High Band Antenna Research Articles

Self-Decoupled Dual-Band Dual-Polarized Aperture-Shared Antenna Array

This communication presents a self-decoupled dual-band dual-polarized aperture-shared antenna array. The proposed dual-band array consists of a low-band (LB) antenna element interleaved into a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 2$ </tex-math></inline-formula> high-band (HB) antenna array with compact elements spacing. To mitigate the blockage effect resulting from the LB element, shorted patches are loaded on the LB radiator for suppressing its scattering in the HB frequency range. In addition, high cross-band isolation between the closely spaced LB and HB elements is obtained due to their intrinsic filtering response. Moreover, the in-band mutual coupling between the HB elements is reduced by adding cross-shaped strips on them. As a result, the cross-band and in-band couplings and the antenna blockage effect can be simultaneously reduced without adding an extra decoupling component. A prototype of the proposed array operating in 1.71–2.17 GHz (LB) and 3.3–3.8 GHz (HB) is fabricated and measured. The radiation patterns of the LB and HB elements are almost unaffected by each other. Furthermore, all of the port-to-port isolations are higher than 20 dB. These merits make the proposed design suitable for the multiband base station.

Aperture-Shared Dual-Band Antennas With Partially Reflecting Surfaces for Base-Station Applications

A new approach is proposed to develop aperture-shared dual-band antennas using partially reflecting surfaces (PRSs). A fully aperture-shared dual-band antenna consisting of a low-band antenna (LBA) and a high-band antenna (HBA) right below the LBA is first investigated. The aperture sharing has little influence on the LBA but causes a severe shielding effect on the HBA. To alleviate the shielding effect, a low-pass PRS is inserted between the LBA and HBA, forming an air-filled quasi-Fabry&#x2013;Perot cavity, which can make compensation for the radiation from the HBA but with little influence on the LBA. The second PRS is introduced above the LBA to enhance the bandwidth (BW) for HBA. It is shown that the fully aperture-shared dual-band antenna achieves an impedance BW (<inline-formula> <tex-math notation="LaTeX">$\vert \text{S}11\vert &lt; -15$ </tex-math></inline-formula> dB) of &#x007E;43.7&#x0025; (0.68&#x2013;1.07 GHz) for the LBA and &#x007E;45&#x0025; (1.7&#x2013;2.7 GHz) for the HBA with an average gain of &#x007E;8 dBi and stable radiation patterns. Based on the PRSs, an aperture-shared dual-band antenna subarray is finally developed for base-station applications, which covers the frequency bands of 0.68&#x2013;0.96 and 1.7&#x2013;2.7 GHz for 2G/3G/4G mobile communications.

Coplanar Dual-Band Dual-Polarized Shared-Aperture Antenna Array For 4/5G Base Station Applications

A novel dual-band dual-polarized shared-aperture antenna array is presented for fifth generation wireless communication application. This novel antenna array is a combination of a low-band antenna and four high-band antennas. In order to simplify the design model while achieving broadband, antenna working on low-band (from 0.68 to 0.99 GHz) and antenna working on high-band (from 3.3 to 5.1 GHz) are both loop-shape radiator fed by Y-shape feed. Coplanar, dual-band and shared-aperture are resolved by embedding four HB antenna elements in the LB antenna. From simulation, the proposed coplanar antenna array respectively achieves 43% (from 0.68 to 0.99 GHz) and 37% (from 3.3 to 5.1 GHz) impedance bandwidth in the low-band and high-band.

An Electromagnetic-Transparent Cascade Comb Dipole Antenna for Multi-Band Shared-Aperture Base Station Antenna Array

In multi-band shared-aperture base station antennas, cross-band mutual couplings always lead to unwanted radiation characteristics. In this article, the influence of the cross-band mutual coupling is investigated from the point of view of induced currents and their secondary radiation fields. A low-band (L-band, 0.76–0.88 GHz) cascade comb dipole antenna is proposed to passively cancel the induced current and therefore suppress the secondary radiation field. Owing to the passive current cancellation capability, the proposed L-band cascade comb antenna does not affect the radiation performance of the middle-band (M-band, 1.9–2.7 GHz) and high-band (H-band, 3.3–3.9 GHz) antennas in stacked multi-band shared-aperture antenna arrays. The proposed L-band antennas are thus electromagnetic-transparent to the M- and H-band antennas. Based on the novel electromagnetic-transparent cascade comb dipole antenna, a triple-band shared-aperture antenna array that consists of the L-, M-, and H-band antennas are developed, manufactured, and measured. Both simulated and measured results validate the attractive features of the cascade comb dipole antenna in multi-band shared-aperture massive MIMO base station antenna array developments.

Frequency Selective Surface-Based Dual-Band Dual-Polarized High-Gain Antenna

This article presents a frequency selective surface (FSS)-based dual-band dual-polarized antenna with compact size and high gain. The proposed dual-band antenna consists of a low-band (LB) and high-band (HB) antenna elements. The HB element is coaxially located under the radiator of the LB element, minimizing the overall size. The proposed LB element consists of a two-layer-FSS-based radiator (FSS-radiator) and two pairs of differential feeding probes. Besides, the passband of the proposed FSS-radiator is designed to cover the operating band of the HB element. As a result, the radiation of the HB element can transmit through the FSS-radiator of the LB element. Without reducing HB element gain, instead, the average HB element gain can be improved more than 1 dB by the feeding probes of the LB element. For demonstration, a prototype of the dual-band antenna operating in the LB of 2.3–2.7 GHz and the HB of 3.3–3.8 GHz is fabricated and measured. The dual-band prototype features high gain (~9.1 dBi in the LB and ~8.6 dBi in the HB) in a compact radiation aperture of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.32\lambda _{lc} \times 0.32\lambda _{lc}$ </tex-math></inline-formula> or <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.45\lambda _{hc} \times 0.45\lambda _{hc}$ </tex-math></inline-formula> (where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{lc}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{hc}$ </tex-math></inline-formula> , respectively, denote the guided wavelengths at the central operating frequencies in LB and HB). Moreover, easy fabrication and low cost are achieved. These merits make the proposed antenna suitable for multiband aperture-shared array.

The LOFAR Two-metre Sky Survey

In this data release from the ongoing LOw-Frequency ARray (LOFAR) Two-metre Sky Survey we present 120–168 MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44°30′ and 1h00m +28°00′ and spanning 4178 and 1457 square degrees respectively. The images were derived from 3451 h (7.6 PB) of LOFAR High Band Antenna data which were corrected for the direction-independent instrumental properties as well as direction-dependent ionospheric distortions during extensive, but fully automated, data processing. A catalogue of 4 396 228 radio sources is derived from our total intensity (Stokes I) maps, where the majority of these have never been detected at radio wavelengths before. At 6″ resolution, our full bandwidth Stokes I continuum maps with a central frequency of 144 MHz have: a median rms sensitivity of 83 μJy beam−1; a flux density scale accuracy of approximately 10%; an astrometric accuracy of 0.2″; and we estimate the point-source completeness to be 90% at a peak brightness of 0.8 mJy beam−1. By creating three 16 MHz bandwidth images across the band we are able to measure the in-band spectral index of many sources, albeit with an error on the derived spectral index of &gt; ± 0.2 which is a consequence of our flux-density scale accuracy and small fractional bandwidth. Our circular polarisation (Stokes V) 20″ resolution 120–168 MHz continuum images have a median rms sensitivity of 95 μJy beam−1, and we estimate a Stokes I to Stokes V leakage of 0.056%. Our linear polarisation (Stokes Q and Stokes U) image cubes consist of 480 × 97.6 kHz wide planes and have a median rms sensitivity per plane of 10.8 mJy beam−1 at 4′ and 2.2 mJy beam−1 at 20″; we estimate the Stokes I to Stokes Q/U leakage to be approximately 0.2%. Here we characterise and publicly release our Stokes I, Q, U and V images in addition to the calibrated uv-data to facilitate the thorough scientific exploitation of this unique dataset.

High-resolution international LOFAR observations of 4C 43.15

It has long been known that radio sources with the steepest spectra are preferentially associated with the most distant galaxies, the α − z relation, but the reason for this relation is an open question. The spatial distribution of spectra in high-z radio sources can be used to study this relation, and low-frequency observations are particularly important in understanding the particle acceleration and injection mechanisms. However, the small angular sizes of high-z sources together with the inherently low resolution of low-frequency radio telescopes until now has prevented high angular resolution low-frequency observations of distant objects. Here we present subarcsecond observations of a z = 2.4 radio galaxy at frequencies between 121 and 166 MHz. We measure the spatial distribution of spectra, and discuss the implications for models of the α−z relation. We targeted 4C 43.15 with the High Band Antennas of the International LOFAR Telescope with a range of baselines up to 1300 km. At the central frequency of 143 MHz we achieve an angular resolution of ~0.3″. By complementing our data with archival Very Large Array data we study the spectral index distribution across 4C 43.15 between 55 MHz and 8.4 GHz at resolutions of 0.4″ and 0.9″. With a magnetic field strength of B = 5.2 nT and fitted injection indices of αinjnorth = −0.8 and αinjsouth = −0.6, fitting a Tribble spectral ageing model results in a spectral age of τspec = 1.1 ± 0.1 Myr. We conclude that our data on 4C 43.15 indicates that inverse Compton losses could become comparable to or exceed synchrotron losses at higher redshifts and that inverse Compton losses could be a viable explanation for the α−z relation. Statistical studies of these objects will become possible in the future when wide-area subarcsecond surveys start.

Low Scattering Element-Based Aperture-Shared Array for Multiband Base Stations

This article presents a dual-wideband dual-polarized aperture-shared antenna array with low blockage effect between cross-band elements, including a low-band (LB) element with a <inline-formula> <tex-math notation="LaTeX">$4 \times 4$ </tex-math></inline-formula> high-band (HB) antenna array underneath. The radiator of the proposed LB element consists of four slim loops with loaded periodic split-ring resonators (SRRs) for achieving low scattering property in the high band. As a result, the blockage effect of the LB element on the HB antenna array is nearly indistinct. The radiation performance, as well as the S-parameters of the HB elements in the dual-band array, is almost the same as the isolated case. For demonstration, the dual-band aperture-shared antenna array, operating in the LB of 0.69&#x2013;0.96 GHz (32.7&#x0025;) and the HB of 3.3&#x2013;4.2 GHz (24&#x0025;), is fabricated and measured. Besides, the isolated LB antenna and HB array are designed for comparison. Compared with the isolated case, the measured peak gain variation of a <inline-formula> <tex-math notation="LaTeX">$1 \times 4$ </tex-math></inline-formula> HB array in one column is lower than 0.7 dB after adding the proposed LB element. In addition, the radiation pattern variation within 3 dB beam coverage is less than 1.5 dB. Therefore, the proposed dual-band aperture-shared array is very suitable for tackling the integration of 5G and 4G/3G/2G base station antennas.

Measurement of the LOFAR-HBA beam patterns using an unmanned aerial vehicle in the near field

An unmanned aerial vehicle (UAV) is exploited to characterize in situ the high-band antennas (HBAs) of the low-frequency array (LOFAR) CS302 station located in Exloo, The Netherlands. The size of an HBA array is about 30 m. The Fraunhofer distance (a few kilometers) is not reachable in the frequency band (120 to 240 MHz) within the flight regulation limits. Therefore, far-field patterns cannot be directly measured. The UAV, equipped with an radio frequency synthesizer and a dipole antenna, flies in the near-field region of the considered array. Measurement of three different frequencies (124, 150, and 180 MHz) is efficiently made during the same UAV flight. The near-field focusing method is exploited to validate the far-field pattern of the array under test within an angular range around the beam axis. Such a technique avoids both the time consuming λ / 2 sampling of the aperture field and the further application of computationally heavy near-field to far-field transformations. The array beam is well reconstructed in the main lobe and first sidelobes within a 2D scan plane sampled with a radial raster. A further postprocessing technique is proposed and validated on a subarray of HBAs. It suggests efficient ways for the future characterization of regular aperture arrays for SKA-MID Phase 2.

Follow-up of 27 radio-quiet gamma-ray pulsars at 110–190 MHz using the international LOFAR station FR606

Context. The Fermi Large Area Telescope has detected over 260 gamma-ray pulsars. About one quarter of these are labeled as radio-quiet, that is they either have radio flux densities &lt; 30 μJy at 1400 MHz, or they are not detected at all in the radio domain. In the population of nonrecycled gamma-ray pulsars, the fraction of radio-quiet pulsars is higher, about one half. Aims. Most radio observations of gamma-ray pulsars have been performed at frequencies between 300 MHz and 2 GHz. However, pulsar radio fluxes increase rapidly with decreasing frequency, and their radio beams often broaden at low frequencies. As a consequence, some of these pulsars might be detectable at low radio frequencies even when no radio flux is detected above 300 MHz. Our aim is to test this hypothesis with low-frequency radio observations. Methods. We have observed 27 Fermi-discovered gamma-ray pulsars with the international LOw Frequency ARray (LOFAR) station FR606 in single-station mode. We used the LOFAR high band antenna band (110−190 MHz), with an average observing time of 13 h per target. Part of the data had to be discarded due to radio frequency interference. On average, we kept 9 h of observation per target after the removal of affected datasets, resulting in a sensitivity for pulse-averaged flux on the order of 1−10 mJy. Results. We do not detect radio pulsations from any of the 27 sources, and we establish stringent upper limits on their low-frequency radio fluxes. These nondetections are compatible with the upper limits derived from radio observations at other frequencies. We also determine the pulsars’ geometry from the gamma-ray profiles to see for which pulsars the low-frequency radio beam is expected to cross Earth. Conclusions. This set of observations provides the most constraining upper limits on the flux density at 150 MHz for 27 radio-quiet gamma-ray pulsars. In spite of the beam-widening expected at low radio frequencies, most of our nondetections can be explained by an unfavorable viewing geometry; for the remaining observations, especially those of pulsars detected at higher frequencies, the nondetection is compatible with insufficient sensitivity.

A High Gain Ku Band Antenna with Circular Polarization Using Hybrid Choke Ring Structure

A Novel Hybrid Choke High Gain Antenna (HCHGA) that operates at 15 GHz is presented in this article, HCHGA is a circularly polarized one that effectively mitigates the multipath effects. The proposed antenna has good impedance matching with S11 of −20.99 dB with a gain of 10.26 dB, and an axial ratio of 1.74 dB. The radiation patterns of the antenna are orthogonal to the antenna surface without any nulls. The proposed antenna is fabricated using RT Duroid 5880 substrate. Simulations are done on a basic patch antenna mounted on different types of ground planes including extended flat ground plane, concentric choke ring, and Hybrid choke ring. The effects of variation in the parameters of grooves thickness, grooves height, and grooves shape on antenna performance parameters are observed. The design of the hybrid chokes facilitated higher values of gain and front-to-back ratio development for mitigating the multipath. From the prototype of the proposed antenna, the measured and simulated results are in good agreement with each other and this proposed antenna is suitable for satellite communication applications.

Investigating ionospheric calibration for LOFAR 2.0 with simulated observations

Context. There are a number of hardware upgrades for the Low-Frequency Array (LOFAR) currently under development. These upgrades are collectively referred to as the LOFAR 2.0 upgrade. The first stage of LOFAR 2.0 will introduce a distributed clock signal and allow for simultaneous observations using all the low-band and high-band antennas of the array. Aims. Our aim is to provide a tool for obtaining accurate simulations for LOFAR 2.0. Methods. We present our software for simulating LOFAR and LOFAR 2.0 observations, which includes realistic models for all important systematic effects such as the first- and second-order ionospheric corruptions, time-variable primary-beam attenuation, station-based delays, and bandpass response. The ionosphere is represented as a thin layer of frozen turbulence. Furthermore, thermal noise can be added to the simulation at the expected level. We simulate a full eight-hour simultaneous low- and high-band antenna observation of a calibrator source and a target field with the LOFAR 2.0 instrument. The simulated data are calibrated using readjusted LOFAR calibration strategies. We examine novel approaches of solution-transfer and joint calibration to improve direction-dependent ionospheric calibration for LOFAR. Results. We find that the calibration of the simulated data behaves very similarly to a real observation and reproduces certain characteristic properties of LOFAR data, such as realistic solutions and image quality. We analyze strategies for direction-dependent calibration of LOFAR 2.0 and find that the ionospheric parameters can be determined most accurately when combining the information of the high-band and low-band in a joint calibration approach. In contrast, the transfer of total electron content solutions from the high-band to the low-band shows good convergence but is highly susceptible to the presence of non-ionospheric phase errors in the data.

Coplanar Dual-Band Base Station Antenna Array Using Concept of Cavity-Backed Antennas

In this article, we propose a novel reflector-back cavity-low-band (LB)/high-band (HB) configuration for the dual-band dual-polarized base station antenna array design. The HB antennas are embedded in the opening sections of the loop radiating arms of the LB antennas. Clear and independent radiating environments are fulfilled by using two different types of reflectors (large planar reflector and back cavity). The mutual effects between the LB and HB antennas help to acquire good impedance matching and stable radiation patterns. Loop-like LB radiators work as directors in the HB, thereby effectively suppressing the side and back radiations of the HB antennas. Dual-function open slots are loaded on the ground stubs of the LB baluns, serving as impedance transformer in the LB but filtering structures in the HB. The experimental results validate that the proposed antenna array successfully covers bandwidths of 0.69–0.96 and 3.3–3.8 GHz (VSWR < 1.5) for 4G/5G applications. The port isolations are higher than 38 and 25 dB in the LB and HB, respectively. With the two independent radiating environments formed in the two bands, both LB and HB antennas exhibit stable unidirectional radiation patterns. The antenna gains and HPBWs are also satisfactory in both bands.

The LOFAR Two-meter Sky Survey: Deep Fields Data Release 1

The Low Frequency Array (LOFAR) is an ideal instrument to conduct deep extragalactic surveys. It has a large field of view and is sensitive to large-scale and compact emission. It is, however, very challenging to synthesize thermal noise limited maps at full resolution, mainly because of the complexity of the low-frequency sky and the direction dependent effects (phased array beams and ionosphere). In this first paper of a series, we present a new calibration and imaging pipeline that aims at producing high fidelity, high dynamic range images with LOFAR High Band Antenna data, while being computationally efficient and robust against the absorption of unmodeled radio emission. We apply this calibration and imaging strategy to synthesize deep images of the Boötes and Lockman Hole fields at ~150 MHz, totaling ~80 and ~100 h of integration, respectively, and reaching unprecedented noise levels at these low frequencies of ≲30 and ≲23μJy beam−1in the inner ~3 deg2. This approach is also being used to reduce the LOTSS-wide data for the second data release.

A LOFAR census of non-recycled pulsars: extending to frequencies below 80 MHz

We present the results from the low-frequency (40–78 MHz) extension of the first pulsar census of non-recycled pulsars carried out with the LOw-Frequency ARray (LOFAR). We used the low-band antennas of the LOFAR core stations to observe 87 pulsars out of 158 that had been previously detected using high-band antennas. We present flux densities and flux-calibrated profiles for the 43 pulsars we detected. Of this sample, 17 have not, to our knowledge, previously been detected at such low frequencies. Here we recalculate the spectral indices using the new low-frequency flux density measurements from the LOFAR census and discuss the prospects of studying pulsars at very low frequencies using current and upcoming facilities, such as the New Extension in Nançay Upgrading LOFAR (NenuFAR).

Searching for the largest bound atoms in space

Context. Radio recombination lines (RRLs) at frequencies ν &lt; 250 MHz trace the cold, diffuse phase of the interstellar medium, and yet, RRLs have been largely unexplored outside of our Galaxy. Next-generation low-frequency interferometers such as LOFAR, MWA, and the future SKA will, with unprecedented sensitivity, resolution, and large fractional bandwidths, enable the exploration of the extragalactic RRL universe. Aims. We describe methods used to (1) process LOFAR high band antenna (HBA) observations for RRL analysis, and (2) search spectra for RRLs blindly in redshift space. Methods. We observed the radio quasar 3C 190 (z ≈ 1.2) with the LOFAR HBA. In reducing these data for spectroscopic analysis, we placed special emphasis on bandpass calibration. We devised cross-correlation techniques that utilize the unique frequency spacing between RRLs to significantly identify RRLs in a low-frequency spectrum. We demonstrate the utility of this method by applying it to existing low-frequency spectra of Cassiopeia A and M 82, and to the new observations of 3C 190. Results. Radio recombination lines have been detected in the foreground of 3C 190 at z = 1.12355 (assuming a carbon origin) owing to the first detection of RRLs outside of the local universe (first reported in A&amp;A, 622, A7). Toward the Galactic supernova remnant Cassiopeia A, we uncover three new detections: (1) stimulated Cϵ transitions (Δn = 5) for the first time at low radio frequencies, (2) Hα transitions at 64 MHz with a full width at half-maximum of 3.1 km s−1 the most narrow and one of the lowest frequency detections of hydrogen to date, and (3) Cα at vLSR ≈ 0 km s−1 in the frequency range 55–78 MHz for the first time. Additionally, we recover Cα, Cβ, Cγ, and Cδ from the −47 km s−1 and −38 km s−1 components. In the nearby starburst galaxy M 82, we do not find a significant feature. With previously used techniques, we reproduce the previously reported line properties. Conclusions. RRLs have been blindly searched and successfully identified in Galactic (to high-order transitions) and extragalactic (to high redshift) observations with our spectral searching method. Our current searches for RRLs in LOFAR observations are limited to narrow (&lt;100 km s−1) features, owing to the relatively small number of channels available for continuum estimation. Future strategies making use of a wider band (covering multiple LOFAR subbands) or designs with larger contiguous frequency chunks would aid calibration to deeper sensitivities and broader features.

Metamaterial cell inspired high gain multiband antenna for wireless applications

In this paper, a novel metamaterial inspired high gain dual band antenna has been designed. The unit cell is highly reflective, resonating at 5.9 GHz and 9.9 GHz with 3 dB stopband characteristic from 2 to 4.5 GHz. The unit cell structure has been characterized by deriving its equivalent circuit model in ADS, where the circuit simulation shows good matching with the full-wave analysis. The unit cell is placed at a height of 0.152λo above the patch antenna forming a resonant cavity which causes gain enhancement of 4 dBi and 3.1 dBi along with an increase in bandwidth of 90.63% and 179.63% respectively at the operating frequency of 5.7 GHz and 10.3 GHz. Furthermore, the prototype antenna has been fabricated and measured where the experimental results are in good agreement with the simulated responses.

The LOFAR Tied-Array All-Sky Survey (LOTAAS): Survey overview and initial pulsar discoveries

We present an overview of the LOFAR Tied-Array All-Sky Survey (LOTAAS) for radio pulsars and fast transients. The survey uses the high-band antennas of the LOFAR Superterp, the dense inner part of the LOFAR core, to survey the northern sky (δ &gt; 0°) at a central observing frequency of 135 MHz. A total of 219 tied-array beams (coherent summation of station signals, covering 12 square degrees), as well as three incoherent beams (covering 67 square degrees) are formed in each survey pointing. For each of the 222 beams, total intensity is recorded at 491.52 μs time resolution. Each observation integrates for 1 hr and covers 2592 channels from 119 to 151 MHz. This instrumental setup allows LOTAAS to reach a detection threshold of 1–5 mJy for periodic emission. Thus far, the LOTAAS survey has resulted in the discovery of 73 radio pulsars. Among these are two mildly recycled binary millisecond pulsars (P = 13 and 33 ms), as well as the slowest-spinning radio pulsar currently known (P = 23.5 s). The survey has thus far detected 311 known pulsars, with spin periods ranging from 4 ms to 5.0 s and dispersion measures from 3.0 to 217 pc cm−3. Known pulsars are detected at flux densities consistent with literature values. We find that the LOTAAS pulsar discoveries have, on average, longer spin periods than the known pulsar population. This may reflect different selection biases between LOTAAS and previous surveys, though it is also possible that slower-spinning pulsars preferentially have steeper radio spectra. LOTAAS is the deepest all-sky pulsar survey using a digital aperture array; we discuss some of the lessons learned that can inform the approach for similar surveys using future radio telescopes such as the Square Kilometre Array.

Radiation Pattern Improvement of Cross-Band Dipoles Using Inductive-Loading Mode-Suppression Method

A ground-backed two-dipole antenna system is analyzed. The two antennas operate at different frequency bands. For the higher-band antenna (HBA), the radiation patterns are significantly distorted by the strong mutual coupling between two antennas. A method of mode suppression by inductive loading is proposed to improve the distorted radiation patterns of the HBA based on the characteristic-mode analysis (CMA). Modal significances and characteristic currents calculated by CMA help to identify the modes contributing to the pattern distortion and determine the loading locations on the lower band antenna. With the unwanted modes suppressed, the original 3 dB angular coverage range in H-plane is fully restored, and the boresight gain in E-plane is compensated by 1 dB. The results are experimentally validated.

Mitigation of Non-Narrowband Radio Frequency Interference Incorporating Array Imperfections

In a recent paper, we presented a non-narrowband spatial radio frequency interference (RFI) mitigation algorithm for radio astronomy arrays. The algorithm constructs a 2nd-order filter by combining a 1st-order subspace subtraction method with a non-narrowband signal model. The model is based on the assumption that the frequency response is approximately flat and that the array is calibrated. In this paper, we consider the effects of array imperfections such as unknown complex gains and mutual coupling, incorporate these into the non-narrowband signal model and extend the RFI mitigation algorithm to include a calibration step. With a calibration step and no mutual coupling, the proposed algorithm was able to process twice the bandwidth per channel when compared to conventional narrowband techniques. This performance declines to 1.6 times greater bandwidth when the effect of mutual coupling is included. The evaluation of the algorithm was done using the layout of a Low Frequency Array (LOFAR) High Band Antenna (HBA) station and a digital audio broadcast recorded with a software defined radio.