12 Antenna Properties - Hong Kong Polytechnic …em/hdem06pdf/12 Antenna Properties.pdfLinear dipole...
Transcript of 12 Antenna Properties - Hong Kong Polytechnic …em/hdem06pdf/12 Antenna Properties.pdfLinear dipole...
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Radiation resistance
Antennas are designed for effective radiation of electromagneticenergy.
– Equivalent circuit of an antenna
– input radiation resistance Rr
• Represents radiated energy
– input loss resistance RL
• Represents conduction and dielectric losses of the antenna
– input reactance XA
• represents the energy stored in the field near the antenna
rR
inI
2
Radiation resistance
The power radiated is equal to:
The power losses is 2
2rin
radRIW =
If Win is the input power, the radiation efficiency is:
Lr
r
in
radr RR
RWW
+==η
2
2Lin
lossRIW =
rR
inI
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Directive gain, directivity and gainStronger in some directions
Isotropic Antenna (the reference antenna)
Same intensity for all directions
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Let Pavg be the average Poynting vector which is the power flow density per unit area,
( )*Re21 HEPavg ×=
The total power radiated Wrad is then
φθθφθ
φθθ
ddddUW
ddRddW
Srad
Srad
sin),(
sin2
=ΩΩ=
=⋅=
∫
∫ SSPavg
where U(θ,φ) is the power flow through a unit solid angle, and is called the radiation intensity (W/sr).
avgPrU 2),( =φθ
Directive gain, directivity and gain
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Directive gain, directivity and gain
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DirectivityMaximum value of the directive gain in a certain direction.
Power GainRatio of the radiation intensity in a given direction to the radiation intensity of a lossless isotropic radiatorthat has the same input power.
πφθφθ4/
),(),(in
p WUG =
Directive gain GD(θ,φ)Ratio of the radiation intensity in a particular direction(θ,φ) to the average radiation intensity.
πφθφθφθ4/
),(),(),(radavg
D WU
UUG ==
Directive gain, directivity and gain
),( φθdGMaxD =
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Example
Find the directive gain of a Hertzian dipole.
( ) φθ HE21*Re
21
=×= HEPavg
I
θ
θE
φH
( ) θβηπ
2222
2
sin32 or
Idl=avgP
( ) θβηπ
222
22 sin
32 oIdlrU == avgP
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Example
and then
( )
θ
πφθθθ
θ
φθφθ
π π
2
2
0 0
2
2
sin23
4/sinsin
sin
),(),(
=
=
=
∫ ∫ dd
UUG
avgD
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Example
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Example
Directive gain and the directivity of the Hertzian dipole
Suppose the radiation efficiency is 46%,
I
θφθ 2sin23),( =DG θ
θ
dB 76.123),2/( ===∴ φπdGD
dB 16.069.046.0 −===∴ DGp
)46.0/( =inrad WW
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Example
Find the radiation resistance of a Hertzian dipole
Suppose, Poor radiator !!
( )
( )
=
=
=
=
∫ ∫
∫ ∫
22
2
22
22
2
0 0
322
22
2
0 0
802
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sin32
sin
λπ
βηπ
φθθβηπ
φθθ
π π
π π
dlI
dlI
dddlI
ddPP
o
o
avgr
2280
=∴λ
π dlRr
Ω=⇒= 08.001.0 rRdlλ
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Linear dipole antenna
In dipole antennas, the current magnitude along the dipole can be represented like in a transmission line, where 2h is the length of the dipole and z=0 at the center feed-point of the dipole. ( )[ ]zhIzI m −= βsin)(
I 2h
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Linear dipole antenna
Knowing the current distribution I(z), we can sum up the fields due to the infinitesimal segments on the antenna using the results of the Hertzian dipole.
I 2h ( )
θβθβθ
θπ
φ
θπ
ηθ
φ
sincoscoscos)(
)(2
ˆ
)(2
ˆ
hhF
FR
ejI
FReIj
jkRm
jkRmo
−=
=
=
−
−
aH
E
F(θ) is called the pattern function
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Linear dipole antenna
Antenna pattern– E-plane pattern (pattern function versus θ for a
constant φ)
– H-plane pattern (pattern function versus φ for a constant θ=π/2)
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Linear dipole antenna
At certain dipole lengths (≅ λ/2, λ…) called resonant lengths, the input impedance is purely resistive. For half-wavelength dipole,
The pattern pattern for a half-wavelength dipole is
Ω=≈ 73rin RZ
( )
θ
θπθ
βθβθ
sin
cos2
cos
sincoscoscos)(
=
−=
hhF
64.1=D
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Example -- Monopole
A thin quarter-wavelength vertical antenna over a conducting ground is excited by a sinusoidal source at its base. Find the (a) radiation pattern, (b) resistance, and (c) directivity.
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Example -- Monopole
(a) The electromagnetic field in the upper half-space due to the quarter-wave vertical antenna is the same as that of the half-wave antenna.
(b) The magnitude of the time-average Poynting vector holds for but the quarter-wave antenna radiates only into the half-space, its total radiated power is only half of a half-wave dipole. Therefore, the radiation resistance is
(c) Same as half-wave dipole
2/0 πθ ≤≤
Ω==−= 5.362/73dipole)/2 wavehalf(rr RR
64.1=D
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Effective Area and Friis Equation
Effective Area
The effective area Ae of a receiving antenna is the ratio of the time-average power received to the time-average power density of the incident wave at the antenna.
It may be shown that is Ae related to the directive gain as:
avgLe PPA /=
),(4
2
φθπλ
De GA =
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Effective Area and Friis Equation
Friis Equation
The time-average power density at the receiving antenna is
Consider two antennae separated by a distance r. The transmitting antenna transmits a total power Pt.
Ae1, GD1, PtAe2, GD2, PL
r
124 Dt
avg Gr
PPπ
=
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Effective Area and Friis Equation
The power received to the load is
(Friis Equation)
( ) tDD
avgD
eavgL
PGGr
PG
APP
212
2
2
2
2
4
4
πλπλ
=
=
( ) 212
2
4 DDt
L GGrP
Pπλ
=∴