The Atmospheric Propagation Medium between 45 and 75 GHz
Abstract
The clear atmosphere (dry air mass) represents a unique filter over the 45 to 75 GHz range with frequency-dependent absorption properties caused by the microwave spectrum of oxygen not found at any lower frequency. The basic information necessary for determining molecular line, band, and continuum absorption is contained in a transfer function model. Close to sea level, an unstructured band exists, centered at 60 GHz with a half-width of 8 GHz and a pressure-dependent intensity that is corrected for pressure-induced line overlap effects. At elevations above 12 km, the lines separate and radio channels with up to 400 MHz bandwidth can be accommodated between lines. Above 30 km, the line properties become polarization and orientation (i.e., with respect to the direction of a magnetic field) dependent due to the Zeeman effect. Each oxygen line splits proportionally with geomagnetic field strength (0.2-0.8 Gauss) into numerous sub-lines, which conform to Zeeman patterns extending over a megahertz scale around each line center. Using this model in computer routines in conjunction with distributions of pressure, temperature, and humidity along a radio path makes it possible to predict frequency-dependent signal attenuation and delay effects of the neutral atmosphere (h=0 to 100 km). Further analysis investigates possible bandwidth limitations of the propagation medium by employing short-pulsed (< or = 1 ns) test signals with carriers in the 45 to 75 GHz range. Examples are presented of specific attenuation (dB/km) and excess delay (ps/km) rates, of zenith path attenuation (dB), and of pulse distortion over a terrestrial link (60 GHz, 35 km) for Gaussian, rectangular, and BPSK modulation. Benefits from the frequency-variable atmospheric properties include a fairly well defined signal range, affording transmission security and trade-offs between range and susceptibility to detection and interference.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 01, 1982
- Accession Number
- ADA116435
Entities
People
- Hans J. Liebe
Organizations
- Institute for Telecommunication Sciences