In the last decade, the proliferation of new 3D printing technologies has enabled the fabrication of complex geometries in manifold materials for novel applications. One discipline that has been explored extensively in the context of additive manufacturing is electromagnetic devices such as antennas. Difficultto-fabricate geometries are now possible and can deliver new antenna functionality and extend performance (e.g., lower frequency resonance in small volumes, wider bandwidth, narrow-beam directionality, and so on). Coupled with accurate 3D electromagnetic simulations, a new paradigm is emerging for antenna design and manufacture. Starting from a seed geometry, the state space can now be explored to identify new combinations and permutations of electromagnetically-beneficial shapes through multiple simulation iterations. Subsequently, the identified structures can be further validated and improved through rapid manufacturing using 3D printing for hardware evaluation in an anechoic chamber. However, to fully benefit from this emerging paradigm, an up-to-date survey of the most recent metal processes is required. This survey would determine which processes are well suited for building the next generation of antennas. For this purpose, a variety of metal 3D printing was employed to fabricate benchmark antennas with pathological geometries, including thin walls, overhanging features, and large aspect ratios. This survey can inform designers about potential structures to serve in novel antennas. A total of five processes have been preliminarily explored including selective laser melting, binder jetting, and plated vat photopolymerization, all of which delivered different advantages and disadvantages in terms of mechanical and electromagnetic performance. INDEX TERMS 3D printing, 3D printed antennas, 3D Hilbert curve, additive manufacturing, antenna radiation pattern, binder jetting, dipole antennas, fractal antenna, multifrequency antennas, powder-bed fusion, ultra high frequency, UHF, vat photopolymerization.

Site of StudyTo characterize atmospheric propagation effects at Ka-band (20 GHz) and Q-band (40 GHz), a dual Ka/Q-band beacon receiver was deployed to Milan, Italy in a collaboration between NASA Glenn Research Center and the Politecnico di Milano, utilizing the beacons onboard the Alphasat satellite (launched July 2013).

NASA Glenn Research Center (GRC) and the Politecnico di Milano (POLIMI) have initiated a joint propagation campaign within the framework of the Alphasat propagation experiment to characterize rain attenuation, scintillation, and gaseous absorption effects of the atmosphere in the 40 GHz band. NASA GRC has developed and installed a K/Q-band (20/40 GHz) beacon receiver at the POLIMI campus in Milan, Italy, which receives the 20/40 GHz signals broadcast from the Alphasat Aldo Paraboni Technology Demonstration Payload (TDP) #5 beacon payload. The primary goal of these measurements is to develop a physical model to improve predictions of communications systems performance within the Q-band. Herein, we describe the design and preliminary performance of the NASA propagation terminal, which has been installed and operating in Milan since June 2014. The receiver is based upon a validated Fast Fourier Transform (FFT) I/Q digital design approach utilized in other operational NASA propagation terminals, but has been modified to employ power measurement via a frequency estimation technique and to coherently track and measure the amplitude of the 20/40 GHz beacon signals. The system consists of a 1.2-m K-band and a 0.6-m Q-band Cassegrain reflector employing synchronous open-loop tracking to track the inclined orbit of the Alphasat satellite. An 8 Hz sampling rate is implemented to characterize scintillation effects, with a 1-Hz measurement bandwidth dynamic range of 45 dB. A weather station with an optical disdrometer is also installed to characterize rain drop size distribution for correlation with physical based models.

The rain rate data and statistics of a location are often used in conjunction with models to predict rain attenuation. However, the true attenuation is a function not only of rain rate, but also of the drop size distribution (DSD). Generally, models utilize an average drop size distribution (Laws and Parsons or Marshall and Palmer [1]). However, individual rain events may deviate from these models significantly if their DSD is not well approximated by the average. Therefore, characterizing the relationship between the DSD and attenuation is valuable in improving modeled predictions of rain attenuation statistics. The DSD may also be used to derive the instantaneous frequency scaling factor and thus validate frequency scaling models Since June of 2014, NASA Glenn Research Center (GRC) and the Politecnico di Milano (POLIMI) have jointly conducted a propagation study in Milan, Italy utilizing the 20 and 40 GHz beacon signals of the Alphasat TDP#5 Aldo Paraboni payload. The Ka- and Q-band beacon receivers provide a direct measurement of the signal attenuation while concurrent weather instrumentation provides measurements of the atmospheric conditions at the receiver. Among these instruments is a Thies Clima Laser Precipitation Monitor (optical disdrometer) which yields droplet size distributions (DSD); this DSD information can be used to derive a scaling factor that scales the measured 20 GHz data to expected 40 GHz attenuation. Given the capability to both predict and directly observe 40 GHz attenuation, this site is uniquely situated to assess and characterize such predictions. Previous work using this data has examined the relationship between the measured drop-size distribution and the measured attenuation of the link [2]. The focus of this paper now turns to a deeper analysis of the scaling factor, including the prediction error as a function of attenuation level, correlation between the scaling factor and the rain rate, and the temporal variability of the drop size distribution both within a given rain event and across different varieties of rain events

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