Transmitting Antennas

Question 1

What produces EM waves

Answer 1

time varying currents

Question 2

Isotropic

Answer 2

uniform in all directions

Question 3

Properties of a real antenna

Answer 3

Input impedance, gain and radiation pattern, polarization

Question 4

Antenna input impedance

Answer 4

ZA=RA+jXA

Question 5

Components of resistive antenna input impedance

Answer 5

Rrad (radiation/heat)
Rloss (energy storage)

Question 6

Gain/Radiation Pattern

Answer 6

Some directions are favored over others

Question 7

A high gain antenna is

Answer 7

highly directional

Question 8

Polarization

Answer 8

E-field direction depends on antenna geometry and excitation. What shape does the E-field trace?

Question 9

conductivity of copper

Answer 9

5.8(10^7)

Question 10

HPBW

Answer 10

Half power beam-width, bandwidth of signal at -3dB level

Question 11

SLL

Answer 11

Side Lobe Level, dB level at the first lobe

Question 12

Assumptions for the Hertzian dipole

Answer 12

filamentary wire, negligible feed gap, dipole lies along z-axis, high conductivity

Question 13

Antenna directivity

Answer 13

a direction-dependent scaling factor applied to the otherwise isotopically radiated power

Question 14

Formula to switch to dBi

Answer 14

10log(value)

Question 15

dBi

Answer 15

dB relative to an isotropic radiator

Question 16

omnidirectional

Answer 16

there is no dependence in a direction, therefore it is omnidirectional in that direction

Question 17

Principal Plane Pattern

Answer 17

the radiation patterns plotted in the theta-plane and phi-plane

Question 18

FBR

Answer 18

Front-to-back ratio, dB level at pi or 180 degrees

Question 19

FNBW

Answer 19

First Null Beamwidth, bandwidth from the first nulls

Question 20

Half-wavelength dipole

Answer 20

-Rrad increases faster with length than Rloss
-I(z) becomes sinusoidal
-XA approaches a cross-over at 0 (less capacitive -> inductive)

Question 21

intrinsic impedance

Answer 21

ratio of E-field and H-field within a medium, the impedance of the medium

Question 22

Quarter-wavelength monopole theory

Answer 22

We can apply image theory to the half-wavelength dipole antenna: cut the dipole in half (at the
feed-point) and mount the upper arm over a perfectly conductive half-space. Image theory dictates
that the fields above the half-space will be identical to those of a half-wavelength dipole in free
space, while the fields within the half-space go to zero

Question 23

Quarter-wavelength monopole input impedance

Answer 23

The impedance is reduced by half 𝑍A = 36.5 + 𝑗21 Ω for 𝑙 = 𝜆/4, where 𝑅rad =
36.5 Ω. By trimming the length by 4-6%, the reactive component is tuned out,
leaving 𝑍A ≅ 𝑅rad ≅ 35-36.5 Ω

Question 24

Quarter-wavelength monopole gain and directivity

Answer 24

The directivity (and gain) is double that of a half-wavelength dipole because all the power
is radiated above the boundary and none below it.

Question 25

Broadside

Answer 25

a type of radiation pattern for a linear array antenna where the axis of the main lobe is perpendicular to the axis of the array

Question 26

900 MHz ISM band

Answer 26

902-928 MHz

Question 27

ISM band

Answer 27

The ISM radio bands are portions of the radio spectrum reserved internationally for industrial, scientific, and medical (ISM) purposes, excluding applications in telecommunications.[1]

Question 28

End fire array

Answer 28

An endfire array is a linear array in which the direction of radiation is along the line of the antennas

Question 29

Skin effect

Answer 29

If the radius of the conductor is much less than the skin depth, we can assume that the current is uniform to one skin depth