Propeller Modulation Effects on a Scanning-Beam Microwave Landing System

Jack M. Pope, William N. Staehle
1975 SAE Technical Paper Series   unpublished
This report presents the results of an investigation to assess the modulation effects on microwave signals transmitted through rotating propeller blades. Interruption of the antenna "line-ofsight" signal by the rotating propeller causes a variation of path loss, which produces essentially an amplitude modulation of the received signal. This interruption or blockage effect is generally only partial because of edge diffraction around the particular interfering propeller blade. Signals reflected
more » ... om the rotating propeller will also cause Doppler frequency shifts present in the received signals. A scanning beam microwave landing system (MLS) known as MODILS (modular instrument landing system) was used to process the received signals for display. The effects of propeller modulation were studied by varying the following parameters: * Spacing between propeller and receiving antenna * Propeller dimensions * Propeller speed (rpm) * Number of propeller blades * System data rate * Receiver response time * Receiver antenna aperture A ground station simulator and a variable speed fan with different size blades were used to simulate the actual MODILS ground station and a rotating propeller. Results of the tests are compared with data obtained using the MODILS ground station and the variable speed fan, and also using an actual aircraft whose rotating propeller caused observed propeller modulation. Data are also presented showing the effects of propeller modulation on a typical Doppler MLS.
doi:10.4271/750521 fatcat:gmgi6iltwredpla7gmrz6h7ggm