Editing Free-space path loss (section)

==Influence of distance and frequency==
[[File:Inverse square law.svg|thumb|In free space the intensity of electromagnetic radiation decreases with distance by the [[inverse square law]], because the same amount of power spreads over an area proportional to the square of distance from the source.]] 

The free-space loss increases with the distance between the antennas and decreases with the wavelength of the radio waves due to these factors:<ref name="Cerwin">{{cite book
 | last1  = Cerwin
 | first1 = Steve 
 | title  = Radio Propagation and Antennas: A Non-Mathematical Treatment of Radio and Antennas
 | publisher = Author House
 | date   = 2019
 | pages  = 31–35
 | url    = https://books.google.com/books?id=L-ilDwAAQBAJ&dq=%22free+space%22+%22path+loss%22&pg=PT31
 | isbn   = 9781728320328
 }}, Section 1.8</ref>
*'''[[Intensity_(physics)|Intensity]]''' (<math>I</math>) – the [[power density]] of the radio waves decreases with the square of distance from the transmitting antenna due to spreading of the electromagnetic energy in space according to the [[inverse square law]]<ref name="SensorsLowPower"/>
*'''[[Antenna_aperture|Antenna capture area]]''' (<math>A_\text{eff}</math>) – the amount of power the receiving antenna captures from the radiation field is proportional to a factor called the ''antenna aperture'' or antenna capture area, which increases with the square of wavelength.<ref name="SensorsLowPower"/>  Since this factor is not related to the radio wave path but comes from the receiving antenna, the term "free-space path loss" is a little misleading.
*'''Directivity of receiving antenna'''- while the above formulas are correct, the presence of Directivities Dt and Dr builds the wrong intuition in the FSPL Friis transmission formula. The formula seems to say that "free space path loss" increases with frequency in vacuum, which is misleading. The frequency dependence of path loss does not come from free space propagation, but rather from receiving antenna capture area frequency dependence. As frequency increases, the directivity of an antenna of a given physical size will increase. In order to keep receiver antenna directivity constant in the formula, the antenna size must be reduced, and a smaller size antenna results in less power being received as it is able to capture less power with a smaller area. In other words, the path loss increases with frequency because the antenna size is reduced to keep directivity constant in the formula, and has nothing to do with propagation in vacuum. 
*'''Directivity of transmitting antenna''' - the directivity of transmitting antenna does not have the same role as directivity of receiving antenna. The difference is that the receiving antenna is receiving the power from free space, and hence captures less power as it becomes smaller.  The transmitting antenna does not transmit less power as it becomes smaller (for example half wave dipole), because it is receiving its RF power from a generator or source, and if the source is 1 Watt or Pt, the antenna will transmit all of it (assuming ideal efficiency and VSWR for simplicity).

* '''System Loss Factor (L) :''' Miscellaneous loses or system loses (L=>1) are usually due to transmission line attenuation, filter loses, and antenna loses in communication system. A value of L = 1 indicates no loss in the system hardware.<ref>{{Cite book |last=Rappaport |first=Theodore S. |title=Wireless communications: principles and practice |date=2010 |publisher=Pearson India Education Services |isbn=978-81-317-3186-4 |edition=Second edition, twentieth impression 2019, Indian subcontinent adaption |location=Noida |pages=107}}</ref>