As an RF signal propagates it becomes weaker at any given measurement point as it gets farther away from the transmitter. What concept is described?

Free Space Path Loss (FSPL) is the fundamental concept describing the reduction in signal strength (attenuation) of an electromagnetic wave as it propagates through a line-of-sight path in free space. This loss is not due to absorption by a medium but is a natural consequence of the signal's energy spreading out over a larger area as it moves away from the transmitter. According to the inverse-square law, the power density of the RF signal decreases proportionally to the square of the distance from the source. FSPL is a critical, predictable component in calculating the link budget for any wireless system.

A. RF latency: This refers to the time delay for a signal to travel from the transmitter to the receiver, not the loss of signal strength.

C. Beamwidth: This is a specification of an antenna, defining the angular width of its main radiation lobe, which relates to directivity, not signal loss over distance.

D. Diffraction: This is a specific RF behavior where waves bend around an obstacle, which is different from the general signal weakening that occurs in unobstructed space.

1. Goldsmith, A. (2005). Wireless Communications. Cambridge University Press. In Chapter 2, Section 2.5, "Path Loss Models," the text introduces the free-space path loss model, stating, "This model predicts that received power decays as the square of the T-R separation distance." (p. 39).

2. Rappaport, T. S. (2002). Wireless Communications: Principles and Practice (2nd ed.). Prentice Hall. Chapter 4, "Mobile Radio Propagation: Large-Scale Path Loss," introduces the Free Space Propagation Model, explaining that it is used to "predict received signal strength when the transmitter and receiver have a clear, unobstructed line-of-sight path between them." The Friis free space equation is presented to quantify this loss as a function of distance and frequency. (pp. 102-104).

3. MIT OpenCourseWare. (2011). 6.02 Introduction to EECS II: Digital Communication Systems, Fall 2011. Massachusetts Institute of Technology. Lecture 15 notes, "Wireless Communication," describe path loss: "As the signal propagates from the transmitter, its energy spreads out. This is a 'good' effect... but it also means that the power of the signal falls with distance." The notes explicitly link this to the inverse square law. (p. 2). Available at: https://ocw.mit.edu/courses/6-02-introduction-to-eecs-ii-digital-communication-systems-fall-2011/resources/lecture-15-wireless-communication/