by ya bao1 antennas and propagation. 2 by ya bao introduction an antenna is an electrical conductor...
TRANSCRIPT
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by Ya Bao 1
Antennas and Propagation
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2By Ya Bao
Introduction An antenna is an electrical conductor or
system of conductors Transmission - radiates electromagnetic energy
into space Reception - collects electromagnetic energy
from space In two-way communication, the same
antenna can be used for transmission and reception
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3By Ya Bao
Radiation Patterns Radiation pattern
Graphical representation of radiation properties of an antenna
Depicted as two-dimensional cross section Beam width (or half-power beam width)
Measure of directivity of antenna Reception pattern
Receiving antenna’s equivalent to radiation pattern
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4By Ya Bao
Radiation patterns
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5By Ya Bao
Types of Antennas Isotropic antenna (idealized)
Radiates power equally in all directions Dipole antennas
Half-wave dipole antenna (or Hertz antenna) Quarter-wave vertical antenna (or Marconi
antenna) Parabolic Reflective Antenna
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6By Ya Bao
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7By Ya Bao
Antenna Gain Antenna gain
Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)
Effective area Related to physical size and shape of antenna
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8By Ya Bao
Antenna Gain Relationship between antenna gain and effective
area
G = antenna gain Ae = effective area f = carrier frequency c = speed of light (» 3 ´ 108 m/s) = carrier wavelength
2
2
2
44
c
AfAG ee
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9By Ya Bao
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10By Ya Bao
Propagation Modes Ground-wave propagation Sky-wave propagation Line-of-sight propagation
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11By Ya Bao
Ground Wave Propagation
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12By Ya Bao
Sky Wave Propagation
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13By Ya Bao
Line-of-Sight Propagation
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14By Ya Bao
Line-of-Sight EquationsEffective, or radio, line of sight
d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction, rule of
thumb K = 4/3
hd 57.3
Maximum distance between two antennas for LOS propagation:
2157.3 hhd
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15By Ya Bao
LOS Wireless Transmission Impairments
Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise
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16By Ya Bao
Attenuation Strength of signal falls off with distance over
transmission medium Attenuation factors for unguided media:
Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal
Signal must maintain a level sufficiently higher than noise to be received without error
Attenuation is greater at higher frequencies, causing distortion
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17By Ya Bao
Free Space Loss Free space loss, ideal isotropic antenna
Pt = signal power at transmitting antenna
Pr = signal power at receiving antenna = carrier wavelength d = propagation distance between antennas c = speed of light (» 3 ´ 10 8 m/s)
where d and are in the same units (e.g., meters)
2
2
2
2 44
c
fdd
P
P
r
t
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18By Ya Bao
Free Space Loss Free space loss equation can be recast:
d
P
PL
r
tdB
4log20log10
dB 98.21log20log20 d
dB 56.147log20log204
log20
df
c
fd
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19By Ya Bao
Free Space Loss Free space loss accounting for gain of other
antennas can be recast as
rtdB AAdL log10log20log20
dB54.169log10log20log20 rt AAdf
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20By Ya Bao
Categories of Noise Thermal Noise Intermodulation noise Crosstalk Impulse Noise
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21By Ya Bao
Thermal Noise Amount of thermal noise to be found in a
bandwidth of 1Hz in any device or conductor is:
N0 = noise power density in watts per 1 Hz of bandwidth
k = Boltzmann's constant = 1.3803 10-23 J/K T = temperature, in kelvins (absolute temperature)
W/Hz k0 TN
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22By Ya Bao
Thermal Noise Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz
(in watts):
or, in decibel-watts
TBN k
BTN log10 log 10k log10 BT log10 log 10dBW 6.228
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23By Ya Bao
Noise Terminology Intermodulation noise – occurs if signals with
different frequencies share the same medium Interference caused by a signal produced at a frequency
that is the sum or difference of original frequencies Crosstalk – unwanted coupling between signal
paths Impulse noise – irregular pulses or noise spikes
Short duration and of relatively high amplitude Caused by external electromagnetic disturbances, or
faults and flaws in the communications system
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24By Ya Bao
Expression Eb/N0
Ratio of signal energy per bit to noise power density per Hertz
The bit error rate for digital data is a function of Eb/N0 Given a value for Eb/N0 to achieve a desired error rate,
parameters of this formula can be selected As bit rate R increases, transmitted signal power must
increase to maintain required Eb/N0
TR
S
N
RS
N
Eb
k
/
00
dBWTRSN
EdBW
dBo
b 6.228log10log10
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Multipath Propagation