We present a novel, experimentally based method for determining the plasma frequency reduction factor in a traveling-wave tube (TWT) using measured small-signal gain characteristics. We used broadband impulse excitation to obtain single-pass, ripple-free experimental gain versus frequency. The approach extracts the normalized growth rate from this experimental gain data and applies the third-order Pierce model to obtain the frequency-dependent Pierce space-charge parameter, QC. From this parameter, the reduced plasma frequency $\omega_{q}$ is determined. Combining it with known values of beam voltage, current and radius enables direct evaluation of the plasma frequency reduction factor $R_{s c}=\omega_{q} / \omega_{p}$. The method inherently captures realistic beam–circuit interactions present in practical TWTs without relying on idealized geometric assumptions. This experimentally based approach provides a practical means of characterizing space-charge effects and supports improved modeling of beam–wave interaction in TWTs.

We present a novel, experimentally based method to extract the plasma frequency reduction factor in a wideband traveling-wave tube (TWT) by analyzing the gain characteristics. We used broadband impulse excitation to obtain single-pass, ripple-free experimental gain versus frequency. The approach involves extracting the normalized growth rate from this experimental gain data and applying the fourth-order Pierce model to calculate the frequency-dependent Pierce space-charge parameter $QC$ . We used this parameter to back-calculate the reduced plasma frequency $ \omega _{q} $ . Combining it with the known values of beam voltage, current and radius enable direct evaluation of the plasma frequency reduction factor $R_{sc} = \omega _{q}/\omega _{p}$ . Our method accounts for realistic beam and circuit nonidealities that are challenging to incorporate in analytic theoretical models of space charge reduction effects. In the illustrative case of a helix circuit TWT, these include the effects of finite-thickness and width (wire and tape) helices, dielectric supports and dispersion control vanes, finite beam radius, and rapid variations in beam to slow wave circuit (SWC) proximity, which are not all fully captured by traditional analytic models, such as the sheath helix approximation. The comparison with the classical analytic sheath helix model shows that our methodology predicts higher $R_{sc}$ values than the analytic model. We attribute this difference to the finite, discrete nature of the helix wires, which allows more space-charge field lines to remain longitudinal rather than being drawn laterally outward from the beam by image charges on a completely continuous, close-proximity, conductive SWC. Our plasma frequency reduction factor extraction method enables one to 1) conduct detailed, direct studies of how $R_{sc}$ and $\omega _{q}$ vary with circuit, beam, and electromagnetic wave properties; 2) compare with prior approximate theoretical model predictions that estimate $R_{sc}$ and $\omega _{q}$ ; and 3) in general, increase the accuracy of the parametric Pierce model as a first-pass estimator tool for designing new TWTs.

We present the design and experimental characterization of a three-dimensional (3D) printed, ultra-wideband transmitarray (TA) antenna with beam scanning capability. The TA incorporates a polarization-rotating unit cell (PRUC) that performs linear polarization rotation and functions as a wideband 1-bit spatial phase shifter impedance-matched to free space. Each PRUC comprises two orthogonal double-ridge horn antenna (DRHA) sections sandwiching a cross-dipole structure. The double-ridge horn antenna sections enable the wideband impedance matching. The cross-dipole layers in the center of the unit cell are hard-wired to be either connected or disconnected. Manually changing the orientation of the cross-dipole structure reconfigures the unit cell between two operating states, enabling polarization rotation of +90 degrees and-90 degrees. Simulation results predicted that the PRUC operates effectively over 6-12 GHz (one-octave bandwidth), with a transmission coefficient of |T-xy| > -0.5 dB, |R-yy| < -10 dB (corresponding to VSWR < 1.93), and a relative phase difference of 180 degrees between the two operating states. A 400-element TA antenna with a size of 9 lambda(0)& times; 9 lambda(0) at 9 GHz was fabricated and experimentally characterized. The main body of the TA antenna was fabricated using stereolithography (SLA) 3D printing and metallized with copper using a commercial electroplating process. We experimentally evaluated the beam scanning potential of this TA antenna between 6-12 GHz by manually reconfiguring the spatial phase shifters. The measurements between 6-10 GHz demonstrate wide-angle two-dimensional (2-D) beam scanning capability ranging from-60 degrees to +60 degrees, corresponding to a fractional bandwidth of 50%.

We present an adaptive beam pointing and tracking system based on an electronically reconfigurable reflectarray antenna (ERRA) with 2-bit phase quantization, developed for potential use in troposcatter communications applications. The system addresses the challenges of beam pointing and tracking in dynamic environments—where platform movements, vibrations, and tropospheric effects can rapidly degrade link quality—by enabling fast, electronically controlled steering of the beam without mechanical movement. The system integrates 2-bit quantized phase control of each individual element of the ERRA with real-time feedback to achieve precise beam pointing and tracking over a wide angular range of up to ±60° in both azimuth and elevation planes. Two tracking algorithms—monopulse-like and conical tracking—were implemented, characterized, and compared using the same system. We experimentally validated the operation of the proposed system at 5.9 GHz by tracking a beacon carried by a drone. Two sets of experiments, including a static accuracy test and a dynamic tracking test, were conducted to quantitatively evaluate the two algorithms. In the former, the drone hovered at multiple programmed positions, evenly covering ±60° azimuth and 0-60° elevation angles, and 100 tracking samples were collected at each position to calculate the RMS Euclidean error. In the dynamic test, the drone was flown along a preselected trajectory while the system continuously tracked the moving target. Our experimental results demonstrate that the monopulse-like tracking algorithm outperforms the conical scan algorithm in this system.

Traditional RF telemetry faces critical limitations in hypersonic environments and secure space applications, including signal blackout and susceptibility to interception. X-ray Communication (XCOM) overcomes these challenges by enabling plasma sheath penetration during hypersonic flight and reentry, while also offering ultra-narrow beams for high-bandwidth, low-probability-of-intercept (LPI) deep-space links. We experimentally validated an XCOM transmitter that uses a helix traveling wave tube (TWT) to generate X-ray pulses from a titanium target. We selected Ti to produce high-intensity characteristic X-rays at accessible energies (${\lt}10 \mathrm{keV}$). We first measured 50 $\mu$ S X-ray pulses to verify the link. Simulations of the aluminumfiltered spectra supported these measurements, confirming that the signals matched the physics of X-ray transmission. We also drove the TWT with a 500 MHz modulation signal. By comparing the signal response under thin ($17 \mu \mathrm{~m}$) and thick $(1.8 \mathrm{~mm})$ filters, we identified electromagnetic interference (EMI) that currently obscures the high-frequency X-ray signal. We present our methodology for isolating these signals. In the coming months, we anticipate presenting data from our exploration into high-frequency modulation of these X-ray signals.

The high-frequency (HF) band, ranging from 3 to 30 MHz, is widely used in a number of applications such as beyond-line-of-sight (BLOS) communications and over-the-horizon radar systems. The long-range propagation characteristics of the HF band rely on the Earth’s ionosphere as a refracting medium to establish BLOS wireless links. However, traditional HF communications links often use channels with narrow bandwidths (typically 3 kHz), which limits the amount of data that can be transmitted. In this perspective article, we discuss the challenges of establishing wideband HF communications links with emphasis on the ionosphere, noise characteristics, antenna performance, and available bandwidth. Solar radiation drives ionospheric layering, where fluctuating electron densities determine usable frequencies that change with time of day, seasons, and latitude. Since background noise at HF decreases with increasing frequency, operating at the upper HF limit is advantageous. We present a noise prediction model following ITU Recommendation P.372-17 and highlight the role of high-directivity antennas such as Log-Periodic Dipole Arrays, in optimizing the signal-to-noise ratio of the link. Using ray tracing techniques, we model several representative wideband HF links that employ near-vertical-incidence skywave (NVIS) or longrange skywave modes over distances ranging from approximately 390 to 5,200 km. We examine the performance of these links for bandwidths up to 1 MHz and present their day and night ionograms, with transmitter and receiver locations spanning near-equatorial to high northern latitudes. The modeled maximum usable frequencies are corroborated against crowdsourced FT8 reception data and direct ionosonde measurements over the same period for links for which propagation data was available.

We present experimental energy and spectral characterization of large impulse amplification in a wideband traveling wave tube (TWT) using metrics spanning both timeand frequency-domain responses. These include input-output energy conversion, and power spectral density comparisons that distinguish in-band amplification from out-of-band spectral growth. We present the results for both positive and negative bipolar impulse excitations to assess waveform asymmetry, distortion, and polarity-dependent electron-wave interaction effects in the large-signal regime. Additionally, during the talk we will discuss the influences of beam energy and impulse center frequency on impulse amplification, revealing their roles in shaping gain saturation, temporal compression, and spectral broadening. This work aims to identify the key parameters governing efficient wideband impulse amplification in TWTs and provide a framework for comparing transient amplification performance beyond traditional small-signal metrics.

This paper introduces a machine learning-based framework for modeling and utilizing an electronically reconfigurable matching and decoupling network (MDN) for two-element arrays across the high-frequency (HF) band. The proposed neural network surrogate incorporates prior physical knowledge and is trained on measured datasets, enabling it to capture the non-idealities and imperfections of the physical circuit that are intractable to consider with purely theoretical models. With this surrogate, a vast number of valid MDN configurations predicted by theory but previously inaccessible in practice become available for exploration. This unprecedented access to the solution space allows the identification of configurations that achieve high port-to-port isolation, as well as the flexibility to optimize for additional performance criteria such as low insertion loss, wide bandwidth, and out-of-band interference suppression, thereby significantly enhancing the practical utility of reconfigurable MDNs.

In this work, we present an ultra-thin, selftriggered, diode-integrated energy selective surface (ESS) that acts in the S-band as a band-stop limiter under high-power incident fields while preserving low-loss transmission at low power. The conceived design consists of a PCB with a single, bias-free layer etched on a 0.2-mm thick $\left(\approx 0.20 \% \lambda_{0}\right.$ at the band center) FR-4 substrate. In the band-stop configuration, the dog-bone ESS unit-cell shape achieves a -10 dB fractional bandwidth (FBW) of 37%, with rejection up to 45 dB in fullwave simulations. We discuss the equivalent circuit retrieval, the ON/OFF physics of the Schottky element, and the agreement over 2.2-3.8 GHz between measured and simulated results. Waveguide measurements on the fabricated prototype showed a stable passband with reduced insertion loss at -28 dBm. At +25 dBm, the device operated in a self-actuated regime, with diode turn-on and limiting behavior, without the need for detectors or bias networks. Future developments will be directed to investigate the response of the ESS under pulsed high-power microwaves. To conclude, the proposed work combines electronic circuit front-end survivability with compact, fabrication-friendly geometry, thus representing a viable solution within the open literature.

This article presents a machine learning (ML)-based framework for modeling and utilizing an electronically reconfigurable matching and decoupling network (MDN) for two-element arrays operating across the high-frequency (HF) band (3-30 MHz). The proposed neural network model incorporates prior physical knowledge of the system and serves as a reliable digital replica of the physical MDN, accurately capturing the nonidealities and imperfections that arise in practice and are too complex to model using purely theoretical approaches. The dataset used for model development was collected through an automated measurement setup consisting of the MDN, a network analyzer, and computer control software. Leveraging this digital replica, we propose a solution-generation methodology that uncovers a vast number of valid MDN configurations for any given load and frequency-a capability that theoretical analysis predicted but conventional methods failed to achieve in practice. In addition, as an optional step, a few hardware fine-tuning adjustments may be used to further increase port-to-port isolation to levels exceeding 40 dB, which were not previously attainable in practice. Moreover, the method allows designers to incorporate additional performance criteria besides impedance matching and decoupling such as low insertion loss, wide matching and decoupling bandwidths, and out-of-band interference suppression, significantly expanding the practical utility of reconfigurable MDNs.

We report on our ongoing work on impulse amplification using a custom, wideband helix traveling wave tube (TWT) amplifier. We have specifically conducted experiments and simulations of large signal amplifications of bipolar differentiatedGaussian impulses. Our results confirm that the saturated peak power for an impulse surpasses the saturated power obtained with CW sinusoidal signals, aligning with earlier observations. In particular, we experimentally observed similar to 1.4 kW of instantaneous peak power with a large differentiated-Gaussian impulse, despite a saturated power limit for CW sinusoidal excitation of similar to 150W.

Recent advances in critical application areas such as high-power radar, electronic warfare, directed-energy weapons, and satellite communications have created a significant demand for antenna arrays capable of rapid and accurate beam steering at high continuous wave (CW) and pulsed power levels. Traditionally, radar and electronic warfare applications have used active electronically scanned array (AESA) antennas for beam scanning. AESAs are effective but expensive, as each element of the array requires a transmit/receive (T/R) electronics module, significantly increasing the cost and leading to high power consumption by the array. Furthermore, the peak power levels achievable from AESAs generally fall significantly short of what is needed for directed energy or high-power-microwave (HPM) applications.

In recent years, phased-array antennas with high-power-handling capabilities have become a keystone of research and development activities in high-power radar, electronic warfare, satellite communications, directed energy systems, and other application areas requiring precise and/or rapid beam scanning at high continuous wave (CW) or pulsed power levels. Currently, our group at the University of WisconsinMadison (UW-Madison) is researching a novel, high-power-capable, electromagnetic lens consisting of a periodic surface of polarization-rotating unit cells (PRUCs) for use in hybrid phased-array architectures offering rapid, electronic beam steering. Our previous research resulted in the design and fabrication of a 1-bit electronically-reconfigurable PRUC phase shifter (Z. Zhang, M. Gao, M. M. Honari, J. Wu, J. H. Booske, and N. Behdad, “A Wideband, 1-bit, Electronically Reconfigurable Phase Shifter for High-Power Microwave Phased-Array Applications,” IEEE Trans. Plasma Sci., vol. 51, issue 7, July 2023) with a maximum CW and peak power handling capability of $>23 \mathrm{W} / \text{cm}^{2}$ and $347 \mathrm{W} / \text{cm}^{2}$, respectively, over an octave bandwidth from 6-12 GHz. Our current research, which we will present at this conference, focuses on investigating the impacts of manufacturing and assembly techniques on the CW power-handling capability of this PRUC phase shifter.

We exploit the effect of two-tone excitations on two-surface multipactor to mitigate multipactor for a microstrip transmission line, and to determine the SEY of copper samples. Simulations predict that adding a second tone with a certain frequency and amplitude relative to the primary operating signal will shift the multipactor region to higher power, thereby suppressing multipactor for the primary operating tone and power level. Simulations also indicate that whether or not multipactor occurs under two-tone excitation is highly sensitive to the SEY of the surfaces, such that a modest reduction in the SEY may be sufficient to suppress multipactor for a specific two-tone combination. We experimentally investigate this by testing an oxidized copper sample, and a de-oxidized version of that oxidized copper sample. We are also investigating these effects further using controlled-porosity samples as a means of varying the effective SEY.

We present the design and measurement results of a non-linear-time-invariant (non-LTI) electrically small antenna (ESA) operating at the high-frequency (HF) band. This non-LTI ESA addresses the bandwidthefficiency product limitations of passive, linear-time-invariant (LTI) ESAs. Due to the long wavelengths of electromagnetic waves in the HF band, practical HF antennas are typically electrically small, resulting in large quality factors and limited bandwidths of passive ESAs. These constraints are governed by wellestablished physical limits and have been thoroughly investigated and confirmed by many ESA designs. Different techniques, such as non-foster impedance matching, active matching, and direct-antenna modulation among others, have been investigated to overcome these limitations. These techniques do not violate the laws of physics. Rather, they violate the linear-time-invariance or passive operation assumptions used in the derivation of the fundamental physical limits.

This paper reviews recent advances in wide-band phased-array antennas for high-average power microwave (HAPM) applications, focusing on the challenges of simultaneously achieving high-power handling, electronic beam scanning, and ultra-wide bandwidth performance. Traditional active phased arrays encounter significant challenges in meeting these demands, largely due to the high cost of transmit/receive (T/R) modules with moderate to high power-handling capabilities, as well as the thermal management challenges associated with high-average power. Passive phased-array architectures have emerged as a more effective solution for meeting these requirements for HPM applications. Recent research at the University of Wisconsin-Madison has led to the development of ultra-wideband, high-power-capable, electronically reconfigurable lens antennas utilizing polarization-rotation phase-shifting technology. This innovation not only enables the antennas to handle extreme power levels but also simplifies design and reduces associated costs. The potential of this technology is significant for applications in fields such as electronic warfare, high-power radar, satellite communications, and long-range wireless power transfer.

In this paper, we present a review of the applications of polarization rotating surfaces (PRS) in designing beam-steerable antenna arrays. We discuss the principles of operation of PRS in both transmissive and reflective modes and demonstrate how the geometric phase shift offered by these surfaces can be used to build a simple 1- bit, 0 degrees/180 degrees spatial phase shifter. The use of such spatial phase shifters in designing fixed beam transmitarray and reflectarray antennas is discussed. An example of a fully electronically reconfigurable reflectarray antenna using PRS unit cells is presented. Finally, the challenges of designing electronically tunable polarization rotating transmitarray antennas are presented and an example design of such a structure is presented. The latest results of the design of an electronically reconfigurable polarization rotating transmitarray antennas will also be presented at the symposium.

High-data-rate x-ray carrier communication (XCOM) is a new frontier for wireless data transmission. We investigated using a helix TWT to produce high repetition-rate electron bunches as a source for generating high repetition-rate x-ray pulses. Simulations illustrate the advantage of leveraging the accessible low energy K-shell characteristic lines produced by titanium to improve the bit-error rate of the x-ray intensity signal when compared to a target which produces spectra composed of pure bremsstrahlung radiation. The simulations use a time-dependent Monte Carlo code to predict x-ray signals expected from an experimentally measured, density-modulated, TWT spent electron beam. Experiments are in progress to test the accuracy of the simulation predictions. The experiments will use an avalanche photodiode as the receiver x-ray sensor.

We present a review of recent progress towards development of wideband phased-array antennas for high-power microwave (HPM) applications and discuss the challenges involved in designing these phased arrays. Achieving high-power handling, electronic beam scanning, and wide bandwidths is challenging using current active phased-array antennas. Passive phased-array antennas may be a viable solution for meeting these requirements for HPM applications that require radiating pulsed waveforms with hundreds of megawatts to gigawatts of peak power levels. After presenting a brief review of the state-of-the-art in phased-array antenna technologies, we present an overview of the recent research conducted at the University of Wisconsin-Madison in developing high-power-capable, wideband, electronically reconfigurable transmitarray antennas. We discuss the capabilities of this technology as well the challenges in using it to develop gigawatt-class phased-array antennas.

Co-site radio frequency (RF) interference often occurs when several RF transceivers operate in the vicinity of each other. It can degrade communication quality, particularly when a high-power transmitter is working near a receiver attempting to detect a low-power signal. Co-site interference can be particularly troublesome in the HF band (3–30 MHz) where the wavelengths of electromagnetic waves are large (10–100 m) and large antennas are often needed. The near-field regions of such antennas can extend to long distances away from them encompassing other nearby antennas and transceivers. This close proximity exacerbates the co-site interference issues among different antennas mounted on the same platform. Some traditional methods commonly used to reduce co-site interference are less effective at the HF band. For instance, mechanically movable antennas with directional radiation patterns are harder to realize at HF where real estate on platforms is generally very limited. Moreover, HF communications and over-the-horizon radar systems often experience changes in the HF propagation channel as a function of the time of the day, season, locations of the transmitter/receiver, etc. These changes require the frequency of operation within the 3–30 MHz frequency range to vary over the course of a single day. This frequency agility requirement makes it more challenging to use a fixed frequency allocation strategy for different transceivers to minimize interference. Additionally, fixed filters or other passive interference mitigation techniques have a limited application in such frequency agile applications. Finally, the radiated power levels of many HF systems range from tens to hundreds of Watts for communications systems to kW power levels for over-the-horizon radar systems. In such situations, out-of-band interference caused by high-power emissions of powerful transmitters can cause significant problems to nearby receivers, if not attenuated sufficiently.