Quasi-optical Filters Research Articles

Deuterated ammonia toward the Orion Nebula

The detection of transitions in NH2D at 85926.2 and 110153.4 MHz from the Orion molecular cloud is reported. An 11-m radio telescope with an 80-120 GHz cooled Cassegrain receiver and a tunable quasi-optical rejection filter was used in the investigations. Equations of statistical equilibrium for the lowest 34 energy levels of NH2D were solved. It seems unlikely that the observed lines arise in the very extended CO cloud. However, they could originate either in the molecular 'ridge' or the unresolved (Kleinmann-Low) core of the cloud or both. A column density of about 50 trillion/sq cm is deduced. The NH2D spectra, with 1 MHz resolution, are shown in a graph. Using previously reported column densities for NH3, a value of 0.05 with an estimated maximum uncertainty factor of 10 is found for the ratio NH2D/NH3.

Quasioptical band-rejection filter at 100 GHz

We describe a tunable Fabry–Perot type filter which has application to radio astronomy in the frequency band 80–110 GHz. The filter causes image sideband suppression when used with a heterodyne spectral line receiver having an intermediate frequency of 4.75 GHz. The transmission loss in the signal sideband is about 0.4 dB, while the image sideband rejection is more than 15 dB; there appears to be little if any problem with scattering or distortion of the antenna radiation pattern.

An Adjustable Quasi-Optical Bandpass Filter---Part I: Theory and Design Formulas

A quasi-optical bandpass filter suitable for millimeter and submilliieter wavelengths and in the far infrared region is described. It consists of three or more wire-grid polarizers with quarter-wave spacings. The filter has the advantage over conventional quasi-optical filters, e.g., Fabry-Perot filters, that its bandwidth and the shape of its frequency response are adjustable. This is achieved by changing the angular orientations of the wires of the different polarizers. The filter requires the input electric field to be linearly polarized in a direction perpendicular to the wires of the first grid. The theory of operation is presented and design formulas for the filter are given, under the assumption that ideal wire-grid polarizers are employed. The effects of using realistic grids on the performance of the filter are dealt within another paper.

An Adjustable Quasi-Optical Bandpass Filter---Part II: Practical Considerations

This paper investigates the effects of using realistic grids on the performance of the adjustable quasi-optical bandpass filter presented in Part I. The theory given here is in excellent agreement with measurements performed on a three-grid filter in the 50-75-GHz band.

Bandpass and Pseudo-High-Pass Quasi-Optical Filters

Two forms of quasi-optical filters are discussed herein for use at millimeter-wave frequencies and possibly for far infrared frequencies. One form consists of metallic grids with intervening dielectric support material which forms a quasi-optical filter analogous to the inductively coupled waveguide bandpass filter. Because of dielectric losses, the relatively weak upper stopband, and the rapidly repeating passbands, this type of structure leaves much to be desired as a bandpass quasi-optical filter. However, when designed for wide bandwidth, it makes a very good pseudo-high-pass filter. For moderate- to wide-band bandpass applications, pseudo-high-pass filters of this type can be designed to match with a form of quasi-optical low-pass filter previously treated by the authors, in order to give a bandpass filter with strong, broad stopbands on both sides of the passband. Design principles, computed performance, and experimental results are presented for both pseudo-high-pass and bandpass structures.

Quasi-Optical Low-Pass Filters Which Attenuate by Absorption

A type of filter structure is discussed which has application for quasi-optical systems using focused beams or oversized waveguide. The filter structure consists of a number of focusing reflectors which in the passband of the filter focus the energy from one reflector to the next so that the energy is beamed in a zig-zag fashion. However, the focusing reflectors are made from arrays of spaced-apart metal plates with the edges of the plates parallel to the E field. The focusing face of each reflector is thus formed by the front edges of the plates, When the frequency is suffcientIy high so that the spacing between the plates is greater than a half-wavelength, the energy passes between the plates and is absorbed by dissipative material placed therein. Thus above a certain frequency the reflectors are absorptive and a stopband is formed. A trial three-reflector structure was fabricated and tested with very encouraging results. The minimum loss points in the passband were about 1 dB, and the stopband was broad and free from spurious responses. There were 3- or 4-dB attenuation spikes in the passband due to "trapped modes," but it should be possible to eliminate these by improved mode launchers.

Low-Pass Quasi-Optical Filters for Oversized or Focused-Beam Waveguide Applications

A form of Iow-pass quasi-optical filter for millimeter-wave and possibly infrared applications is described which consists of thin layers of dielectric interspersed with arrays of thin metallic strips in a manner similar to an artificial dielectric. Equivalent circuits for filter half-sections of the type used were developed, and formulas and graphs are presented from which image parameters for such half-sections can be obtained. A design procedure is given from which multisection fiters of this type can be designed from low-pass lumped-element prototypes in order to give prescribed transmission characteristics. Some trial designs were worked out and fabricated for use in oversized waveguide, and their responses were computed in detail with a digital computer. Except for some higher order mode effects believed to be due to the standard-size to oversize waveguide tapers which were used, the measured responses were in good agreement with the theory. Stopband edge ratios between 3:1 and 4:1 are possible. Filter structures of this type are amenable to fabrication by techniques similar to those used in integrated circuit technology.

Quasi-optical waveguide filters for millimeter and submillimeter wavelengths

Quasi-optical filters consisting of iterations of dielectric slabs housed in oversized rectangular waveguide are described. Filters consisting of dielectric slabs separated by air of the same electrical length have been analyzed and constructed for the cases of normal incidence (two-port filters) and 45° incidence (four-port filters). The two-port filters have low-pass properties while the four-port filter has simultaneous low and high-pass properties. Experimental data on 9 mm and 0.9 mm models have shown good agreement with the theory, thus demonstrating their use as filters for millimeter and submillimeter wavelengths.