A Wideband Resistive Beam-Splitter Screen

Abstract

We present the design, construction and measurements of the electromagnetic performance of a wideband space beam splitter. The beam splitter is a sheet in free space that is designed to divide incident radiation into reflected and transmitted components for interferometer measurement of spectral features in the mean cosmic radio background. Analysis of a 2element interferometer configuration with a vertical beam splitter between a pair of antennas leads to the requirement that the beam splitter be a resistive sheet with sheet resistance ηo/2, where ηo is the impedance of free space. The transmission and reflection properties of such a sheet are computed for normal and oblique incidences and for orthogonal polarizations of the incident electric field. We have constructed such an electromagnetic beam splitter as a square soldered grid of resistors of value 180 Ohms (approximately ηo/2) and a grid size of 0.1 m. We measured the reflection and transmission coefficients over a wide frequency range between 50 and 250 MHz in which the wavelength well exceeds the mesh size. Measurements of the coefficients for voltage transmission and reflection agree to within 5% with physical optics modeling of the wave propagation, which takes into account edge diffraction.

ON THE DETECTION OF GLOBAL 21-cm SIGNAL FROM REIONIZATION USING INTERFEROMETERS

“…This coherence would be well above that without a beam splitter in between. The performance of space beam splitters was analyzed in Mahesh et al (2015) where it was shown that the sheet impedance was required to be resistive and of value half the impedance of free space (377/2Ω) for maximum coherence and hence interferometer response. Mahesh et al (2015) also proposed a method for the construction of such a screen as a resistor grid, Figure 7.…”

“…The performance of space beam splitters was analyzed in Mahesh et al (2015) where it was shown that the sheet impedance was required to be resistive and of value half the impedance of free space (377/2Ω) for maximum coherence and hence interferometer response. Mahesh et al (2015) also proposed a method for the construction of such a screen as a resistor grid, Figure 7. Schematic of a configuration with a beam splitter sheet in between the interferometer elements (Mahesh et al 2015).…”

“…Mahesh et al (2015) also proposed a method for the construction of such a screen as a resistor grid, Figure 7. Schematic of a configuration with a beam splitter sheet in between the interferometer elements (Mahesh et al 2015).…”

“…A resistive sheet is in between the inline dipoles and serves as a space beam splitter. As discussed in Mahesh et al (2015), for a resistive sheet with sheet impedance equal to half the impedance of free space, the reflected and transmitted powers received by an interferometer element are equal and each is one-fourth of the incident power. Further, half the incident power is absorbed in the resistive sheet.…”

“…Motivated by the formidable challenge of discriminating against instrument related internal systematics in singleelement radiometers, there has been recent work on interferometer based detection of the global signal (Vedantham et al 2015;Mahesh et al 2015;Presley et al 2015). Compared to single-element radiometers, interferometers are relatively insensitive to receiver noise and noise originating internally in ohmic losses and passive components in the signal path.…”