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Free Space Propagation

1) The received power in free space decreases as the inverse square of the distance from the transmitter. It is affected by factors like transmitter power, antenna gains, wavelength, and path loss. 2) The received power at a reference distance can be used to calculate the power decay at greater distances according to an inverse square law relationship. 3) Power flux density represents the amount of power traveling through a given area in the far field. Larger antennas have a greater effective aperture and are able to capture more power according to this density.

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Bikash Kumar Behera
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181 views2 pages

Free Space Propagation

1) The received power in free space decreases as the inverse square of the distance from the transmitter. It is affected by factors like transmitter power, antenna gains, wavelength, and path loss. 2) The received power at a reference distance can be used to calculate the power decay at greater distances according to an inverse square law relationship. 3) Power flux density represents the amount of power traveling through a given area in the far field. Larger antennas have a greater effective aperture and are able to capture more power according to this density.

Uploaded by

Bikash Kumar Behera
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Free Space Propagation:

In the “far field” (distances many wavelengths from the antenna), the received
power Pr in free space at a path length d is given

where Gt and Gr are the transmitter and receiver antenna gains, respectively; Pt is the
transmit power; and λ is the wavelength.

• The effective isotropic radiated power (EIRP) is PtGt.

The path loss is Lp.

Received Power Reference

where Pr (d0) is the received power at the reference distance d0. Now, we see that
whatever the received power in free space is at distance d0, the power at d decays as
(d0/d)2 beyond that distance.

where Π0(dBm) = 10 log10 Pr(d0).

There is a concept in propagation of power flux density, the amount of


power that travels through a given area. This is a far-field concept only.
Power flux is denoted Pd in and has units of Watts per square meter, W/m2 .
In free space,

where η is the intrinsic impedance of free space, 120πΩ = 377Ω, and |E|2 is the
magnitude squared of the electric field. The idea is that an antenna “captures” some of
this power, according to, effectively, how large the antenna is. We call this the
effective antenna aperture, and denote it Ae, with units m2 . In short, physically larger
antennas are capable of larger Ae, although there is no exact proportionality. In this
case the definition of the received power is Pr(d) = Pd Ae

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