Steerable Biconical Antennas with Multi

Steerable Biconical Antennas with Multi-Port
Excitation
Technical University Dresden
Communication Laboratory
Gerald Staats
Abstract
During the past decade, wireless communications systems have experienced a tremendous
and world-wide increase in demand. The requirements on mobility and connectivity can be
expected to push further growth in this sector
in the years to come. At the same time, interactive contents lead to a significant increase of
data rate per connection, a fact seriously challenging the capacity of existing networks. One
promising concept to overcome these limitations
is the usage of antennas with steerable antenna
characteristics.
By means of steerable antennas every cell frequency can be used multiple times, for independent connections into different directions within
the same cell, thus significantly increasing channel capacity. This principle is also referred to as
SDMA (space division multiple access) since all
radio links are mutually isolated in space for the
reason that directive beams are used.
In this work a multimode biconical antenna with
electronic beam steering capabilities in the azimutal plane which is also suited for polarisation
diversity has been investigated.
1 Introduction
The multimode biconical antenna consists of a
biconical horn incorporating a radial waveguide
with a central reflector and a circular configuration of excitation elements, which eliminates the
need of having an external feeding network in order to determine the beam direction. By principle, it offers a 360-degrees steering in the azimutal plane. For synthesis of the desired radiation
pattern, the field distribution at the position of
the excitation elements has been developed into
a series of modes of the radial waveguide (64 for
the prototype case). The fields are transformed
to biconical modes prior to radiation into free
space. It has to be noted, that very large operating bandwidth in excess of an octave has been
achieved with this antenna type.
Based on the results gained during the work on
the multimode biconical antenna several further
design variations could be derived.
One type comprises of the basic configuration
with centre-fed radial guide and biconical horn,
additionally, supplemented by an electronically
controllable slot array. The latter has been positioned at the transition point between the radial and the biconical waveguide. Controlling
the transmission properties of the slots by means
of electronic elements selects the desired beam
characteristics.
Further, a biconical antenna including a parallel
plate Luneburg lens has been presented. Here,
the field distributions at the inner radius of the
biconical antenna, required to obtain a certain
radiation pattern, are determined by a Luneburg
lens surrounded by the excitation elements. By
selecting a feed element the beam direction can
be directly adjusted.
2 Theoretical Background
The further descriptions are primarily focused
on the biconical antenna with central reflector.
The function of this antenna is based on development of field distributions in the radial and
biconical waveguide in series of eigenmodes. Basis therefore is the degeneration of the modes
in circumference direction after Stratton [Str41,
S.399-404]. For the field distribution of the magnetic vector potential for TM-Modes at the excitation elements Equation (1) with Coefficients
following Equation (2) and the standardization
in Equation (3) has been derived. The investigation of the electric vector potential of the TEModes yields similar equations.
1
h



M X
N 

X
i
(1)
(2)
C1 Hm (βρ ρ) + D1 Hm (βρ ρ)





Az,radial (ρ, ϕ, z) =
,
 × [ATMmn,RL cos (mϕ) + BTMmn,RL sin (mϕ)] 

m=0 n=1 




×C30 cos nπ
h (h/2 + z)


Az,Stift (ρS , ϕ, z)






R
R
h/2
2π
1
× cos (mϕ)
dzdϕ,
ATMmn,RL = 1/2
ΛTMmn,RL ϕ=0 z=−h/2 





×C30 cos nπ
h (h/2 + z)


Az,Stift (ρS , ϕ, z)






R
R
h/2
2π
1
×
sin
(mϕ)
BTMmn,RL = 1/2
dzdϕ,
ΛTMmn,RL ϕ=0 z=−h/2 





×C30 cos nπ
h (h/2 + z)
Z
2π
Z
(1)
(2)
h/2
nh
i
nπ
o2
(1)
(2)
C1 Hm
(βρ ρS ) + D1 Hm
(βρ ρS ) cos (mϕ) C30 cos
(h/2 + z)
ρS dzdϕ.
h
ϕ=0 z=−h/2
(3)
After propagation the field distribution at the transition to the biconical waveguide is similary
developed into a series of spherical harmonics, resulting in the magnetic vector potential for TMModes after Equation 4 with Coefficients following Equation 5 and the normalization in Equation
6. Again the investigation of the electric vector potential of the TE-Modes yields similar equations.
h
i


0 Ĥ (1) (βr) + D 0 Ĥ (2) (βr)


C


1 ν(n)
1 ν(n)




M
N


XX
h
i
m
m
Ar,biconical (r, ϑ, ϕ) =
,
(4)
× A2 Pν(n) (cos ϑ) + B2 Pν(n) (− cos ϑ)



m=0 n=1 






× [ATMmn,BL cos (mϕ) + BTMmn,BL sin (mϕ)]
ΛTMmn,RL =
ATMmn,BL =
BTMmn,BL =
R 2π R π
1
1/2
ΛTMmn,BL
ϕ=0 ϑ=0 



R 2π R π
1
1/2
ΛTMmn,BL
Z





2π
ΛTMmn,BL =
ϕ=0





ϕ=0 ϑ=0 



Z
π






ϑ=0 








h
i 
m
m
× A2 Pν(n) (cos ϑ) + B2 Pν(n) (− cos ϑ)
sin ϑdϑdϕ,




× cos (mϕ)

Ar,biconical (rT , ϑ, ϕ)



h
i 
m (cos ϑ) + B P m (− cos ϑ)
× A2 Pν(n)
sin ϑdϑdϕ,
2 ν(n)




× sin (mϕ)
Ar,biconical (rT , ϑ, ϕ)
2




h
i 
m (cos ϑ) + B P m (− cos ϑ)
sin ϑdϑdϕ.
× A2 Pν(n)
2 ν(n)





× cos (mϕ)
h
(5)
i
(1)
(2)
C10 Ĥν(n) (βrT ) + D10 Ĥν(n) (βrT )
Naturally the border conditions for both regions must be fulfilled.
2
(6)
3 Measurements and Results
The measurements have been carried out on the
prototype of the multimode biconical antenna in
Figure 1.
Figure 2: Explosion drawing of the biconical antenna with central reflector.
dBi
90
25
20
15
10
5
0
-5
-10
-15
-20
-25
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
120
60
Vertikal polarisiert,
H-Ebene
Messung
Phys. Optik
Analytisch
150
30
180
0
330
210
240
300
270
(a) TM-modes, H-plane pattern.
90
dBi
Figure 1: Multimode biconical antenna with central reflector at the measurement setup
for the vertical plane.
25
20
15
10
5
0
-5
-10
-15
-20
-25
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
120
150
180
60
Vertikal polarisiert,
E-Ebene
Messung
Phys. Optik
Analytisch
30
0
330
210
The mechanical construction is shown in Figure
2. The antenna consists of a central reflector,
240
300
surrounded by excitation elements, both located
270
in a radial waveguide and a biconical waveguide
with the aperture of the antenna. For feeding
(b) TM-modes, E-plane pattern.
the excitation elements with correct phases delay lines are necessary. In this way a large band- Figure 3: Measurement results for a) H-plane
width was obtained. Most measurements have
and b) E-plane vertical polarization
been carried out with 8 excitation elements fed.
pattern compared with analytical and
In this configuration the vertical polarisation ranumerical computed radiation patdiation pattern after Figure 3(a) was measured
terns. The pattern for horizontal pofor the H-plane and after Figure 3(b) for the Elarization looks similar.
Plane, respectively.
3
30
0
Anpassung
S11 Stift
S11a Stift
S22 Loop
Loops&Stifte
bestückt
-5
25
20
15
10
Meßhorn
Horngain
8 Stifte mit Horn
EE(1)
HE(1)
HH(1)
EH(1)
8 Stifte ohne Horn
EE(2)
HE(2)
HH(2)
EH(2)
nur Stifte bestückt
-10
dBi
dB
5
0
-5
-10
-15
-15
-20
-25
-30
-20
1
2
3
4
5
6
7
8
2
9 10 11 12 13 14 15 16 17 18 19 20
3
4
5
6
7
GHz
8
9
10
11
12
13
14
15
GHz
(a) Input reflection coefficient.
(b) Gain, 8 TM-launchers only.
Figure 4: a) Input reflection |S11a | (shaped rod) in the frequency range f = (1 − 20) GHz and b)
Gain in the frequency range f = (2 − 15) GHz with and without horn respectively.
4 Conclusions
Table 1 shows a short summary of the obtained measurement results.
parameter
Gain
max. gain
side lobes
crosspolarization
V/R-ratio
input reflection
rel. band width
obtained value
∼ 15dBi at frequency f = (4 − 12) GHz vertical polarization,
G=
G∼
= 15dBi at frequency f = (8 − 12) GHz horizontal polarization
Gmax ∼
= 25dBi vertical and horizontal polarization
∼
aSL = 15dB vertical and horizontal polarization
≤ 30dB vertical polarization,
≤ 25dB horizontal polarization
≥ 35dB vertical and horizontal polarization
|S11 | ≤ −10dB at frequency f = (7 − 18) GHz vertical polarization,
|S11 | ≤ −5dB at frequency f = (7 − 12) GHz horizontal polarization
Brel,10dB ∼
= 100% vertical polarization,
∼
Brel,5dB = 40% horizontal polarization
Table 1: Summary of the measurement results of the multimode biconical antenna.
References
[Sta00] Staats, G.: Steuerbare bikonische Antenne mit kreisförmiger Anordnung der Erregerelemente. In: Kleinheubacher Tagung 2001, Kleinheubacher Berichte 2002, Band 45. Darmstadt : T-Systems GmbH, Technologiezentrum, September 2000, S. 81–92
[Sta01] Staats, G.: Steuerbare bikonische Antenne mit zentraler Anregung und steuerbarer
Blende. In: Kleinheubacher Tagung 2001, Kleinheubacher Berichte 2002, Band 45. Darmstadt : T-Systems GmbH, Technologiezentrum, September 2001, S. 74–80
[Str41] Stratton, J. A.: Electromagnetic Theorie. first. New York, London : McGraw-Hill Book
Company, Inc., 1941
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