A Resistive Wideband Space Beam Splitter

We present the design, construction and measurements of the electromagnetic performance of a wideband space beam splitter. The beam splitter is designed to power divide the incident radiation into reflected and transmitted components for interferometer measurement of spectral features in the mean cosmic radio background. Analysis of a 2-element 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 {\eta}o /2, where {\eta}o is the impedance of free space. The transmission and reflection properties of such a sheet is 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 {\eta}o /2) and a grid size of 0.1 m, and present measurements of the reflection and transmission coefficients over a wide frequency range between 50 and 250 MHz in which the wavelength well exceeds the mesh size. Our 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.Comment: 14 pages,17 figure

An octave bandwidth frequency independent dipole antenna

Precision measurements of the spectrum of the cosmic radio background require frequency independent antennas of small electrical dimensions. We describe the design of a wide-band fat-dipole antenna with a sinusoidal profile having a frequency independent performance over the octave band 87.5 to 175 MHz. The input return loss exceeds 15 dB and the radiation power pattern is frequency invariant and close to cosine square over the octave bandwidth. The structure has been optimized using electromagnetic modeling, and the design has been validated by constructing a prototype

A wideband resistive beam-splitter screen

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 two-element 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 η_o/2, where η_o 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 η_o/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

Design of frequency independent profiled discone antenna for detecting spectral ripples from the Epoch of Recombination

Detection of distortions in the spectrum of Cosmic Microwave Background Radiation (CMBR) is the current challenging problem in the modern observational cosmology. It holds key to understand the evolution of early universe. These distortions are predicted to be 8-9 orders of magnitude weaker than CMBR temperature and hence require an antenna with smoothly varying radiation and impedance characteristics without any inflexions, for their detection. We describe in this paper the design of a sinusoidally profiled discone antenna having the frequency independent performance in the frequency range 2-4 GHz. The structure is electromagnetically modeled and optimized. The return loss of the prototype built is better than 15 dB and has non-uniformity at < 1% level. The half power beam width has a max. variation of 10% across the operating band

Wideband antennas for precision spectral radiometers for cosmology

Precision cosmology has been made possible by differential microwave radiometers that have imaged the anisotropy in the cosmic microwave background radiation. Continued progress is expected from measurement of spectral distortions in this cosmic radiation. A key distortion in the global sky spectrum is in the 40-250 MHz band and expected from variations in the hydrogen spin temperature and ionization state during Cosmic Dawn and Epoch of Reionization when the first stars and galaxies form. Electrically-small shaped ultra-wideband antennas are described that have been designed for the detection of these global or all-sky spectral distortions; a key design feature is the making of the transfer function Maximally Smooth