Transmitting Antennas Flashcards
What produces EM waves
time varying currents
Isotropic
uniform in all directions
Properties of a real antenna
Input impedance, gain and radiation pattern, polarization
Antenna input impedance
ZA=RA+jXA
Components of resistive antenna input impedance
Rrad (radiation/heat)
Rloss (energy storage)
Gain/Radiation Pattern
Some directions are favored over others
A high gain antenna is
highly directional
Polarization
E-field direction depends on antenna geometry and excitation. What shape does the E-field trace?
conductivity of copper
5.8(10^7)
HPBW
Half power beam-width, bandwidth of signal at -3dB level
SLL
Side Lobe Level, dB level at the first lobe
Assumptions for the Hertzian dipole
filamentary wire, negligible feed gap, dipole lies along z-axis, high conductivity
Antenna directivity
a direction-dependent scaling factor applied to the otherwise isotopically radiated power
Formula to switch to dBi
10log(value)
dBi
dB relative to an isotropic radiator
omnidirectional
there is no dependence in a direction, therefore it is omnidirectional in that direction
Principal Plane Pattern
the radiation patterns plotted in the theta-plane and phi-plane
FBR
Front-to-back ratio, dB level at pi or 180 degrees
FNBW
First Null Beamwidth, bandwidth from the first nulls
Half-wavelength dipole
-Rrad increases faster with length than Rloss
-I(z) becomes sinusoidal
-XA approaches a cross-over at 0 (less capacitive -> inductive)
intrinsic impedance
ratio of E-field and H-field within a medium, the impedance of the medium
Quarter-wavelength monopole theory
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
Quarter-wavelength monopole input impedance
The impedance is reduced by half πA = 36.5 + π21 β¦ for π = π/4, where π
rad =
36.5 β¦. By trimming the length by 4-6%, the reactive component is tuned out,
leaving πA β
π
rad β
35-36.5 β¦
Quarter-wavelength monopole gain and directivity
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.
Broadside
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
900 MHz ISM band
902-928 MHz
ISM band
The ISM radio bands are portions of the radio spectrum reserved internationally for industrial, scientific, and medical (ISM) purposes, excluding applications in telecommunications.[1]
End fire array
An endfire array is a linear array in which the direction of radiation is along the line of the antennas
Skin effect
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
