Nil Apaydin

Experimental Validation of Frozen Modes Guided on Printed Coupled Transmission Lines

Previous work has theoretically demonstrated that nonreciprocal slow-wave modes, namely, “frozen modes,” can be supported on a pair of coupled transmission lines printed on a magnetic substrate. Small antennas have also been designed by exploiting these modes. However, to date, we have yet to demonstrate and observe their existence experimentally. To this end, we construct two printed prototypes comprised of several unit-cells and employ the “T-matrix method” to determine the dispersion properties by measuring the S-parameters of these finite periodic prototypes. The printed unit-cell is designed to exhibit a unique stationary inflection point in the dispersion diagram corresponding to a frozen mode with almost zero group velocity. Through careful measurements and calculations, the frozen mode is observed to propagate at a significantly slower speed (286 times slower) than the speed of light. Importantly, this extraction method can be applied to any other periodic layout to obtain related dispersion properties.

Nonreciprocal Leaky-Wave Antenna Based on Coupled Microstrip Lines on a Non-Uniformly Biased Ferrite Substrate

Previous work demonstrated that nonreciprocal slow waves can be guided on a pair of coupled transmission lines printed on a magnetic substrate. This was done by designing the unit-cell geometry to exhibit spectral asymmetry near the band edge (

Nonreciprocal and Magnetically Scanned Leaky-Wave Antenna Using Coupled CRLH Lines

K = pi

Half-Ring Helical Structure for Traveling Wave Tube Amplifiers

) frequency in its dispersion (K-ω) diagram. In this effort, the same concept is adapted to attain nonreciprocal radiation by operating the coupled lines in the fast wave region (|β| <; k0) of the K-ω diagram. To validate the nonreciprocal radiation, an 8-unit-cell leaky-wave antenna (LWA) was constructed and tested in presence of a non-uniform external magnetic field. Specifically, 5 dB contrast was observed between the measured transmit and receive gain in the E-plane. Importantly, good agreement was observed between measurements and simulations once non-uniformities in the magnetic field were taken into account. We note that the external magnetic field can be changed to achieve beam scanning. This is unlike traditional LWAs that achieve beamscanning via frequency control.

Nonreciprocal and magnetically scanned leaky-wave antenna using coupled microstrip lines

A new class of magnetoelectrics-based leaky-wave antennas (LWA) is proposed for wide angle beam-steering via magnetic tuning. This new LWA is based on coupled composite-right-left-handed microstrip lines printed on a ferrite substrate. To accomplish radiation, the printed coupled transmission lines are operated in the fast wave region (|β|<;k0). Beam-steering was attained by changing the magnetic properties of the ferrite, controlled by the external bias field. Specifically, the beam was successfully scanned in the E-plane by 80° via changing the bias field by ±50Oe. The associated gain varies between 3.5dB and 5dB at the operating frequency 1.85 GHz as the beam is scanned.

Demonstration of unidirectional printed structures emulating Magnetic Photonic Crystals

We introduce a new slow wave structure referred to as half-ring helix for traveling wave tube (TWT) applications. This new structure is shown to achieve 27% size reduction with concurrent nondispersive response across the S-band. A 10-dB gain improvement was also observed as compared with the standard helix TWT. Moreover, the designed traveling wave tube can attain a maximum saturated output power of 1 KW and a bandwidth of 0.75 GHz (2.5-3.25 GHz). This improved gain and power handling, along with a satisfactory bandwidth, makes the structure attractive as compared with conventional ones. In this paper, we present the TWT design with numerically computed cold test results and evaluate its hot test performance using particle-in-cell simulations.

Metamaterial-based slow wave structure for travelling wave tubes

A periodic nonreciprocal leaky wave antenna is presented based on coupled microstrip lines on a ferrite substrate. This printed layout was validated to exhibit nonreciprocal transmission properties in its guided-wave region (|β|>;k0). To accomplish radiation, the printed dual transmission lines are operated in the fast wave region (|β|<;k0), where strong spectral asymmetry (ω(K)≠ω(2π-K)) is also observed. This leads to novel nonreciprocal radiation characteristics. Specifically, more than 10 dB difference is observed between the simulated transmit and receive gain in the E-plane. The antenna can be also magnetically scanned between 20° and 90° (endfire) in the elevation plane by changing the external magnetic field strength. The associated gain varies between 8.34 dB and 9.95 dB at the operating frequency 2.8 GHz as the beam is scanned.

3-D artificial media exhibiting degenerate band edge and frozen modes

Tunable dispersion characteristics of periodic electromagnetic structures provide a new direction in RF design [1]. Among them, relevant efforts have focused on Electromagnetic Band Gap (EBG) structures [1], used for high impedance ground planes, FSS, PBG based guiding structures [2]. Artificial or composite periodic media have also been successfully employed to tune small antennas into exceptionally directive radiators [3]. Inclusion of anisotropic and ferrimagnetic materials into periodic dielectric layers (i.e. RBE) were also considered. By engineering dispersion characteristics, significantly slower group velocity modes can be realized [4]-[5]. The ferrites presence is practically attractive as it supports nonreciprocal mode propagation. Such magnetic layered structures, namely Magnetic Photonic Crystals (MPC), display unidirectional propagation at the frozen mode and Stationary Inflection Points (SIP) [6].

ELECTRONIC DEVICE COMPONENTS AS ANTENNAS

The advent of strong relativistic electron beams has led to the development of high power microwave sources. These devices generate high peaks of microwave power by transferring the kinetic energy in the electron beam to electromagnetic waves guided within a slow wave structure (SWS). Among these devices, traveling-wave tubes (TWTs) and backward wave oscillators (BWOs) are based on this principle, referred to as Cerenkov radiation. Strong interest also exists in employing the Cerenkov Maser (microwave amplification via stimulated emission of radiation) as a TWT due to its simplicity and tunability. This TWT is lined with a dielectric coating to slow down the electromagnetic waves. However, as noted by Shiffler et al. (Shiffler et al., IEEE Trans. Plasma Sci., 2010), dielectrics become vulnerable to charging and surface breakdowns. A way to avoid charging is to employ purely metallic metamaterial-based slow wave structures that emulate the behavior of a dielectric liner.

Slot antenna integrated into a resonant cavity of an electronic device case

Tunable dispersion characteristics of periodic structures are providing new direction in RF design [1]. Among them, are the negative index media based on the split-ring resonator and left handed transmission lines [2, 3]. Several miniature antenna implementations based on substrates with negative index metamaterials (NIM) have also been prepared [2]. Also a large body of work has been published using printed transmission line realizations of left handed propagation modes preceding the developments in NIMs.