miniaturized and integrated microwave systems.
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