POWER DENSITY

Question 1

is defined as the travel of electromagnetic waves through or along a medium

Answer 1

Radio Frequency (RF) propagation

Question 2

For RF propagation between approximately _______ and _______, radio waves travel in a direct line of sight

Answer 2

100 MHz and 10GHz

Question 3

dielectric constant of space

Answer 3

1

Question 4

dielectric constant of air at sea level

Answer 4

1.000536

Question 5

is a theoretical, lossless, omnidirectional (spherical) antenna. That is, it radiates uniformly in all directions.

Answer 5

isotropic radiator

Question 6

The power of a transmitter that is radiated from an isotropic antenna will have a ____________ (power per unit area) in all directions

Answer 6

uniform power density

Question 7

The power density at any distance from an isotropic antenna is simply the ____________

Answer 7

transmitter power divided by the surface area of a sphere (4ʌR2) at that distance

Question 8

The surface area of the sphere increases by the square of the radius, therefore
the power density, PD, (watts/square meter) decreases by the square of the radius.

Question 9

use directional antennas to channel most of the radiated power in a particular direction.

Answer 9

Radars

Question 10

is the ratio of power radiated in the desired direction as compared to the power
radiated from an isotropic antenna

Answer 10

Gain (G) of an antenna

Question 11

The power density at a distant point from a radar with an antenna gain of Gt is the power density from
an isotropic antenna multiplied by the radar antenna gain.

Question 12

Another commonly used term is effective isotropic radiated power (EIRP), where EIRP = Pt Gt. ERP is also used but EIRP is preferred because it specifically defines the type of reference antenna as
isotropic.

Question 13

A receiving antenna captures a portion of this power determined by its

Answer 13

effective capture Area (Ae)

Question 14

The received power available at the antenna terminals is the

Answer 14

power density times the effective capture area (Ae) of the receiving antenna.

Question 15

For a given receiver antenna size the capture area is constant no matter how far it is from the transmitter. Also notice from Figure 1 that the received signal power decreases by 1/4 (6 dB) as the distance doubles. This is due to the R2 term in the denominator of equation.

Question 16

For safety (radiation hazard) and EMI calculations, power density is usually expressed in

Answer 16

milliwatts per square cm