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This paper presents a dual band Band Pass Filter (BPF) operating at both the downlink and uplink frequency bands for Ku-band satellite applications. The commonly used frequency band in mobile communications satellites is the Ku-band. These mobile satellite systems help connect remote regions, vehicles, ships, people and aircraft to other parts of the world and/or other mobile or stationary communications units, in addition to serving as navigation systems. The structure of the proposed filter is based on parallel coupled microstrip lines and four sections are used. Tuning the two operational bands can be achieved using two open-circuited stubs at the first and last sections of the parallel coupled microstrip lines. The proposed filter is adjusted to operate at 12.54 GHz and 14.14 GHz for downlink and uplink bands, respectively. The proposed dual band BPF is fabricated, measured, and good agreement is obtained between simulated and measured results.

In modern wireless communications, there is a great need for dual band operation and consequently dual band filters are required. Ku-band satellite transmitters and receivers usually need dual band Band Pass Filter (BPF) at downlink and uplink frequency bands to properly receive intended signals and weed out unwanted interfering ones in order to ensure communication does not get mixed up [

There are several approaches reported in the literature to design an integrated dual band Band Pass Filter (BPF) [

Different BPFs have been proposed for Ku-Band applications. A compact BPF for Ku-band applications has been proposed in [

In this paper, a dual band BPF operating at both the downlink and uplink frequency bands for Ku-band satellite applications is presented. The filter structure is based on parallel coupled lines. The dual band operation is generated by loading the first and last sections with open circuited stubs. Good return loss, compact size, good insertion loss, and high in-between-band isolation can be achieved. The paper is organized as follows. Section II describes the analysis and design of the parallel coupled line BPF. Section III is devoted to results of the proposed filter. The paper is concluded in Section IV.

The design of the presented band pass filter (BPF) is based on parallel coupled line half-wavelength resonator. The order of the parallel coupled line filter is 3. The designed prototype is fabricated on Rogers RT6010 substrate with dielectric constant ε_{r} = 10.2, thickness (h) of 1.9 mm, and loss tangent (tanδ = 0.0023). A three-pole (n = 3) filter was selected for our design. The design processes are explained in three steps as follows.

Starting from the conventional shape of the microstrip parallel coupled line BPF, a single band BPF can be simply designed. The amplitude ripple in the passband is selected to be 0.5 dB and the order of the filter is 3. This means that it is a three-pole filter. Designing such coupled line filter is achieved by transforming the cascaded networks by its equivalent RLC circuit using transmission line theory. The following design equations have to be considered [

β = ln [ coth ( G r 17.37 ) ] (1)

where G_{r} is the ripple in the passband.

γ = sinh ( β 2 n ) (2)

a k = sin [ ( 2 k − 1 ) π 2 n ] , k = 1 , 2 , 3 , ⋯ , n (3)

b k = γ 2 + sin 2 [ k π n ] , k = 1 , 2 , 3 , ⋯ , n (4)

g 0 = 1 , g 1 = 2 a 1 γ (5)

In our case, and at n = 3, β = 3.548, γ = 0.62643. So, a_{1} = a_{3} = 0.5, a_{2} = a_{4} = 1. Then b_{1} = b_{2} = 1.142, b_{3} = 1.3924.

g k = 4 a k − 1 a k b k − 1 g k − 1 , k = 2 , 3 , 4 , ⋯ , n (6)

g m + 1 = 1 if n is odd (7)

g m + 1 = coth 2 [ β 4 ] if n is even (8)

From the mentioned equations, the normalized lowpass Chebyshev filter elements with ripple 0.5 dB are calculated and the values are as follow:

g 1 = g 3 = 1.5963 , g 2 = 1.0967 , g 4 = 1

In addition, the center frequency and the fractional bandwidth are calculated as follow:

w 0 = w 1 w 2 and fractional BW ( Δ ) = ( w 2 − w 1 ) / w 0

where w 0 = f c , w 1 = f c l , and w 2 = f c h .

In order to design the parallel coupled line filter, the following equations are used:

For the 1st coupling structure:

Z 0 J 1 = π Δ 2 g 1 (9)

For the intermediate structure:

Z 0 J n = π Δ 2 g n g n − 1 (10)

For the final coupling structure:

Z 0 J n + 1 = π Δ 2 g n g n + 1 (11)

where Z_{0} is the characteristic impedance, and J is the inverter of admittance.

The characteristic impedance of the parallel coupled lines is divided into odd impedance (Z_{0o}) and even impedance (Z_{0e}), which can be calculated as:

( Z 0 o ) j , j + 1 = 1 Y 0 [ 1 − J j , j + 1 Y 0 + ( J j , j + 1 Y 0 ) 2 ] (12)

( Z 0 e ) j , j + 1 = 1 Y 0 [ 1 + J j , j + 1 Y 0 + ( J j , j + 1 Y 0 ) 2 ] (13)

According to the Ku-band satellite applications, the frequency band from 11.7 GHz to 12.2 GHz are assigned for downlink fixed satellite service (FSS) and frequencies from 12.2 GHz to 12.7 GHz are assigned for downlink broadcasting satellite services (BSS). In addition, the frequencies from 14 GHz to 14.5 GHz are assigned for uplink FSS [

In order to operate such presented filter at a single band, for example at 12.54 GHz downlink frequency, the calculated values of the odd and even impedances are given in

The next step is to calculate the parallel coupled line dimensions for the four sections according to the calculated even and odd impedance values. These dimensions have been optimized using the Advanced Design System (ADS) electromagnetic momentum simulator, ver. 2019. The substrate material specifications have been defined correctly to the ADS simulator (ε_{r} = 10.2, h = 1.9 mm, and tanδ = 0.0023) and the optimized dimensions are given in

The ADS structure for these parallel coupled line sections that constitute the downlink BPF is shown in

J | J_{j}_{,j+1}/Y_{0} | (Z_{0e})_{j}_{,j+1} (Ω) | (Z_{0o})_{j}_{,j+1} (Ω) |
---|---|---|---|

0 | 0.22938 | 64.0997 | 41.1615 |

1 | 0.12696 | 57.1531 | 44.457 |

2 | 0.12696 | 57.1531 | 44.457 |

3 | 0.22938 | 64.0997 | 41.1615 |

Stage | W (mm) | S (mm) | L (mm) |
---|---|---|---|

1 | 2.07566 | 1.19907 | 2.0803374 |

2 | 2.28765 | 2.27238 | 2.02678 |

3 | 2.28765 | 2.27238 | 2.02678 |

4 | 2.07566 | 1.19907 | 2.0803374 |

with the feed transmission lines, which have 50 Ω impedance. The transmission lines are constructed from the same substrate material (RT/Duroid 6010).

On the same way, the calculated values of the odd and even impedances at 14.14 GHz uplink frequency are given in

The optimized dimensions of the four sections of the parallel coupled line are given in

The ADS structure for these parallel coupled line sections that constitute the uplink BPF is shown in

Following the same procedure, a wide band BPF is proposed with a 3-dB cut off frequencies, f_{CL} = 12.4 GHz and f_{CH} = 14.8 GHz. The values of the odd and even impedances for the proposed wide band BPF have been calculated and presented in

J | J_{j}_{,j+1}/Y_{0} | (Z_{0e})_{j}_{,j+1} (Ω) | (Z_{0o})_{j}_{,j+1} (Ω) |
---|---|---|---|

0 | 0.2602 | 66.3801 | 40.381 |

1 | 0.1623 | 59.492 | 43.172 |

2 | 0.1623 | 59.492 | 43.172 |

3 | 0.2602 | 66.3801 | 40.381 |

Stage | W (mm) | S (mm) | L (mm) |
---|---|---|---|

1 | 2.10135 | 0.845421 | 1.8149076 |

2 | 1.46138 | 1.46138 | 1.80468 |

3 | 1.46138 | 1.46138 | 1.80468 |

4 | 2.10135 | 0.845421 | 1.8149076 |

J | J_{j}_{,j+1}/Y_{0} | (Z_{0e})_{j}_{,j+1} (Ω) | (Z_{0o})_{j}_{,j+1} (Ω) |
---|---|---|---|

0 | 0.4672 | 84.2806 | 37.5535 |

1 | 0.2634 | 66.6405 | 40.2984 |

2 | 0.2634 | 66.6405 | 40.2984 |

3 | 0.4672 | 84.2806 | 37.5535 |

The optimized dimensions of the four sections of the proposed wide band parallel coupled line BPF are given in

A layout for such filter has been generated using ADS for preparing to the fabrication process. The proposed filter layout is shown in

14.8 GHz. The achieved measured insertion loss is 0.71 dB and 0.78 dB at 12.5 GHz and 14.6 GHz, respectively, and remains better than 3 dB across the mentioned band. A good agreement is found between simulated and measured results.

Stage | W (mm) | S (mm) | L (mm) |
---|---|---|---|

1 | 0.743 | 0.718 | 1.946 |

2 | 1.8 | 0.773 | 1.879 |

3 | 1.984 | 0.46 | 1.996 |

4 | 1.085 | 0.325 | 2.126 |

The simulated S-parameters are centered at 13.6 GHz while the measure results are shifted slightly and centered at 13.45 GHz. Any small difference between simulated and measured results may come from standard manufacture impacts and the copper surface roughness.

In this part, a dual band BPF can be generated by adding lumped capacitors to the coupled line sections.

Z L = 1 j w c , where w = 2 π f (14)

The initial values of the lumped capacitors have been calculated using equation (14) and optimized using ADS simulator in order to design a dual band BPF at 12.54 GHz and 14.14 GHz for downlink and uplink frequencies for Ku-band satellite applications. The optimization goals are directed toward improving the

selectivity and the stop band rejection ratio. The ADS structure for such dual band BPF is shown in _{1} = 2.1 pF and C_{2} = 0.19 pF.

In this part, the lumped capacitors are replaced with the open circuited transmission lines. The lumped elements are not preferred due to their parasitics above 1 GHz and the limited available commercial values of these inductors and capacitors. Therefore, a dual band BPF has been proposed using distributed elements without degradation in the performance compared to lumped ones.

The impedances of the two capacitors C_{1} and C_{2} are calculated and their equivalent Z_{in}_{1} and Z_{in}_{2} for the two open stubs transmission lines using the normal transmission line impedance equation [_{0}cot(θ). For θ = 45˚, Z_{in} equals –jZ_{0}. This is equivalent to λ_{g}/8 transmission line.

The initial values of the two open stubs are calculated using this procedure, then they are optimized using ADS simulator to achieve acceptable performance compared to the lumped capacitors. The optimization limits are imposed in order to obtain realizable and acceptable dimensions for the open stubs and also the coupled line sections. The ADS structure for such dual band BPF using open stubs instead of lumped capacitors is shown in

A layout for the proposed dual band parallel coupled line BPF loaded by transmission line has been generated using ADS for preparing to the fabrication process. The proposed filter layout is shown in

Both measured and simulated results of the proposed dual band BPF are illustrated in

This paper presented a dual band BPF operating at both the downlink and uplink frequency bands for Ku-band satellite applications. The proposed filter was adjusted to operate at 12.54 GHz and 14.14 GHz for downlink and uplink bands, respectively. The proposed dual band BPF was fabricated, measured, and good agreement is obtained between simulated and measured results. The filter dimensions are 25 × 20 × 1.9 mm^{3}. The measured results achieved 70 MHz bandwidth centered at 12.53 GHz (12.49 - 12.56 GHz) for the downlink and 50 MHz bandwidth centered at 14.15 GHz (14.14 - 14.19 GHz) for the uplink. The proposed

dual band BPF is suitable to be used inside recent microwave satellite receivers used in Ku-band mobile communications satellite transmitters and receivers. In order to complete the RF front end receiver, designing dual band antenna and wideband low noise amplifier can be considered interesting future research studies.

The authors declare no conflicts of interest regarding the publication of this paper.

Amar, A.S.I., Dardeer, O.M.A. and Zekry, A.A. (2019) Dual Band BPF Based on Parallel Coupled Lines Loaded by Open Stubs for Ku-Band Satellite Applications. Journal of Electromagnetic Analysis and Applications, 11, 148-160. https://doi.org/10.4236/jemaa.2019.119010