Nowadays, the balanced circuits and antenna feeding systems are widely used to reduce environment noise, avoid pattern degradation, and ease connection with other balanced components. As a result, the balun plays an important role in converting signals between unbalanced and balanced structures and vice versa. The classical form of balun is a transmission-line transformer with electromagnetic coupling [1][2][3][4], named as Marchand balun. In this case, the perfect matching at each port and perfect isolation between two output ports can be achieved. In addition to baluns with microstrip line, a lot of other types of baluns have been proposed, such as substrate integrated waveguide (SIW) structure [5][6][7], low-temperature cofired ceramic (LTCC) structure [8][9], substrate integrated suspended line (SISL) structure [10] and vertical transition [11][12][13].
Each of these balun structures [5][6] consists of a 3 dB SIW power divider and microstrip lines. The microstrip lines are reversely placed on the SIW output ports, results in different electric field orientations. In [7], the longitudinal apertures are located on the same side of the SIW. A pair of fork-type microstrip lines placed in opposite directions are used to obtain the out-of-phase feature.
To further reduce the size, a LTCC balun filter is designed by applying a vertical split ring resonator in [8]. A half-wavelength resonator is utilized, generating the out-of-phase characteristic at the two open ends in order to obtain the balanced outputs in [9]. To further reduce the fabrication cost and insertion loss, a compact self-packaged balun using compensated interdigital capacitor (CIDC) based on substrate integrated suspended line (SISL) platform is presented in [10].
On the other hand, the spatial structure of balun has also been reported. A pair of microstrip lines with opposite directions are used to couple the energy in the cavity through slots to obtain out-of-phase outputs for the balun [11][12]. A wideband vertical transition between grounded coplanar waveguide (GCPW) and parallel-strip (PS) was presented in [13]. Each line of PS is connected to the central conductor and the ground of the GCPW separately to obtain the transition as well as balun. Transitions between two distinct transmission lines involving planar structures are seen in many circuit applications. A hybrid rectangular waveguide (RWG) to GCPW transition with a built-in diplexer is presented in [14]. The planar functional circuit is placed in an area located within the aperture of the
Table I
Dimension of the Proposed Balun
Parameters
|
Value
|
Parameters
|
Value
|
a
|
7.12
|
b
|
3.56
|
c
|
2.5
|
Wo1
|
1.1
|
L1
|
1.4
|
L2
|
0.75
|
W1
|
0.1
|
W2
|
0.6
|
D1
|
1.1
|
D2
|
0.8
|
Dt1
|
0.4
|
Dt2
|
0.4
|
Do1
|
0.4
|
Do2
|
0.74
|
Do3
|
1.6
|
Do4
|
1.3
|
d
|
0.4
|
p
|
0.6
|
Hsiw1
|
0.508
|
t
|
0.035
|
RWG, yielding a very compact design compared to conventional cascaded circuit topologies.
In this letter, a balun integrated in vertical transition from WR28 waveguide to planar circuit is designed and fabricated. The proposed component features a vertical transition between a common RWG port and two balanced microstrip line ports. The shorted probes together with ground and via-holes formed closed loops, which conform to the Faraday’s Law, making the electromagnetic field realize the transition. A pair of L-shape probes are centrosymmetric. The current direction on the probes are same and the opposite position of the output ports gained the balanced ports.