Very High Frequency Band Research Articles

High angular momentum coupling for enhanced Rydberg-atom sensing in the very-high frequency band

Recent advances in Rydberg-atom electrometry detail promising applications in radio frequency communications. Presently, most applications use carrier frequencies greater than 1 GHz where resonant Autler–Townes splitting provides the highest sensitivity. This letter documents a series of experiments with Rydberg atomic sensors to collect and process waveforms from the automated identification system (AIS) used in maritime navigation in the very high frequency (VHF) band. Detection in this band is difficult with conventional resonant Autler–Townes based Rydberg sensing and requires a new approach. We show the results of a method called high angular momentum matching excited Raman (HAMMER), which enhances low frequency detection and exhibits superior sensitivity compared to the traditional AC Stark effect. From measurements of electromagnetically induced transparency in rubidium and cesium vapor cells, we show the relationship between incident electric field strength and observed signal-to-noise ratio and find that the sensitivity of the HAMMER scheme in rubidium achieved an equivalent single VHF tone sensitivity of 100μV/m/Hz. With these results, we estimate the usable range of the atomic vapor cell antenna for AIS waveforms given current technology and detection techniques.

Comparison of Coverage-Prediction Models for Modern Mobile Radio Networks

The accurate prediction of a signal’s attenuation is essential for the development of reliable, modern, mobile radio-communication networks. In this study, the accuracies of four propagation models in diverse terrains and environments were analyzed using field measurements along a comprehensive test route. We evaluated the ability of the models—Egli, Okumura, Hata–Davidson, and Longley–Rice—to predict signal propagation in the Very-High-Frequency (VHF) and Ultra-High-Frequency (UHF) bands. Based on a meticulous comparison, we present valuable insights into the strengths and limitations of these models, enhancing coverage-prediction methodologies for evolving mobile radio networks. The Egli model, despite its simplicity, introduces significant inaccuracies due to its assumptions and a lack of consideration for the terrain. The Okumura model, which is widely used in urban areas, requires careful correction selection, while the Hata–Davidson model improves upon the former’s weaknesses. The Longley–Rice model excels in flexibility and accuracy, especially in the VHF bands, using topographical data, though it can overestimate the attenuation in shadowed areas. The study concluded that no single model was universally accurate, as each model has its strengths and limitations. It highlights the need for informed model selection based on the terrain’s characteristics and specific requirements. The results will be useful to network planners, helping them to design efficient, mobile communication networks with reliable coverage and optimal spectrum utilization. The Longley–Rice model emerged as particularly powerful, offering detailed predictions across diverse environments.

High-Efficiency GaN-Based Power Amplifiers for Envelope Nonlinearities’ Mitigation in VHF Wideband Polar-Mode Transmitters

Space-based communications at the very high frequency (VHF) band for air traffic management is a new technology application under development that requires energy-efficient architectures to mitigate the power limitations of satellite platforms. The usage of high-efficiency radiofrequency (RF) transmitters can help reduce the power consumption, but nonlinearities concerning the amplified signal in wide fractional bandwidth systems are a problem to solve. This paper proposes a high-efficiency (RF) power amplifier (PA) for satellite communications at the VHF band that aims to reduce the envelope distortion inherent to wide fractional bandwidth multicarrier polar-mode transmitters. Its design is based on a solution called hybrid-coupled switching voltage PA in combination with gallium nitride (GaN) high electron mobility transistor (HEMT) technology. The developed VHF PA prototype delivers up to 95 W from a 28 V power supply, with a drain efficiency about 80% within the 118 MHz to 138 MHz operating band. To test its linearity performance, operating in a polar-mode configuration, a GaN-based wideband envelope amplifier (EA) has been developed to modulate the RF PA supply port. This EA improves its power efficiency by combining it with a slow envelope power supply (SEPS). Some measurements have been taken for a 100 W peak envelope power (PEP) and 10 MHz (maximum carrier spacing) four-tone digitally-modulated test signal, where any distortion product is attenuated 46 dB below the average power of the amplified signal without applying any digital predistortion (DPD) technique.

Electrodermal activity spectral and nonlinear analysis – potential biomarkers for sympathetic dysregulation in autism

Autism spectrum disorder (ASD) is a neurodevelopmental disease characterized by emotional and social deficits, which can be associated with sympathetic dysregulation. Thus, we aimed to analyze the electrodermal activity (EDA) using time, and novel spectral and nonlinear indices in ASD. The cohort consisted of 45 ASD boys and 45 age-matched controls. EDA was continuously recorded at rest. The EDA indices were evaluated by time-, spectral-, and nonlinear-domain analysis. Our results revealed increased non-specific skin conductance responses, spectral parameters in high and very-high frequency bands, approximate and symbolic information entropy indicating sympathetic overactivity in ASD vs. controls (p < 0.05, for all). Surprisingly, the nonlinear index from detrended fluctuation analysis α1 was lower in ASD vs. controls (p = 0.024) providing thus distinct information about qualitative features of complex sympathetic regulation. Concluding, the complex time, spectral, and nonlinear EDA indices revealed discrete abnormalities in sympathetic cholinergic regulation as one of the potential pathomechanisms contributing to cardiovascular complications in ASD.

A VHF Band Small CRLH Antenna Using Double-Sided Meander Lines

In this paper, a miniaturized very-high frequency (VHF) band antenna using both top and bottom meander lines is proposed. To design a compact size antenna in the VHF band, a Composite Right/Left-Handed (CRLH) transmission line is applied to antenna structure; additionally, both top and bottom meander lines were used to achieve a greater inductance. The CRLH transmission line unit cell operates at 88 MHz, and the fabricated antenna is designed by arranging 7-unit cells. The overall size of the proposed antenna is 0.087λ × 0.02λ × 0.0003λ at the lowest operating frequency, and the antenna operates at 84 MHz. The VSWR 3.5:1 reference operating bandwidth of the antenna is 2%. The received power of the proposed CRLH antenna was measured to verify the antenna performance.

A Circular Sector-Shaped Dipole Antenna with Meandered Arms and Added Ferrite-Loaded Artificial Magnetic Conductor

A circular sector-shaped dipole antenna with meandered arms based on an artificial magnetic conductor (AMC) is proposed in this study. A compact unit cell of the AMC operating within 19–485 MHz is designed using a ferrite material, which exhibits high-permeability characteristics in the high-frequency (HF), very-high-frequency (VHF), and ultra-high-frequency (UHF) bands. The configuration of the meandered arms is optimally designed using a binary genetic algorithm, so that the antenna combined with the AMC meets the voltage standing wave ratio (VSWR) specification of 3.5:1 or less within the HF, VHF, and UHF bands. The proposed antenna with the AMC is fabricated and measured. The measured VSWR frequency bandwidth (3.5:1 or less) is 25.8:1 (14–361 MHz), and the measured result matches with the simulated result well. Furthermore, to verify the performance of the proposed antenna with the AMC, the measured received power results are compared with a commercial reference antenna operating in HF, VHF, and UHF bands.

Low-Profile VHF Antenna Based on Quarter-Mode Substrate-Integrated Waveguide Structure

A traditional whip antenna is suitable for terrestrial wireless communication due to its omnidirectional radiation pattern which is capable of transmitting and receiving signals from all directions. However, the length of a whip antenna is long for the very high frequency (VHF) band applications; for example, the ideal length is 80 cm at 90 MHz, which restrains the mobility when mounted on a vehicle. Many studies have been conducted to develop a low-profile antenna that can substitute a whip antenna but most of them are for relatively high frequency (GHz) applications. In this paper, a planar low-profile antenna operating in the VHF band is proposed. The height of the antenna is only 9 cm by utilizing a quarter-mode substrate-integrated waveguide (QMSIW) structure with shorting pins at the edges. The latter promotes omnidirectional radiations despite its short height. The antenna design was optimized by 3D full-wave simulation software. Subsequently, an antenna prototype was fabricated and tested. The results demonstrated that the antenna resonated at 86 MHz with a maximum gain of 1.7 dB along the azimuth direction.

Design and Implementation of High Directivity Directional Coupler at VHF Band Using Grounding Composed of Strip Lines Technics

This paper proposes a design of high directivity directional coupler (D.C) based on grounding composed of strip lines for high power Radar transmitter at Very High Frequency (VHF), (150 – 200 MHz). The directional coupler is used to check and verify the transmitter output high power, frequency, and reflected signal from an antenna connected with the Radar transmitter. The performance requirements of directional couplers are a strong coupling to reduce the effect on the transmitted output power and high directivity to suppress the interference of the reflected signal from the antenna. So far, various architectures have been proposed to gain high directivity, and there have been many studies used to obtain a strong coupling and higher directivity. However, the conventional architecture of the directional coupler has a directivity of only about 20 dB, and there have been difficulties to achieve the higher directivity of more than 20 dB. In this paper, the proposed architecture of directional coupler based on grounding composed of strip lines is discussed and compares the test results of the proposed directional coupler with the conventional one. The high directivity directional coupler is designed using a computer-aided design Simulation program; Advance Design System (ADS 2016), using Rogers 4003 substrate. The directional coupler was fabricated on printed circuit board (PCB) technology and measured using a vector network analyzer (VNA). The results show that the proposed directional coupler has directivity between -25 to -24 dB inside the working bandwidth and is adequate for a high-power radar transmitter.

Very large group delay in VHF band using coupled high temperature superconducting resonators

Storing a very high frequency (VHF) band (30–300 MHz) electromagnetic wave has many potential applications, such as phase modulation, buffering, and radio frequency memory. It can be effectively achieved by applying coupled resonator-based electromagnetically induced transparency (EIT) due to its slow light effect. However, the wavelength in the VHF band is too long to design resonators, and the group delay is limited by the high resistive loss of metal. The practical application of EIT in the VHF band is still a big challenge. In this work, we propose and experimentally demonstrate EIT response in a high-temperature superconducting (HTS) microwave circuit with coupled-resonator-induced transparency. The chip size of the HTS circuit is only 34 mm × 20 mm with a very low transparency frequency of 198.55 MHz. In addition, we implement very large group delay higher than 12.3 μs and 16.2 μs with working temperatures of 65 K and 50 K separately, which is much longer than the previous reported works on slow wave. The fabricated circuit is planar with working temperature about 65 K, and thus can be easily integrated into other microwave devices under the cryogenic conditions provided by a commercial portable Stirling cryocooler. Our proposed method paves a way for studying EIT in the microwave region due to the high quality factor of the HTS resonator, which has great potential use for radio-frequency memory in the future.

A Compact Four-Pole Tunable HTS Bandpass Filter at VHF Band

This article presents a compact, low loss, wide tuning range, high temperature superconducting (HTS) bandpass filter at very high frequency (VHF) band. The filter consists of four zigzag open-loop resonators loaded with gallium arsenide (GaAs) varactors as tuning elements. Each resonator is biased independently. The filter circuit is fabricated on a 3-inch HTS thin film deposited on a LaAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate. The filter demonstrates a frequency tuning range of 73.2-118 MHz, which is a tunability of 48%, an insertion loss of 0.6-2.2 dB, and a 3-dB fractional bandwidth of 3.2-4.3%. Besides, the return loss of the filter is better than 16.6 dB in the whole tuning range, with slight adjustment of bias voltages for different resonators.

Influence of Aircraft Power Electronics Processing on Backup VHF Radio Systems

The paper describes the influence of power electronics, energy processing, and emergency radio systems (ERS) immunity testing on onboard aircraft equipment and ground stations providing air traffic services. The implementation of next-generation power electronics introduces potential hazards for the safety and reliability of aircraft systems, especially the interferences from power electronics with high-power processing. The paper focuses on clearly identifying, experimentally verifying, and quantifiably measuring the effects of power electronics processing using switching modes versus the electromagnetic compatibility (EMC) of emergency radio systems with electromagnetic interference (EMI). EMI can be very critical when switching power radios utilize backup receivers, which are used as aircraft backup systems or airport last-resort systems. The switching power electronics process produces interfering electromagnetic energy to create problems with onboard aircraft radios or instrument landing system (ILS) avionics services. Analyses demonstrate significant threats and risks resulting from interferences between radio and power electronics in airborne systems. Results demonstrate the impact of interferences on intermediate-frequency processing, namely, for very high frequency (VHF) radios. The paper also describes the methodology of testing radio immunity against both weak and strong signals in accordance with recent aviation standards and guidance for military radio communication systems in the VHF band.

A compact low frequency dual band liquid crystal polymer antenna for VHF and UHF band applications

This article presents a compact low frequency dual band Liquid crystal polymer (LCP) antenna for Very High Frequency (VHF) and Ultra High Frequency (UHF) band applications. The proposed antenna is having dimensions of 19 × 19 × 0.1 mm 3, which is compact in size and designed on FR4, Rogers/ RT Duroid, Polyimide and LCP substrate materials with CPW (Coplanar Waveguide) feed having CSRR (Complementary Split Ring Resonator) structure placed on ground plane. Simulation results of the designed antennas are compared with each other and finally LCP substrate material exhibits better properties compared with all other substrate materials. The designed antenna on LCP operates in frequency ranges of VHF and UHF (0.085–0.29 GHz and 0.805–2.58 GHz) respectively. The designed antenna on LCP is resonating at 0.14 GHz and 1.425 GHz frequencies with reflection coefficient of −24.29 dB and –33.39 dB respectively. The designed antenna on LCP has the gain of 12. 82 dB at 0.14 GHz and 6.27 dB at 1.425 GHz respectively. The designed antenna on LCP is fabricated and analyzed by using combinational analyzer. Measured results of proposed antenna on LCP are in good agreement with simulated results.

Thunderstorm High‐Frequency Radio Bursts With Weak Low‐Frequency Radiation

AbstractBrief bursts of high‐frequency (HF) and very high frequency (VHF) radio emissions unaccompanied by strong low‐frequency radiation have been observed during initiation and propagation of lightning or thunderstorm electrical breakdown without leading to fully fledged lightning. This paper investigates a physical mechanism to generate such radio bursts by electrical discharge activity inside a thundercloud. When a discharge consists of many high‐frequency emission sources, such as streamers, that generate currents in random directions, its radiation spectrum peaks in the HF and VHF bands, and the spectral magnitudes in low frequencies are much smaller or even negligible. Combined with recent observational findings, the present study suggests that lightning initiation may begin with a short burst of many randomly occurring small‐scale discharges in a localized thundercloud region.

Aircraft-Integrated VHF Band Antenna Array Designs Using Characteristic Modes

Because of the large wavelength in very high-frequency (VHF) band, conventional design concepts for flush-mounted VHF band antenna arrays over aircraft platforms meet many challenges. This article presents an approach for the design of VHF aircraft-integrated antenna arrays with switchable beams by using the characteristic modes (CMs) of the aircraft platform. The CMs of the aircraft platform provide the modal significances, modal currents, and modal radiation fields of each mode. An in-depth investigation over the useful CMs information is performed to prove the feasibility of design VHF band antenna array using the CMs of the platform. Switchable radiation patterns with horizontal polarization are then synthesized from these CMs. The switchable beams and polarization constraint make it more challenging than CM-based fixed power pattern synthesis. The polarized switchable beam synthesis is carried out with a multiobjective evolutionary algorithm. A trade-off among the synthesized current distributions for different beam steering angles is made to determine the optimal current excitation locations for antenna array development. Meanwhile, a novel bandwidth enhanced triple-couple-line coupling element is proposed to allow aircraft platform flush-mounted designs. A scaled model of the proposed aircraft-integrated antenna array system was fabricated to experimentally verify the effectiveness of the proposed approach.

Design and Characterization of VHF Band Small Antenna Using CRLH Transmission Line and Non-Foster Matching Circuit

In this paper, we propose an electrically small antenna consisting of a composite right/left-handed (CRLH) transmission line (TL) and a non-Foster matching circuit. An interdigital capacitor (IDC) and spiral inductor are used to fabricate the very high frequency (VHF) band antenna based on CRLH TL. The size of the proposed antenna is as small as 0.025 × 0.014 × 0.0008 λ at 145.5 MHz using the zeroth-order resonant generated by the CRLH TL. The antenna operation bandwidth is extended by the non-Foster circuit (NFC) consisting of a pair of transistors in a cross-coupled manner. An antenna prototype is fabricated and the input impedance, the received power, and gain of the proposed antenna are measured. The results show that the broadband characteristic is maintained while the form factor is extremely small compared to the wavelength. The average received power enhancement and increased bandwidth of antenna are 17.3 dB and 335.5 MHz (from 249.2–268.2 to 145.5–500 MHz), respectively. The calculated gain of the proposed antenna with the non-Foster is about −45 dBi at 155 MHz. The proposed antenna can be considered as a potential candidate of a low-profile antenna for military ground communications at the VHF band.

The Use of the Reassignment Technique in the Time-Frequency Analysis Applied in VHF-Based Passive Forward Scattering Radar.

This paper presents the application of the time-frequency (TF) reassignment technique in passive forward scattering radar (FSR) using Digital Video Broadcasting – Terrestrial (DVB-T) transmitters of opportunity operating in the Very High Frequency (VHF) band. The validation of the proposed technique was done using real-life signals collected by the passive radar demonstrator during a measurement campaign. The scenario was chosen to test detection ranges and the capability of estimating the kinematic parameters of a cooperative airborne target in passive FSR geometry. Additionally, in the experiment the possibility of utilizing FSR geometry in foliage penetration conditions taking advantage of the VHF band of a DVB-T illuminator of opportunity was tested. The results presented in this paper show that the concentrated (reassigned) energy distribution of the signal in the TF domain allows a more precise target Doppler rate to be estimated using the Hough transform.

Deployable Inverted-Hat Monopole With 3:1 Constant Gain Bandwidth

A novel deployable inverted-hat monopole (IHM) antenna is presented for satellite applications. This frequency scalable antenna was designed and verified in very high-frequency (VHF) band (40–160 MHz) for 9U CubeSats and ultrahigh-frequency (UHF) band (200–600 MHz) for 1U CubeSats. The VHF band ultrawideband (UWB) monopole antenna operates from 40 to 160 MHz on an infinite ground plane, with emphasis on constant 5 dBi gain in a deployed volume of 60 × 60 × 100 cm. On a finite ground plane, the antenna achieves a 3.2:1 bandwidth with VSWR <3 and an average gain of 0.5 dBi. A capacitive top hat loading was added to the antenna to improve matching and pattern stability. Full-wave simulations are verified with measurements of fabricated prototypes. Storability and deployability of the antenna design is confirmed with the UHF design.

Design and Performance Prediction of a Dual-Band Coupled-Fed Dipole Array Antenna for PCL Systems in the VHF Band

This article proposes the design of a dual-band coupled-fed dipole antenna for passive coherent location (PCL) systems in the very high frequency (VHF) band. The proposed indirect coupled feed mechanism, which is often employed in microstrip patch antennas, is first applied to VHF band dipole elements for dual-band matching. To confirm the effectiveness of the proposed design, we fabricate the coupled-fed dipole element and measure antenna characteristics, such as the voltage standing wave ratio (VSWR) and the antenna gain. The proposed antenna element is then applied to an eight-element circular array to form the reference and surveillance beams for PCL systems. Finally, the target location is estimated by constructing amplitude-range doppler (ARD) maps for one frequency modulation (FM) and two terrain digital multimedia broadcasting (T-DMB) illuminators in the Seoul-Gyunggi urban area. The results confirm that the proposed element is suitable for dual-band PCL systems in the VHF band compared to a conventional dipole antenna.

Highly Portable, Low-Cost SDR Instrument for RF Propagation Studies

Software-defined radio (SDR) instruments can be used to replace bulky and expensive spectrum analyzers for RF field measurements. Using the commercial off-the-shelf (COTS) equipment, a low-cost, portable SDR instrument for measuring RF propagation has been created and tested. In the U.K., parts of the very high-frequency (VHF) spectrum have been repurposed for the use of Internet-of-Things devices; the instrument developed here is designed to meet the use case of performing an urban propagation study in VHF and UHF short range devices bands in a fast and low-cost manner. Design of the hardware and software is discussed, as well as the calibration of the instrument. The results of a test propagation study are given for the completed instrument. It is shown that the SDR instrument is capable of performing the study to a high degree of agreement with a commercial spectrum analyzer, thus validating the approach. The readings of the received power taken by the instrument are shown to agree with the readings taken at the same locations with a commercial spectrum analyzer to within an average of 1.4 dB at 71 MHz and 1.1 dB at 869.525 MHz. From the measurements taken, log-distance models were able to be produced with a path-loss exponent of 2.44 and a log-normal shadowing standard deviation of 8.5 dB at 71 MHz, and a path-loss exponent of 4.06 and a log-normal shadowing standard deviation of 8.8 dB at 869.525 MHz.

Understanding the Radio Spectrum of Thunderstorm Narrow Bipolar Events

AbstractThis paper reports a study to understand the radio spectrum of thunderstorm narrow bipolar events (NBEs) or compact intracloud discharges, which are powerful sources of high‐frequency (HF) and very high frequency (VHF) electromagnetic radiation. The radio spectra from 10 kHz to about 100 MHz are obtained for three NBEs, including one caused by fast positive breakdown and two by fast negative breakdown. The results indicate that the two polarities of fast breakdown have similar spectra, with a relatively flat spectrum in the HF and VHF band. The ratio of energy spectral densities in the very low frequency and HF bands is (0.9–5) × 105. We develop a statistical modeling approach to investigate if a system of streamers can explain the main features of fast breakdown. Assuming that the current moment peak and charge moment change of individual streamers vary in the ranges of 5–10 A‐m and 5–20 μC‐m, respectively, the modeling results indicate that a system of 107–108 streamers can reproduce the current moment, charge transfer, and radio spectrum of fast breakdown. The rapid current variation on a time scale of nanoseconds required for fast breakdown to produce strong HF/VHF emissions is provided by exponentially accelerating and expanding streamers. Our study therefore supports the hypothesis that fast breakdown is a system of streamers. Finally, suggestions are given regarding future streamer simulations and NBE measurements in order to further develop our understanding of NBEs and lightning initiation.