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The D and F Ionosphere Layers: Why are AM Broadcast Signals Mostly Local?

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2020 ASEE Virtual Annual Conference Content Access


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Publication Date

June 22, 2020

Start Date

June 22, 2020

End Date

June 26, 2021

Conference Session

Engineering Physics and Physics Division Technical Session 4

Tagged Division

Engineering Physics and Physics

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Paper Authors


Paul Benjamin Crilly U.S. Coast Guard Academy

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Paul Crilly is a Professor of Electrical Engineering at the United States Coast Guard Academy. He received his Ph.D. from New Mexico State University, his M. S. and B.S. degrees at Rensselaer Polytechnic Institute, all in Electrical Engineering. He was previously an Associate Professor of Electrical and Computer Engineering at the University of Tennessee and was a Development Engineer at the Hewlett Packard Company. His areas of interest include laboratory development, antennas, wireless communications, signal processing, and instrumentation.

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Abstract – Inquisitive students will often ask the question, why are only local AM broadcast signals heard during the day, whereas at night, these signals from thousands of miles are received? Put another way, why is the AM broadcast band so crowded with distant stations at night and why do some stations go off the air at sunset? This paper will describe a project using simple apparatus that will enable students to use signals of opportunity in order to experience how the D and F ionosphere layers affects low and high frequency (LF and HF) radio reception and thus answer these questions and other questions. Thus the student will become more knowledgeable about skywave propagation and HF and LF communication in general. Signals of opportunity include various AM broadcast stations throughout North America, as well as the time signals from NIST’s WWV and Canada’s CHU. This paper will first present the theoretical basis of sky wave radio propagation and then describe the experiments to understand the behavior of the D and F ionosphere layers with respect to radio signals. By having students observe sky wave radio propagation and the effects of the ionosphere on these signals, they will be better positioned to understand the theory of how this part of the earth’s atmosphere enhances or degrades LF and HF radio propagation. In addition to explaining ionospheric propagation or lack thereof, the presented theory and practice will enable the student to understand and observe why LF and HF signals are noisier than VHF and UHF ones, the goodness of a product detector versus an envelope detector for weak signal reception and also consider the problem of multipath caused by signals propagating via ground and sky wave. This exercise will also enable the student to appreciate the value of frequency diversity for reliable HF communication. Finally, in the realm of public policy, this project will provide the student the significant rational for the how and why the Federal Communications Commission (FCC) will limit AM broadcaster’s transmitter power and airtime.

Crilly, P. B. (2020, June), The D and F Ionosphere Layers: Why are AM Broadcast Signals Mostly Local? Paper presented at 2020 ASEE Virtual Annual Conference Content Access, Virtual On line . 10.18260/1-2--35300

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