Single Stub Tuning Application via Semi-Rigid Coaxial Cables for VHF and UHF Monopole

Antennas on RASAT

Emrah ÖNCÜ Communication Systems Group

TUBITAK Space Technologies Research Institute Ankara, TURKEY

emrah.oncu@uzay.tubitak.gov.tr

Volkan AKAN Communication Systems Group

TUBITAK Space Technologies Research Institute Ankara, TURKEY

volkan.akan@uzay.tubitak.gov.tr

Abstract— For maximum power transfer impedance matching is

an important subject in communication systems. Besides,

minimum complexity is another important concept in order to

obtain cheaper, reliable and feasible matching networks. In this

study, a single stub matching implementation applied while

improving the return loss characteristic of blade antennas used in

VHF/UHF communication system of RASAT is presented.

RASAT will be the first observation satellite designed and

manufactured in Turkey.

Keywords-stub matching; input impedance; RASAT, VHF/UHF

monopole antennas; S-parameters

I. INTRODUCTION In communication systems providing maximum power

transfer is so important that various matching techniques are used for tuning antennas, amplifiers and etc. It is known that impedance matching is sensitive to receiver components like antenna and LNA. Particular matching networks include the following factors [1] : complexity, bandwidth, implementation and adjustability. Minimum complexity is the most preferable since simple designs are generally cheaper, more reliable and less lossy than complex ones. Matching a load to used transmission line is required to operate over a band of frequencies. It is known that broadbanding increases the network complexity. According to type of waveguide or transmission line used in the system, matching network to be implemented can be different. In addition for some RF systems adjustable matching network application is more preferable than the others if there is a variable load in the system.

RASAT will be the first observation satellite designed and manufactured in Turkey. UHF/VHF system will be used as the prime system during the commissioning phase due to its omni directional feature in RASAT. In addition, this system will be back-up for S-Band communication system. In order to obtain near omni-directional coverage, UHF and VHF systems employ blade antennas. Furthermore, blade antennas have well-known benefits of low cost, low chance of shadowing and low mass. In this paper, to improve the return loss characteristic of

blade antennas used in VHF/UHF communication subsystem of RASAT single stub matching implementation via semi-rigid cables is presented. The stub parameters have been calculated analytically and then they are verified by a commercially available RF simulator. Finally, they have been implemented and measured experimentally. The obtained results are agree well with each other.

II. THEORY

A. Analytical Relations Various combinations can be selected for single stub

tuning: shunt-short, shunt-open, series-short or series-open circuit. Open stub tuning generally is not preferred due to radiation possibility resulting EMI problems and also series and parallel stub type depends on transmission line used in the application. Having regard to these design criterions shunt-short circuit stub type has been chosen for tuning VHF/UHF blade antennas on RASAT. In addition implementation of the tuning networks have been realized 50Ω semi-rigid coaxial cables and SMA-tees due to low-weight, ease of use, low volume advantages and suitability for space applications where outgassing is an important subject for used materials on spacecrafts.

BjYin += 1

LY

minVod mZ

oL

Figure 1. Single stub tuning for a complex load.

In Fig.1 single stub matching for a complex load is demonstrated simply. At a point from the load where the voltage on the line is minimum, the reflection coefficient is

This study has been supported by RASAT Satellite Development Project,financed by DPT (State Planning Organization of Turkey) under grantcontract no 2007K120030.

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978-1-4244-7243-7/10/$26.00 ©2010 IEEE

negative real quantity and the input admittance is pure real and given by [2],

SY in =Γ+Γ−

=11 (1)

where S is the standing wave ratio on the line. The distance from the voltage-minimum point to the point where

BjYin += 1 is given by,

⎟⎠⎞

⎜⎝⎛

+−= −

11cos

41

SSd o π

λ (2)

B is the normalized input susceptance in equation (2).

The stub length L0 can be calculated by,

⎟⎟⎠

⎞⎜⎜⎝

⎛−

= −

1tan

41

SSL o π

λ (3)

Let the distance of voltage minimum point from the complex load is Zm . Zm can be obtained by [3],

πβθ −= mr Z2 (4)

where θr is the angle of reflection coefficient and β is the phase constant. Hence the distance between the complex load and the stub is given by,

mo Zdd += (5)

B. Calculation of Stub Parameters For single stub tuning both analytical relations given above

and Smith Chart application can be utilized. As it is well known the Smith Chart solutions are fast-applicable and accurate enough for implementation [1]. However, calculation via analytical approach is more accurate and generally used in computer analysis.

For calculation of stub parameters firstly, input impedances of UHF and VHF monopole antennas are measured and noted as 38.8 - j26.73Ω and 30.9- j81.3Ω, respectively. Secondly, stub lengths and stub distances from the loads are calculated for matching with respect to 50Ω characteristic impedance of transmission line used in the system. Based upon above theory lengths of stubs for UHF and VHF monopole antennas are 123.40 and 32.190 respectively and the calculated distances of the stub locations from UHF and VHF antennas are 77.930 and 46.330 respectively.

III. RESULTS According to calculated stub parameters for UHF monopole

antenna implemented tuning network is indicated in Fig.2. CX2 refers to stub length; CX1 refers to stub position for the defined input impedance.

Figure 2. Modelling of single stub matching circuit

Before matching circuit is applied, the measured S11 of UHF monopole antennas is shown in Fig.3. After tuning simulated and measured S11 parameter is shown in Fig.4. The measured normalized input impedance is also shown in Fig.5. Fig.4 indicates that the simulated and measured values are close to each other as expected for the operating frequency.

400 420 440 460 480 500Frequency (MHz)

-40

-30

-20

-10

0

437.4 MHz-10.52 dB

DB(|S(1,1)|)Before Matching

Figure 3. Measured S11 of UHF monopoles to be used on RASAT before

single stub tuning.

Figure 4. Simulated and measured S11 of UHF monopoles to be used on RASAT after single stub tuning.

250 350 450 550Frequency (MHz)

-80

-60

-40

-20

0

437.4 MHz-24.59 dB

437.4 MHz-56.37 dB

DB(|S(1,1)|)Simulated

DB(|S(1,1)|)Measured

45

0 1.0

1.0

-1.0

10.0

10.0

-10.0

5.0

5.0

-5.0

2.0

2.0

-2.0

3.0

3.0

-3.0

4.0

4.0

-4.0

0.2

0.2

-0.2

0.4

0.4

-0.4

0.6

0.6

-0.6

0.8

0.8

-0.8

Swp Max550MHz

Swp Min250MHz

437.4 MHzr 0.900385x 0.0478662

Z(1,1)Measured

Figure 5. Measured normalized input impedance of UHF blades after single

stub tuning .

Similar stub tuning has been applied for VHF monopole antennas. Before matching applied the measured S11 is presented in Fig.6. At the VHF operating frequency S11 has been measured fairly high.

Figure 6. Measured S11 parameter of VHF blades before single stub tuning.

Figure 7. Measured S11 parameter of VHF blades after single stub tuning.

Figure 8. Measured input impedance of VHF blades after single stub tuning.

The measured results are shown in Fig.6, Fig.7 and Fig.8 after stub tuning implementation. Here the improvement is more effective than UHF monopoles as seen from Fig.7.

Tuning circuits applied to UHF /VHF monopoles are shown in Fig.9 and Fig.10. They are just below the earth facing facet and space facing facet of RASAT. All measurements were realized in anechoic chamber of Information and Telecommunication Technologies Authority of Turkey as shown in Fig.11.

Figure 9. Implementation of single stub tuning for UHF monopoles.

Figure 10. Implementation of single stub tuning for VHF monopoles.

lo-stub length d-stub position from load

d-stub position from load

lo-stub length

46

Figure 11. Flight model of RASAT while being tested in anechoic chamber of

Information and Telecommunication Technologies Authority of Turkey.

IV. CONCLUSION RASAT will be the first observation satellite designed and

manufactured in Turkey and UHF/VHF communication system will be used primarily during the commissioning phase. To obtain near omni-directional coverage, UHF and VHF systems employ blade antennas. In this paper, to improve the return loss characteristic of blade antennas used in VHF/UHF communication system of RASAT single stub matching implementation via semi-rigid cables has been presented. The stub parameters have been calculated analytically and then verified by a commercially available RF simulator. Finally, they have been implemented utilizing 50Ω semi-rigid coaxial cables and SMA-tees and then measured experimentally. The obtained results are agree well with each other.

ACKNOWLEDGMENT The authors would like to thank Information and

Telecommunication Technologies Authority of Turkey for their help while testing flight model of RASAT in their anechoic chamber. The authors also would like to thank Vedat Gün and Mukadder Demirbağ for preparing the matching harnesses of UHF/VHF antennas on RASAT.

REFERENCES [1] David M. Pozar, Microwave Engineering, 3rd ed., John Wiley & Sons,

2005, pp.228. [2] Robert E.Collin, Foundations for Microwave Engineering, 2nd ed. Mc

Graw-Hill, 1992, pp.309-311. [3] David K. Cheng, Field and Wave Electromagnetics, 2nd ed., Addison-

Wesley, 1992, pp.462.

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