Concept and Method of High Efficiency in a Precision Ka-band Subminiature Coaxial Connector

During the late 1950s and early 1960s, 7 mm coaxial transmission lines with standard SMA connectors were only used at frequencies between 10 and 12 GHz. In the mid-1960s, the US Department of Commerce established the Joint Industry Research Committee for the Standardization of Miniature Precision Coaxial Connectors (JIRF/SMPCC). The result of that effort yielded a voluntary product standard in 1972.1,2 The air transmission line size was reduced to 3.5 mm to extend the mode-free operation of that line to 36 GHz. With increasing demand for high speed data transmission rates, the operating frequencies of commercial communication systems are currently increasing to achieve wider bandwidth.3 Consequently, the development of microwave products was undertaken in the United States, Europe and Asia. Korea also developed microwave amplifiers, oscillators and mixers. In this article, a Ka-band connector is designed on the basis of transmission line theory and then fabricated. In order to meet the Ka-band operating frequency range, a multi-step impedance technique, utilizing air layers, is applied. Moreover, for commercial purposes, the connector must be able to operate at high temperature in order that the hook structure and step transition satisfy military (MIL) specifications.

Impedance of the Coaxial Line

The common structure of a connector is similar to a coaxial line, as shown in Figure 1. A coaxial line propagates in a TEM mode and is characterized by an inner (a) and outer (b) radius. The primary parameters may be rigorously derived for a coaxial line that is uniformly filled with a dielectric material. The dielectric material has a relative permittivity er with respect to the free space value, e₀ = 8.854 10–12 Fm–1. The coaxial line supports radial electric fields and circumferential magnetic fields, with a longitudinal current flow in the conductors. For the purpose of understanding the impedance characteristics of the air line section of precision coaxial connectors, a lumped constant circuit representation of a low loss coaxial line is a convenient starting point. The general equation for the low loss coaxial line characteristic impedance is3

where
Z0' = characteristic impedance
R= intrinsic resistance (of the conductor) per unit length
G= dielectric conductance per unit length
L= inductance per unit length
C = capacitance per unit length

Considering the special case where R and G are both equal to zero, Equation 1 reduces to

At microwave frequencies, the inductance per unit length is very nearly equal to the external inductance per unit length and the capacitance per unit length can be calculated using2,3

These inductance and capacitance values depend on the ratio of the outer to inner conductor radii of the coaxial cable, b/a. µ is the permeability of the non-magnetic material and ε is the dielectric constant of the insulator. Also, the effective resistance per unit length (R) of the coaxial line is given by4–6 where

Rs = surface resistance of the coaxial line
σ = metal conductivity of the coaxial line
δ = skin depth

For impedance matching to the measurement equipment, the coaxial line should have a characteristic impedance3–6

Design of a Ka-band Narrow Flange SMA Connector

Subminiature coaxial connectors are commonly used in low power applications at higher microwave frequencies, particularly in the Ku-, K- and Ka-bands. In this article, the Ka-band SMA connector is designed using a Teflon-filled 50 Ω coaxial line. The structure of the Ka-band SMA connector is shown in Figure 2. This SMA connector consists of a central conductor, a dielectric and the body.4 The SMA connector is designed to operate in the Ka-band and is capable of providing a much higher operating frequency, free of higher orders, using arbitrary surface discontinuities inside the connector, as shown in Figure 3. In addition, adding an air layer and a step transition to the connector enhances its high frequency performance.

The connector used in the measuring equipment uses an air-layer structure. Although the mechanical strength of the connector is weak, it is better than having a dielectric layer structure at high temperature, since the rigid plastic supporting the center pin is very thin. This problem was solved by using an air layer inside the PTFE dielectric and fixing the center pin. Thus, the narrow flange SMA connector has a good Ka-band frequency performance, due to the multi-air and multi-step layers. Furthermore, for commercial purposes, this connector is designed to have a guaranteed operating temperature up to 120°C. Four mechanical structures were applied in the design of the connector. First, a dual structure having a main body and a cover was used in order to avoid separation of the dielectric from the connector. Second, the hook structure in the center conductor is used to prevent separation between the dielectric and the inner conductor. Third, a thick outer conductor wall is used. This also provides great strength to ensure reliability in repeat matings. Finally, a very thin inner pin was used to obtain high frequency performance, which is a result of the high impedance line section.

When the stepped-discontinuity structure is added, the inductance per unit length of the transmission line is decreased and the capacitance per unit length is increased. The higher order cut-off frequency was also increased as a result of the discontinuity. To avoid the problem of having an infinite VSWR, the arbitrary length between T1 and T2 should be as long as possible. The length l of the discontinuity in the transmission line is a very important parameter controlling the VSWR characteristics of the connector, as it is a factor in deciding the discontinuity size for 50 Ω matching.4

Simulation Results

In this article, the degradation in RF performance of the connector is minimized by simulating the multi-air and hooked structure using a 3D simulator from a Computer Simulation Technology (CST) tool based on the finite element method (FEM). The inner conductor, shown in Figure 4, was fabricated with two air layers to enhance the high frequency performance of the connector. Figure 5 shows the inner structure of the Ka-band NFR SMA connector. Generally, when the coaxial line uses an air dielectric, the mechanical strength of the connector is weak. To solve this problem, a PTFE dielectric was used. As shown in Figure 6, the simulated return loss and VSWR increase as the frequency increases, but they are less than –32 and 1.04 dB, respectively, in the frequency range from DC to 30 GHz. Figure 7 shows the simulation results for the microstrip line used in the measurements. The microstrip line was 10 mm long and 1.3 mm wide, made on a Teflon PCB for high frequency performance. Figure 8 shows a cross-sectional view of the test fixture used for the simulation and measurement of the Ka-band SMA connector using a microstrip line. The interface between the inner conductor and the Teflon dielectric is in accordance with MIL-STD-348. Furthermore, its performance is better than for similar RF connectors, matching the military standards MIL-PRF-39012 and MIL-C-83517, thereby enabling it to be treated as a microwave component.

Measurement Results

Figure 9 shows the test fixture used to measure the Ka-band SMA connector. It uses a microstrip line with a width of 1.3 mm and a length of 10 mm. The dimensions of the microstrip line were optimized after testing. The measurement was performed using a network analyzer (HP 8510C). Figure 10 shows the measured VSWR as a function of frequency at room temperature and after being subjected to 120°C for an hour. The VSWR near 30 GHz is 1.19; therefore, the connector is expected to be useful up to 30 GHz. Furthermore, after subjecting the connector to a temperature of 120°C for one hour, the measurement was performed again to qualify it for use as a commercial product. Very little difference in VSWR was recorded between the two tests. Therefore, its RF performance has been shown to be stable with temperature. Figure 11 shows photographs of the test fixture and of the fabricated connector.

Conclusion

In this article, a reliable miniature SMA connector for use at Ka-band was designed and fabricated using the characteristic impedance of a coaxial line, a discontinuous transmission line, and the characteristics of dielectric bead-air line interfaces. The VSWR is less than 1.2 from DC to 30 GHz and was realized by adopting a slit construction and by optimizing the conductor pattern on a PCB. Its reliability is confirmed by electrical and environmental tests. The use of this SMA connector is possible in Ka-band RF module or system applications.

Acknowledgments

This research was supported by the Ministry of Information and Communication (MIC), Korea, under the Information Technology Research Center (ITRC) support program supervised by the Institute of Information Technology Assessment (IITA) (IITA-2005-(C1090-0502-0034)), by a research grant from Kwangwoon University in 2006, by the Realistic 3D-IT Research Program of Kwangwoon University under the National Fund from the Ministry of

Education and Human Resources Development (2005), and has been performed through the Support Project of Mission Telecom Co. (MTC), Dongjin-TI Co. and A&P Technology Co.

References

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A.E. Sanderson, “A New High Precision Method for the Measurement of the VSWR of Coaxial Connectors,” IEEE Transactions on Microwave Theory and Techniques, Vol. 9, No. 6, November 1961, pp. 524–528.

P.A. Rizzi, Microwave Engineering Passive Circuits, Prentice Hall, Upper Saddle River, NJ, 1988.
N.J. Sladek, “Fundamental Considerations in the Design and Application of High Precision Coaxial Connectors,” IRE International Convention Record, Vol. 13, Part 5, March 1965, pp. 182–189.
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Jae-Ho Yoon received his BS degree in electronic materials of engineering from Kwangwoon University, Korea, in 2004. He was an exchange student in the 2nd IT Global Wireless Communication Course at the University of California San Diego (UCSD) from February to August 2004. He is currently pursuing his MS degree. His research interests include RFIC/MMIC harmonic noise frequency filtering VCOs, mixers, LNAs and CDMA wireless systems design.

Won-Yong Cho received his BS degree in electronics engineering from Kwangwoon University, Korea, in 2006. He is pursuing his MS degree and works at the RFIC Center at Kwangwoon University as a 3D RF tool education teaching assistant.

Ah-Rah Koh received her BS degree in electronics engineering from Kwangwoon University, Korea, in 2005. She is currently pursuing her MS degree and works at the RFIC Center at Kwangwoon University as an MMIC tool education teaching assistant. Her research interests include RFIC/MMIC VCOs, LNAs and CDMA wireless system design.

Jae Ho Yoon

Gary P. Kennedy received his BSc degree in applied physics from Liverpool John Moores University in 1988 and his PhD degree in solid-state physics/micro-electronics from the University of Liverpool in 1993. After completing his studies, he worked as a researcher and research fellow at the University of Liverpool and Southampton, respectively. In January 2000, he began working as a reliability engineer at the European Semiconductor Manufacturing Ltd., UK. He is currently a research professor in the ITRC RFIC Center of Kwangwoon University, Korea. His research interests include SiGe RFIC.

Nam-Young Kim received his BS degree in electronic engineering from Kwangwoon University, Korea, in 1987, and his MS and PhD degrees from the State University of New York at Buffalo in 1991 to 1994, respectively. He was a research scientist of CEEM at SUNY, Buffalo, in 1994. Since September 1994, he has been a professor in the department of electronic engineering at Kwangwoon University, Korea. He received M.Div and DCE degrees in 2001 and 2005, respectively. He has been director of the RFIC Education and Research Center since March 1998. His research interests include semiconductor device modeling, ASIC, RFIC and MMIC design.

Bhanu Shrestha received his BS and MS degrees in electronic engineering from Kwangwoon University, Korea, in 1999 and 2004, respectively. His research interests include RFIC/MMIC/ RFID circuits and system design.