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A self-grounded bowtie antenna arrangement including an antenna structure including a number of antenna petals including arm sections tapering towards a respective end tip portion and being made of an electrically, conducting material, the end tip portions being arranged to approach a base portion on a first side thereof and to be connected to feeding ports, a specific port being provided for each antenna petal. The base portion includes a conducting ground plane or a Printed Circuit Board, and each antenna petal is made in one piece from a metal sheet or similar, and it is adapted to be fabricated as separate units, and to be mountable onto a front or back side of the base portion or ground plane by means of surface mounting. The ground plane may be a Printed Circuit Board, meaning that the bowties can be mounted by automatic placement and soldering machines.

This article gathers useful, simple algorithms and their physical interpretations for field solutions from incremental sources in three-dimensional (3-D) spatial, two-dimensional (2-D) spectral, and one-dimensional (1-D) spectral domains. The interpretations of the 1-D spectral Green's functions are visualized in space as fields from current sheets, tubes, and shells for the planar, circular cylindrical, and spherical cases, respectively. A joint algorithm is presented for solving the multilayer case for all three cases. Similarly, field problems involving cylindrical objects or bodies of revolution (BOR) are structured into spectrums of 2-D spatial solutions from line sources and ring sources, respectively. The formulations and physical images are pedagogical and open up for new creative ways of teaching electromagnetic (EM) field theory as well as structuring numerical algorithms for field solutions that take known symmetries into account. It is also shown that the 3-D spatial Green's functions can be approximated to improve physical interpretation by omitting higher-order 1/r terms when r > 2λ.

We derive a general fundamental directivity limitation formula that applies to nonsuperreactive antennas of any size that fit within a minimum sphere of any given radius rmin. The derivation is done by using a new concept: the degrees of freedom (DoF) of the field radiated by arbitrary sources within the minimum sphere must be twice the maximum directivity. The formula converges to the known bound of the directivity for large rmin. For small spheres, it becomes equal to three, i.e., 4.8 dBi, which is the directivity of the Huygens source. The transition region between these two limiting cases is determined by counting the most significant spherical modes at the surface of the minimum sphere. This is not trivial, because spherical modes have a gradual cutoff when their order approaches krmin. Therefore, we use a weighted summation where the weighting factor is inversely proportional to the radiation-Q of the modes. This extends the DoF from a discrete to continuous function of the minimum sphere radius. The final maximum directivity is similar to a previously published heuristic limit. The formulas are compared to results for measured antennas with large directivity and superdirectivity.

A wideband high-gain high-efficiency $16 \times 16$ -element slot antenna array is presented for 60-GHz band applications. The antenna is designed based on gap waveguide technology. The most important advantage of using this technology is its ability to decrease complexity and cost of fabrication because there is no requirement of electrical contact between the three layers of the antenna structure. The three layers are a corrugated metal plate with radiating slots, a subarray cavity layer, and a ridge gap waveguide (RGW) feed network layer. The corporate feed network is realized by a texture of pins and guiding ridges in a metal plate. Also, in order to excite the antenna with a standard V-band rectangular waveguide, a transition from RGW to WR-15 is designed. The radiating elements, corrugations, cavity layer, power dividers, and transition are designed and optimized to suppress the reflection coefficient at the input WR-15 port over the desired frequency range from 57 to 66 GHz. Finally, the $16 \times 16$ -element slot antenna array is fabricated by the standard milling technology. The measured results demonstrate about 16% of reflection coefficient bandwidth ( $\vert S_{11}\vert dB) covering the 56–65.7-GHz frequency range, and the measured gain is larger than 32.5 dBi over the band with more than 70% antenna efficiency.

This paper numerically validates a simple formula to calculate the aperture efficiency reduction due to grating lobes in planar phased arrays. Array antennas with element spacing greater than one wavelength will produce grating lobes. Grating lobes are not a big problem in most new millimeter-wave applications, except for the fact that they reduce the aperture efficiency and thereby normally the directivity. When the element pattern is known, the simple formula can be used to calculate the aperture efficiency in the presence of grating lobes. The formula is verified by the simulations of a large uniformly excited planar slot array. The agreement is good for both broadside radiation and a phased main beam, even if we use the far field of an isolated slot. Using the far field of the embedded slot element gives almost the same results, except when a grating lobe radiates along the ground plane and the isolated element pattern is nonzero in this direction.

This paper describes the design and realization of a transition from a microstrip line to a ridge gap waveguide operating between 95 and 115 GHz. The study includes simulations, measurements, and a Monte Carlo analysis of the assembly tolerances. The purpose of this tolerance study is to identify the most critical misalignments that affect the circuit performance and to provide guidelines about the assembly tolerance requirements for the proposed transition design, This work was supported in part by the Swedish Research Council VR, the Swedish Governmental Agency for Innovation Systems VIN-NOVA via a project within the VINN Excellence center Chase, and the European Research Council (ERC) under Grant ERC-2012-ADG-20120216.

This paper shows that a universal software radio peripheral (USRP) can be used to measure the complete passive over-the-air (OTA) performance of multiport antennas for multiple-input multiple-output (MIMO) systems. The passive MIMO OTA measurements are performed in a reverberation chamber using the USRP as a measurement instrument. The total embedded radiation efficiencies, the correlation, and the resulting diversity gain are extracted from transmission measurements only. These are in good agreement with S-parameter measurements (both transmission and reflection) obtained with a vector network analyzer (VNA). The proposed measurement approach involving USRPs provide an economical alternative to these type of measurements as compared to high-end equipment.

Radiation leakages are a considerable problem when measuring waveguide structures at high frequencies. In order to maintain good electrical contact, flanges need to be tightly and evenly screwed to the device under test. This can be a time-consuming operation, especially with repeated measurements. We present a metamaterial-based adapter, which prohibits leakage even in the presence of gaps at the interconnects. This so-called gap adapter has been fabricated from a metallized polymer (SU8). The reflection coefficient is below −20 dB throughout the band for a 50- $\mu \text{m}$ gap on both sides of the gap adapter. In comparison, a conventional waveguide with a 50- $\mu \text{m}$ gap on both sides has a reflection coefficient of −10 dB. The gap adapter can be used to perform fast measurements, since the normal flange screws are redundant. We compare the SU8 gap adapter with a Si version and to a smooth metal waveguide reference disc. The SU8 gap adapter performed better than the Si version and much better than the waveguide disc in all test cases. SU8 gap adapters were used to measure on a waveguide component. The SU8 gap adapters with 50- $\mu \text{m}$ gaps performed comparable with the waveguide component with the flange screws carefully tightened. The polymer also makes the gap adapter mechanically robust and easy to mass fabricate. [2015-0113]

This paper presents the design of a high efficiency corporate-fed 8×8-slot array antenna in the 60 GHz band. The antenna is built using three unconnected metal layers based on Artificial Magnetic Conductor (AMC) in gap waveguide technology. A 2×2 cavity-backed slot subarray is designed in a groove gap waveguide cavity. The cavity is fed through a coupling slot from a ridge gap waveguide corporate-feed network in the lower layer. The subarray is numerically optimized in an infinite array environment. The corporate-feed network is realized by a texture of pins and a guiding ridge. There is very good agreement between simulated and measured results. The fabricated antenna shows a relative bandwidth of 14% with input reflection coefficient better than -10 dB and an overall aperture efficiency larger than 65% (i.e. – 2 dB) with about 25 dBi realized gain between 56.2 and 65.0 GHz.

In the 5G system, we foresee the use of LOS-dominated mm-wave radio links to moving users being subject to slow fading resulting from the users’ random locations and orientations. We refer to this as a random-LOS channel. MIMO processing algorithms will be used in 5G to improve performance in slow fading, similar to how they are used in Rayleigh fading. To this end, we study the probability of detection in the random-LOS channel when there are dual-polarized antennas on both sides of the link. We introduce two polarization deficiencies: the polarization non-orthogonality and the amplitude imbalance between the ports of a two-port antenna. The MIMO efficiency is evaluated as a function of these deficiencies. In the analysis, we consider the MRC algorithm for one bitstream, and the ZF and SVD algorithms for two bitstreams. We also present two analytical formulas for the MIMO efficiency that can be used to determine performance. We use the formulas on two ideally orthogonal dipoles, and show by means of coverage plots how much the 1- and 2-bitstream performances degrade due to the polarization deficiencies in off-boresight directions.

The paper describes the design methodology, experimental validation, and practical considerations of two millimeter-wave wideband vertical transitions from two gap waveguide versions (inverted microstrip gap waveguide, and microstrip packaged by using gap waveguide) to standard WR-15 rectangular waveguide. The experimental results show $S_{11}$ smaller than $-{\hbox{10 dB}}$ over relative bandwidths larger than 25% and 26.6% when Rogers RO3003 and RO4003 materials are used, respectively. The vertical transition from standard microstrip line packaged by a lid of pins to WR-15 shows measured return loss better than 15 dB over 13.8% relative bandwidth. The new transitions can be used as interfaces between gap waveguide feed networks for 60-GHz antenna systems, testing equipment (like vector network analyzers), and components with WR-15 ports, such as transmitting–receiving amplifiers. Moreover, the paper documents the losses of different gap waveguide prototypes compared with unpackaged microstrip line and substrate integrated waveguide (SIW). This investigation shows that in $V$ -band, the lowest losses are achieved with inverted microstrip gap waveguide.

Measurements in reverberation chamber (RC) produce data that are random, and therefore they need to be processed from the statistical point-of-view for obtaining the desired characteristics and the accuracy. The complex channel transfer function in the RC follows complex Gaussian distribution provided that the RC is well stirred. The authors have recently presented a new uncertainty model based on the presence of an unstirred component of the transfer function, which was modelled by introducing an average Rician K-factor. The model was validated in two RCs with translating mode-stirring plates, being able to correctly describe the improvement in accuracy by rotating the antenna under test, and by blocking the line-of-sight between this and the fixed RC antenna(s). In the present study, they apply this uncertainty model to four RCs with different settings (e.g. RC volumes, number of plates or fixed RC antennas, translating and rotating mode-stirrers etc.). For each RC, they examine the measurement uncertainty under different loading conditions. To repeat (during the different measurements) the actual mode-stirrer positions at which the transfer function is sampled, they conduct all the measurements with stepwise (instead of continuous) mode-stirring. The model is shown to work well for all the cases.

This paper presents a study of measuring complete over-the-air (OTA) system performance of software-defined radios (SDRs) such as the Universal Software Radio Peripheral (USRP) developed by Ettus Research and National Instruments. We use the reverberation chamber (RC) for measuring the OTA system throughput (TPUT) when the USRP is configured in single-input single-output, 1 x 2 single-input multiple-output, 2 x 1 multiple-input single-output, and 2 x 2 multiple-input multiple-output. The measured and simulated results are compared. The theoretical model of an ideal threshold receiver is used to simulate the OTA TPUT of the USRP. The total radiated power and total isotropic sensitivity of the USRP is also measured inside the RC. According to the authors' knowledge, complete active system performance measurements of USRPs in an RC are being published for the first time.

The characterization of the performance of wireless devices is key to developing new RF products conforming to the latest communications protocols. Traditionally, communication performance tests have focused on the RF performance of the tested devices, e.g., smart phones, pads, laptops, and so on. In particular, the focus has shifted from conducted (i.e., cabled) measurements to more realistic over-the-air (OTA) characterization of the RF performance of these devices in transmit or receive mode. For example, the receiver performance of 2G and 3G wireless devices can be measured in terms of the total isotropic sensitivity (TIS) that depends on the antenna and the receiver parts of a wireless device. These measurements can be performed in a reverberation chamber setup. However, standard TIS measurements can be time-consuming and do not reflect the actual performance gains of multiple-input multiple-output antenna systems operating over orthogonal frequency division multiplexing channels, such as those encountered in 4G long-term evolution (LTE) systems. Therefore, in order to meet both time and cost efficiency requirements, we propose here a new method to determine the TIS, as well as the diversity performance, of an LTE device based on throughput measurements. The proposed method shows that the TIS of an LTE device is characterized much faster directly from OTA throughput measurements than from standard TIS measurements and with excellent accuracy.

The roof of an automobile would appear to be an ideal platform to place multiple-input–multiple-output (MIMO) antennas, having a large ground plane with plenty of space to separate antennas for optimum embedded element radiation efficiency. However, styling concerns often require that the antennas are to be placed close together. Thus, mutual coupling plays an important role in the performance of an MIMO antenna system as it affects the embedded element efficiency and the channel correlation. In this letter, we present a comparison between the measured mutual coupling effect on the ergodic channel capacity of a $2 \times 2$ MIMO system in a reverberation chamber (i.e., Rayleigh channel) and in a suburban outdoor environment (i.e., Rician channel) at Long Term Evolution (LTE) radio band 13. The outdoor MIMO capacity was obtained during a vehicle drive test using commercial over-the-air (OTA) LTE test equipment. All measurements were conducted with two-element monopole antennas on a 24-in-diameter circular ground plane. The impact that mutual coupling has in the degradation of the embedded element radiation efficiency and increased correlation between the antennas, and ultimately the decrease in ergodic MIMO channel capacity, is highlighted by these measurements. In addition, the results provide insight into the differences of channel capacity obtained in the rich isotropic multipath environment of a reverberation chamber and the channel capacity obtained in a real-world suburban environment.

A single-layer spatial power splitter-combiner structure is packaged in gap waveguide technology. The measured metal-only back-to-back structure features an average insertion loss less than 2.3 dB and a return loss larger than 10 to 20 dB over the entire 75–110 GHz W-band. The design procedure is outlined and the measured and simulated results are shown to be in good agreement. The structure can be used as a stand-alone power splitter and/or combiner (single-mode, all-excited in-phase channels), a quasi-optical beamformer to excite an array of slot antennas in the top ground plane by ridge gap waveguides, or as a back-to-back RF component in grid amplifier designs.

This letter demonstrates a packaging concept where a waveguide-to-microstrip transition can be assimilated on a mm-wave MMIC of an arbitrary size and thus avoid the use of bond wires at the high frequency ports of the MMIC circuit. The advantage of the proposed concept is that- it does not require an extra substrate with sub-critical dimensions for the waveguide-to-microstrip transition. The transition can be integrated onto the MMIC, and thus the MMIC can be coupled directly to the waveguide. A Perfect Magnetic Conductor (PMC) surface is used to suppress the unwanted waveguide mode coupling to the oversized circuit cavity. The measured back-to-back transition works over the frequency band 66–98 GHz (relative BW of 38%) with minimum return loss of 12.7 dB. The total insertion loss of the manufactured prototype is found to be 3.26 dB, which also includes a 14.8 mm long microstrip line on an Alumina substrate. The losses in a single transition vary between 0.25 and 0.42 dB.

This letter presents an end-coupled bandpass filter (BPF) based on the recently introduced inverted microstrip gap waveguide technology. The inverted microstrip gap waveguide is advantageous for millimeter wave applications because of its low-loss, self-packaging characteristics, and cost-effectiveness. A fourth order Chebyshev-type end-coupled BPF is designed to provide a 2 GHz bandwidth at 60 GHz center frequency. The fabricated prototype embedded within a 10 cm inverted microstrip gap waveguide, containing two back-to-back transitions to rectangular waveguide, exhibits an insertion loss of 3 dB in the passband. However, the insertion loss of the filter itself is better than 1.6 dB.

The present paper describes the paradigm shift in OTA testing that came with the arrival of the 4G wireless system. The testing was previously mainly done in anechoic Line-Of-Sight (LOS) environments, and now in 4G it is done in reverberation chambers, emulating Rich Isotropic MultiPath (RIMP). Now we are preparing for the 5G wireless system, which is expected to have mm-wave antennas with steerable lobes. This will require OTA testing in anechoic environments again, but with an emphasis on system measurements, i.e. data rate throughput for an arbitrary location and orientation of the user, referred to as Random-LOS.

The papers in this special issue focus on the impact of user-related randomness on antennas and channels. In the early days of telecommunications, most antennas were considered as fixed installations, and their performance was determined in terms of their directivity and radiations patterns when the polarizations of the antennas on the transmitting and receiving sides were aligned. With the birth of satellite communications, there soon became a need for reusing the frequencies by polarization multiplexing, resulting in dual polarized systems with an even stricter requirement to alignment in order to avoid interference between the two communication channels. Furthermore, mobile communications resulted in random channels owing to both fast Rayleigh fading and slow shadow fading. Nowadays, wireless devices and base stations/access points are used more and more in hugely differing contexts and environments for terminals. The papers in this issue have been prepared in this context, the aim being to help make better known the issues raised above from the antennas and propagation community and to contribute to foster research and development in this area.

We present a $V$ -band multilayer corporate-fed slot array antenna with wide impedance bandwidth and high efficiency. The proposed antenna consists of three unconnected metal layers based on the recently introduced gap waveguide technology. A $2\times 2$ cavity-backed slot subarray acts as the unit cell of the array. The top metal layer contains the radiating slots, the intermediate layer contains the cavities, formed by pins, and the third layer is the ridge gap waveguide corporate-feed network. The latter is realized by a texture of pins and guiding ridges to uniformly excite the cavities with the same amplitude and phase. The proposed antenna fulfills the radiation pattern requirement of the ETSI 320 standard. A prototype consisting of $16\times 16$ slots was manufactured by a fast modern planar 3-D machining method, i.e., die-sink electric discharge machining. The fabricated prototype has a relative impedance bandwidth of 17.6% with input reflection coefficient better than −10 dB. The E- and H-planes radiation patterns satisfy the ETSI class II copolar sidelobe envelope, and the measured cross-polar level is more than −30 dB below the copolar level over the 56–75 GHz frequency band. The measured antenna efficiency is better than 60% over the same band.

This paper presents the design of microstrip-ridge gap waveguide using via-holes in printed circuit boards, a solution for high-frequency circuits. The study includes how to define the numerical ports, pin sensitivity, losses, and also a comparison with performance of normal microstrip lines and inverted microstrip lines. The results are produced using commercially available electromagnetic simulators. A WR-15 to microstrip-ridge gap waveguide transition was also designed. The results are verified with measurements on microstrip-ridge gap waveguides with WR15 transitions at both ends.

A wideband 2x2-slot element for a 60-GHz antenna array is designed by making use of two double-sided printed circuit boards (PCBs). The upper PCB contains the four radiating cavity-backed slots, where the cavity is formed in substrate-integrated waveguide (SIW) using metalized via holes. The SIW cavity is excited by a coupling slot. The excitation slot is fed by a microstrip-ridge gap waveguide formed in the air gap between the upper and lower PCBs. The lower PCB contains the microstrip line, being short-circuited to the ground plane of the lower PCB with via holes, and with additional metalized via holes alongside the microstrip line to form a stopband for parallel-platemodes in the air gap. The designed element can be used in large arrays with distribution networks realized in such microstrip-ridge gap waveguide technology. Therefore, the present paper describes a generic study in an infinite array environment, and performance is measured in terms of the active reflection coefficient S11 and the power lost in grating lobes. The study shows that the radiation characteristics of the array antenna is considerably improved by using a soft surface EBG-type SIW corrugation between each 2x2-slot element in E-plane to reduce the mutual coupling. The study is verified by measurements on a 4x4 element array surrounded by dummy elements and including a transition to rectangular waveguide WR15.

The gap waveguide technology was recently introduced as an alternative to hollow waveguides and substrate integrated waveguides for millimeter-wave applications. This paper presents the design of a 4 x 4 planar dual-mode horn array with low loss corporate feed network realized by using an inverted microstrip gap waveguide. The dual-mode horns are compact and designed to reduce the power losses in grating lobes. It is because the diameters of the horn apertures are larger than two wavelengths to allow more space for the feed network and thereby lower conductive losses. The measurements show very good agreement with simulations, with 10% bandwidth of the return loss, 25 dBi realized gain and about 60% aperture efficiency.

Several electrically large phased array feed (PAF) reflector systems are modeled to examine the mechanism of multiple reflections between parabolic reflectors and low-and high-scattering feeds giving rise to frequency-dependent patterns and impedance ripples. The PAF current is expanded in physics-based macro domain basis functions (CBFs), while the reflector employs the physical optics (POs) equivalent current. The reflector-feed coupling is systematically accounted for through a multiscattering Jacobi approach. An FFT expands the reflector radiated field in only a few plane waves, and the reflector PO current is computed rapidly through a near-field interpolation technique. The FEKO software is used for several cross validations, and the convergence properties of the hybrid method are studied for several representative examples showing excellent numerical performance. The measured and simulated results for a 121-element Vivaldi PAF, which is installed on the Westerbork Synthesis Radio Telescope, are in very good agreement.

Single-layer, wideband, and low-loss corporate-feed networks for slot antenna arrays are described. The antenna is built using ridge gap waveguide technology, formed between two parallel metal plates without the requirements of electrical contact between these plates. The corporate-feed network is realized by a texture of pins and a guiding ridge in the bottom plate, and the radiating slots are placed in the smooth top plate. The paper describes two test antennas: a 4 $\,\times\,$ 1 linear slot array and a 2 $\,\times\,$ 2 planar slot array. Both have been fabricated and tested at Ku- band. The linear array shows more than 20% bandwidth and the 2 $\,\times\,$ 2 array shows a bandwidth of 21% for 10-dB return loss. There are good agreements between measured and simulated patterns for both antennas. Measured gain for the planar array is found to be at least 12.2 dBi over 12–15 GHz band.

This paper describes a novel design for a direct-coupled cavity filter realization using a ridge gap waveguide technology. A ridge gap waveguide transmission line with two coaxial feed connectors is designed and operated within the frequency band of 10-13 GHz. A cavity is coupled to the transmission line to achieve bandstop filter characteristic. Then, the structure of the filter is modified in a way to generate a bandpass feature. The final manufactured prototype is a fourth-order bandpass filter, operating at the center frequency of 11.59 GHz with a bandwidth of 72 MHz. The proposed design has potential applications in channeling filters for telecommunication satellites.

In this paper, perfect magnetic conductor (PMC)-based packaging technique was used to improve the isolation performance among various microwave circuit components such as high-gain amplifier chains. In this approach, a periodic structure (such as metal pin rows) together with the ground plane of the substrate created a stopband for unwanted parallel plate or cavity modes as well as substrate modes, and thereby suppressed the problems of circuit resonances and related package phenomena. This paper describes two Ka-band amplifier chains that were tested with this new packaging technique. Firstly, a single amplifier chain was tested for maximum stable gain operation, and it was found that the stable gain of was achieved, whereas traditional metal wall package with RF absorber offered stable gain of 40 dB, thus showing significant isolation improvement. Secondly, two high-gain amplifier chains were placed side by side and their mutual isolation was tested. With the proposed gap waveguide packaging, a minimum isolation of 78 dB was achieved, whereas a complete metal shield provided a minimum isolation of only 64 dB over the band of interest.

Wireless systems have become more and more advanced in terms of handling the statistical properties of wireless channels. For example, the 4G long term evolution (LTE) system takes advantage of multiport antennas [multiple-input multiple-output (MIMO) technology] and orthogonal frequency division multiplexing (OFDM) to improve the detection probability of single bitstream by diversity in the spatial and frequency domains, respectively. The 4G system also supports transmission of two bitstreams by appropriate signal processing of the MIMO subchannels. The reverberation chamber emulates according to previous works rich isotropic multipath (RIMP) and has proven to be very useful for characterizing smart phones for LTE systems. The measured throughput can be accurately modeled by the simple digital threshold receiver, accounting accurately for both the MIMO and OFDM functions. The throughput is equivalent to the probability of detection (PoD) of the transmitted bitstream. The purpose of this paper is to introduce a systematic approach to include the statistical properties of the user and his or her terminal, when characterizing the performance. The user statistics will have a larger effect in environments with stronger line-of-sight (LOS), because the angle of arrival and the polarization of the LOS contribution vary due to the user’s orientation and practices. These variations are stochastic, and therefore, we introduce the term random-LOS to describe this. This paper elaborates on the characterization of an example antenna in both RIMP and random-LOS. The chosen antenna is a wideband microbase transceiver station (BTS) antenna. We show how to characterize the micro-BTS by the PoD of one and two bitstreams in both RIMP and random-LOS, by considering the user randomly located and oriented within the angular coverage sector. We limit the treatment to a wall-mounted BTS antenna, and assume a desired hemispherical coverage. The angular coverages of both one and two bitstreams for the random-LOS case are plotted as MIMO-coverage radiation patterns of the whole four-port digital antenna system. Such characterizations in terms of PoD have never been done before on any practical antenna system. The final results are easy to interpret, and they open up a new world of opportunities for designing and optimizing 5G antennas on system level.

There is a lack of suitable numerical ports for the gap waveguides. This study studies the resemblance between the ridge/groove gap waveguide and the conventional hollow ridge/rectangular waveguide, respectively, by using numerical analysis. The dispersion diagrams and characteristic impedances are compared and found to be similar within the stopband of the parallel-plate modes. Therefore conventional hollow waveguides can be used as ports in numerical analysis of gap waveguides. In addition, this means that the geometry of circuit components in hollow ridge/rectangular waveguides can be used as a good starting point when designing ridge/groove gap waveguide components.

Previously, the throughput for a single-input multiple-output (SIMO) system was successfully tested in a reverberation chamber (RC), and the results were in good agreement with a model based on the ideal threshold receiver. In the present communication, we measure the throughput of a 2 × 2 open loop multiple-input multiple-output (MIMO) system with full spatial multiplexing (i.e., two data streams), and we compare the results with an extended throughput model taking the spatial multiplexing into account. The measured throughput is found to be in agreement with the simulated one of a simple zero-forcing (ZF) receiver. The throughput degradations due to correlation and power imbalance (caused by different embedded radiation efficiencies of antennas) are studied and referred to as a throughput multiplexing efficiency. These can, to a large extent, be explained by the ZF algorithm. The results show clearly that the rich isotropic multipath (RIMP) environment generated by an RC can be used to study how effective different MIMO algorithms are when there is correlation and efficiency imbalance between the antenna elements.

The present paper defines diversity gain for stationary users. This deals in particular with gathering the received signal statistics over possible user positions and orientations in space rather than over time, and to define a meaningful diversity gain related to the cumulative improvement of the performances of the 1% users with the worst receiving conditions. The definition is used to evaluate diversity gain for some typical small antennas in an extreme environment with only line-of-sight (LOS). The LOS environment is regarded as user-distributed 3D-random LOS caused by the statistics of an ensemble of stationary users with arbitrary orientations in the horizontal plane (2D), and with arbitrary orientations of their wireless devices in the vertical plane. Thus, an overall 3D-random distribution of user orientation is assumed.

Gap waveguide technology represents an interesting alternative as low-loss, cost-effective, and high-performance transmission line and package of microwave and millimeter-wave systems. A Ka-band coupled-resonator filter for a radio link diplexer, which requires high selectivity to isolate transmit and receiving channels, is proposed and realized using gap waveguide technology. The band-pass filter, which has a central frequency of 37.37 GHz and a pass bandwidth of 560 MHz, is fabricated between two parallel metal plates, leaving an air gap between them. After milling one of the plates, silver-plating is applied on them. Measurements show a minimum in-band insertion loss of 1 dB and agree quite well with simulations.

It has been shown that diversity gains and MIMO capacity of multi-port antennas can be conveniently determined from reverberation chamber measurements. In this paper, we first present the covariance eigenvalue approach and the embedded far field function method to determine diversity gain and MIMO capacity, respectively, for arbitrary multi-port antennas. Then, we discuss the convergence of the covariance eigenvalue approach. In the end, we apply both methods to compare MRC diversity gains and MIMO capacity measured in a reverberation chamber (based on direct channel measurements) and an anechoic chamber (based on measurements of the embedded far field functions and efficiencies at all antenna ports). The comparison is performed over 2-8 GHz using a wideband multi-port antenna in two configurations. There are in general good agreements between the reverberation and anechoic chamber measurement results.

This letter describes a simple and low-loss microstrip-to-ridge gap waveguide transition with a very compact geometry. The transition transforms the electromagnetic (EM) fields from the microstrip mode to the air-filled ridge gap waveguide mode. This is achievable if the height of the air gap in the ridge gap waveguide is kept almost equal to the thickness of the substrate of the microstrip line. The transition has a pressure contact between the ridge and the microstrip line, so it works without soldering. This is advantageous in systems that require mechanically separable split-blocks or modules and need a lot of transitions. Experimental results of the manufactured back-to-back transition show an insertion loss of 0.32 dB and a return loss of 14.15 dB over 55% relative bandwidth in Ka-band.

A contactless, broadband and low-loss microstrip-to-groove gap waveguide transition operating at W-band is presented. The principle of operation is based on transforming EM fields from the SIW to the ridge gap waveguide mode via electromagnetic coupling. This is advantageous, since the proposed solution avoids the use of metal contact between the SIW and one of the waveguide parts. Furthermore, metamaterial-based gap waveguide technology provides a resonance-free packaging solution.

In this paper a double layer cavity backed slot array antenna at K band is presented. The slot antenna is built using ridge gap waveguide feed network and gap waveguide cavity layer. These antenna blocks are manufactured without the requirements of electrical contact between the metal plates The corporate-feed network is realized by a texture of pins and a guiding ridge in the bottom plate. The 4×4 slot array antenna has been fabricated and measured. The measured reflection coefficient remains below -11dB with reasonably good radiation patterns over the band of interest from 18.2 GHz to 21.2 GHz.

This contribution discusses the relationship between the number of degrees of freedom (DoF) and the maximum directivity for an arbitrary set of sources enclosed by a given spherical surface. In particular, it is shown that the number of DoF is equal to twice the maximum directivity. This provides a directivity limitation for arbitrarily sized antennas, thus, extending the validity of the known formulas for electrically large and electrically small antennas.

We present a new improved design of the self-grounded dual-polarized wideband bowtie antenna for MIMO application. The antenna is evaluated for the case when the original four ports are combined to two orthogonally polarized ports. The new antenna covers the previous bandwidth but has lower gain and wider beamwidth over the frequency band of interest. The 2×2 MIMO performance of the antenna in Random-Line of Sight (Random-LOS) is evaluated for two bitstreams using the threshold receiver model and Zero-Forcing receiver. The simulated results show an improvement in terms of MIMO multiplexing coverage over a hemisphere for two bitstreams, compared to the previous solution.

Currently gap waveguide technology has revealed strong competitiveness for millimeter wave wireless applications. Specifically, this new topology of waveguide technology provides advantages of low loss, high quality factor and easy RF packaging. In this paper we present the design methodology of millimeter-wave transition from standard WR-15 rectangular waveguide to inverted microstrip gap waveguide. Furthermore, a more practical power divider from standard WR-15 rectangular waveguide to inverted microstrip gap waveguide is also introduced. Both structures are vertical and there is no additional need for any complex modification of the waveguide structure to achieve compatibility between the two structures. The proposed two transitions cover the whole unlicensed 60 GHz band from 57 to 66 GHz in order to be widely utilized as an interface between gap waveguide feed networks and equipment components with WR-15 ports. The simulated results show promising S-parameters.

LTE throughput measurements in a Random Line-of-Sight (Random-LOS) setup have been performed. The effect of polarization-randomness on the level of detection threshold of one or two bitstreams has been investigated. Good agreement is shown between theory and experiments for the SISO, SIMO and MIMO cases.

A passive power splitter and combiner has been designed in-between two metallic ground plates using gap waveguide technology. The measured insertion loss is smaller than 2.3 dB and the return loss is larger than 10 dB over the entire W-band (75–110 GHz). The design procedure has been detailed and the simulated and measured results for the proposed structure are shown. The designed structure can be used as a passive power splitter or combiner, as a grid amplifier in a back-to-back RF chain, or as a quasi-optical beamforming component in beam steering antenna arrays.

A multilayer gap waveguide 8×8 slot array antenna with high efficiency for 60 GHz applications is presented in this paper. The proposed antenna is composed of three unconnected metal layers. A ridge gap waveguide corporate distribution network feeds a radiation layer with 4×4 cavity-backed slot subarrays. The co-polar radiation pattern and sidelobe levels of the antenna are improved by using a simple slot-tilting method to fulfill the radiation pattern requirement of the ETSI 302 standard for fixed radio links. The fabricated prototype has a relative bandwidth of 12 % with input reflection coefficient better than −10 dB. The E- and H-planes radiation patterns satisfy the ETSI class II co-polar sidelobe envelope over 57–65 GHz frequency band. The measured total aperture efficiency is better than 65% over the operating frequency band.