Full version in "Microwave System Design Tools and EW Applications" published by Artech House, Inc.
Microwave Signal Propagation Loss
Introduction Based on D.E. Kerr - Propagation of Short Radio Waves - Vol. 13 Radiation Laboratory Series, McGraw Hill, the program plots the path loss over sea water as a function of range.Default values may be modified and the display updated using the relevant Update Button or keyboard 'Enter' key. The display automatically scales, dependent on the parameter values selected. Free space propagation loss curve is displayed for comparison. Moving the mouse in range-dB loss space displays the plotted curves coordinates referred to the mouse x-axis position. All mouse and button clicking/ parameter changes are sent to the Java Console. The Save Button saves plot data to the Java Console ( slow if the console is open) with a summary of the selected parameters. Data may be selected on the Console using the mouse (Shift-click) and copied to the clipboard ( Ctrl C ) and saved in a text file for a permanent record or transferring to Excel for example. If visual plots are required, Alt-Printscreen, copies the current window to MS Clipboard. Power received(dBm) = Tx prime power(dBm)+ Tx antenna gain(dB) - Loss(dB) + Rx antenna Gain(dB) Java Console in MS Explorer is found in 'View' if activated by 'Tools-Internet Options-Advanced; and ticking, Java console enabled. Background Microwave signal propagation loss rate varies with range dependant on geometry and changing significance of the various possible propagation modes, these are divided into four observable regions.1. Interference Zone - this occurs in the line-of-sight-region where signals arrive via the direct path and also via reflection from the sea or ground surface. 2. Diffraction Zone - this is the shadow region where signal loss increases rapidly and occurs as the receiver moves just below the horizon 3. Intermediate Zone - between the interference and diffraction zones, the signal level follows a smooth but mathematically complex curve. In this path loss model a median cubic linear fit approximation is used which closely (<1dB) matches Kerr's examples 4. Tropospheric Scatter Zone - well beyond the horizon, irregularities in the atmosphere causes microwave signals to be scattered to the extent that at long ranges this scattered element exceeds the diffracted component. Transition between the diffraction and tropospheric scatter regions is approximated using a median logarithmic fit. Rain loss for light (0.25 mm/hr) medium (2.5 mm/hr) and heavy (25 mm/hr) figures are also calculated. NotesSea surface reflection amplitude and phase are polarisation dependent and correct choice of polarisation is therefore necessary to ensure accuracy in the near interference zone. Below the Brewster angle, there is very little difference between the polarisations The program validity extends from 100 MHz to 100 GHz and assumes a standard, well mixed atmosphere with the accepted 4/3 Earth radius approximation used to account for the slight bending of electromagnetic waves in average atmospheric conditions. Observations generally show variations about this mean due to time dependant atmospheric or sea surface fluctuations changing direct and reflected or refracted paths producing dynamic fading effects. Also temperature inversions and evaporation ducts can considerably modify the atmosphere refractive index near the surface to produce increased ray curving or signal waveguiding which can considerably increase signal strength over the horizon. This is termed anomalous propagation and is not supported by the model. pwe |