JOURNAL OF TELECOMMUNICATIONS, VOLUME 4, ISSUE 1, AUGUST 2010 31
A Simple and Efficient Microstrip Diplexer with Stubs M. Khalaj-Amirhosseini and S. A. Akbarzadeh-Jahromi Abstract— In this paper, a new microstrip structure for diplexers is proposed. The proposed diplexer has only four stubs and two transmission lines with the same characteristic impedances while it has a good transmission performances and high isolation between the output ports. A microstrip diplexer is designed and fabricated with 1.0/2.0 GHz response. Measured results of the fabricated diplexer have a good agreement with the calculated results. Index Terms— Diplexers; Stubs; Microstrip Transmission Lines.
iplexers are three port devices which are commonly used behind wide-band or multi-frequency antennas in transceiver applications, take normally two frequencies into their input port and separate them to two output ports. They are commonly used. Several structures have been proposed for diplexers such as microstrip diplexers used bandpass and bandstop filters [1-3], diplexer used two band-pass filters , diplexers based on the slow-wave open-loop resonator with high impedance meander line , coupled folded-line resonators , and cross-coupled stepped impedance resonators (SIRs)  and microstrip diplexer combined with only three simple coupled resonators . In this article, a new structure is proposed for diplexers. The proposed structure contains only two transmission lines and four stubs with the same characteristic impedances. Despite the simplicity of the proposed diplexer, a good transmission performances and high isolation between the output ports are achieved. The proposed diplexer is introduced and analyzed and then three ones are designed. Finally, the usefulness of the proposed diplexer is verified by designing, fabrication and measurement of a microstrip diplexer at frequencies 1.0 and 2.0 GHz.
2 ANALYSIS OF DIPLEXER Fig. 1 depicts the proposed microstrip diplexer consisting of two transmission lines, two open circuit stubs and two short circuit stubs, whose characteristic impedances are the same as Z0. For the sake of small size, four stubs can be bended of course. In this figure, λg1 and λg2 are the wavelength of the microstrip medium at two working frequencies f1 and f2, whenever f2 is assumed greater than f1 and also less than 3f1 to assure the length of the lower ————————————————
• M. Khalaj-Amirhosseini is with Iran University of Science and Technology, Tehran, Iran. • S. A. Akbarzadeh-Jahromi is with Iran University of Science and Technology.
short circuit stub to be positive. The input signal at Port 1 is composed of two frequencies f1 and f2. The output signal at Port 2 has to have only the frequency f1 component of the input signal as well as the output signal at Port 3 has to have only the frequency f2 component of the input signal. Here, the behavior of the proposed diplexer is explained qualitatively. The pair of short and open circuit stubs at the upper branch of the proposed diplexer yield infinite and zero impedance, respectively, at frequencies f1 and f2. On the other hand, the pair of short and open circuit stubs at the lower branch of the proposed diplexer yield infinite and zero impedance, respectively, at frequencies f2 and f1. Therefore, the proposed diplexer is reduced to the circuits shown in Fig. 2 at frequencies f1 and f2 and also their odd multiplications. From Fig. 2, it is simply investigable that the input port is impedance matched as well as the isolation between two output ports is infinite at both frequencies f1 and f2. Also, the signal of frequencies f1 and f2 are separately delivered to Ports 2 and 3, respectively. Therefore, it is understandable that the proposed simple structure shown in Fig. 1 can be an ideal diplexer for frequencies f1 and f2 and their odd multiplications. Moreover, the proposed structure can be used as a de-diplexer (combiner) because its output ports are matched at their corresponding frequencies f1 and f2. One can determine the scattering parameters of the proposed diplexer at arbitrary frequency f. The scattering parameters of the diplexer can be obtained using some circuit relations as the followings
Z in − Z 0 Z in + Z 0
S 21 = cos(θ 2 ) − S 31 = cos(θ1 ) −
2 Z in Z0 sin(θ 2 ) Z in1 Z in + Z 0
2Z in Z0 sin(θ1 ) Z in 2 Z in + Z 0
Z S 23 = S 32 = cos(θ1 ) − j 0 sin(θ1 ) Z in 2 2Z 2 Z × cos(θ 2 ) − j 0 sin(θ 2 ) ′ Z2 Z2 + Z0
−1 4 f1
λg 2 f 2
λg 2 4
f1 , f 2
λ g1 λ g1
λ g 1 f1
3 −1 4 f2
f2 Figure 1. The proposed microstrip diplexer
2 f1 π f1 ϕ 21 = − 2 f2
ϕ 31 = −
where Zin is the impedance of the Port 1 and also Zin1 and Zin2 are the impedances of the upper and lower halfcircuits, respectively. Furthermore, Z2 and Z 2′ are the impedance of the Port 2 with and without considering two upper stubs, respectively. Moreover, the θ1 and θ2 are the following electrical lengths at frequency f.
Finally, it can be deduced from the circuits shown in Fig. 2 that the phase of transmission scattered parameters will be as follows
3 EXAMPLES AND RESULTS In this section, three diplexer are designed at frequencies f1 = 1.0 GHz and f2 = 1.5, 2.0 and 2.5 GHz on a substrate of dielectric constant εr = 3.5 and thickness h = 30 mil = 0.762 mm considering Z0 = 50 Ω. The width of all lines is determined w = 2.23 h = 1.70 mm and also λg1 = 181.0 mm. Figs. 3-5 illustrate the scattering parameters of designed diplexers, which confirm the validation and efficiency of the proposed diplexer. It is investigable that to reduce the bandwidth at two frequencies, one can increase the characteristic impedances of the stubs. The second diplexer designed at frequencies f1 = 1.0 GHz and f2 = 2.0 GHz (λg2 = 90.5 mm) was fabricated and measured. Fig. 6 shows the photo of the fabricated diplexer, whose size is reasonable. Fig. 7 illustrates the measured scattering parameters of the fabricated diplexer. From Figs. 4 and 7, one sees an excellent agreement between the results of theory and measurement. The insertion loss of the fabricated diplexer is 0.19 dB and 0.25 dB, respectively at frequencies 1.0 and 2.0 GHz, which is due to the losses of substrate, conducting strips and connectors. Also, the isolation between two ports is 27 dB and 23 dB, respectively at frequencies 1.0 and 2.0 GHz. Finally, Fig. 8 shows the measured reflection parameters of the output ports, which proves the reflectionless property of the output ports at their corresponding frequencies.
Figure 2. The equivalent circuits of diplexer at frequencies f1 (left) and f2 (right)
Figure 3. The scattering parameters of diplexer designed at frequencies f1 = 1.0 GHz and f2 = 1.5 GHz
Figure 4. The scattering parameters of diplexer designed at frequencies f1 = 1.0 GHz and f2 = 2.0 GHz
Figure 5. The scattering parameters of diplexer designed at frequencies f1 = 1.0 GHz and f2 = 2.5 GHz
Figure 7. The measured scattering parameters of diplexer designed at frequencies f1 = 1.0 GHz and f2 = 2.0 GHz
Figure 8. The measured reflection parameters of diplexer designed at frequencies f1 = 1.0 GHz and f2 = 2.0 GHz
4 CONCLUSION A new microstrip diplexer using four stubs connected to two transmission lines was proposed. It was deduced that despite the simplicity of the proposed diplexer, a good transmission performances and high isolation between the output ports are achieved. Three diplexers were designed and one of them was fabricated and measured for frequencies 1.0 and 2.0 GHz. An excellent agreement between the results of theory and measurement is seen. The insertion loss of the fabricated diplexer is 0.19 dB and 0.25 dB and the isolation between two ports is 27 dB and 23 dB, respectively at frequencies 1.0 and 2.0 GHz. The proposed diplexer can be used as de-diplexer, also. Figure 6. The photo of the fabricated diplexer designed at frequencies f1 = 1.0 GHz and f2 = 2.0 GHz
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Mohammad Khalaj Amirhosseini was born in Tehran, Iran in 1969. He received his B.Sc, M.Sc and Ph.D. degrees from Iran University of Science and Technology (IUST) in 1992, 1994 and 1998 respectively, all in Electrical Engineering. He is currently an Associate Professor at College of Electrical Engineering of IUST. His scientific fields of interest are electromagnetic direct and inverse problems including microwaves, antennas and electromagnetic compatibility. Seyed-Abbas Akbarzadeh was born in Jahrom, Iran, on April 13, 1984. He received the B.S. and M.S. degrees (with honors) in electrical engineering from the University of Iran University of Science and Technology “IUST”, Tehran, Iran, in 2007. His research interests include of RF/microwave passive structures and antennas, and circuit components in presence of complex materials.