S. Mohammad Hassan Javadzadeh, S. Mohammad Saeed Majedi, and Forouhar Farzaneh

Quadrature hybrid couplers are useful in communications systems and measurement applications. Many 3-dB, 90-deg. hybrid directional couplers have been developed for narrowband use but, with growing interest in ultrawideband (UWB) communications, a proposed 90-deg. quadrature hybrid coupler has been designed for use from 1 to 10 GHz. It was realized in broadside stripline technology, with the coupled regions connected by means of a tandem structure. The coupler was modeled and simulated using commercial computer-aided-engineering (CAE) simulation programs, such as the Advanced Design System (ADS) from Agilent Technologies and the High Frequency Structure Simulator (HFSS) from Ansoft, with simulations showing close agreement with measurements. A prototype yielded amplitude unbalance of ±1.5 dB, phase unbalance of 90 ±7 deg., and isolation and return loss of better than 14 dB from 1 to 10 GHz.

In a transverse electromagnetic (TEM) system, two coupled parallel lines achieve the highest degree of coupling when they are one-quarter wavelength (λ/4) of the frequency of interest.¹ Meeting this one-quarter-wavelength requirement for optimum coupling usually results in a relatively narrow operating bandwidth (about 10% to 20%). More wideband coupling can be achieved by cascading several λ/4 coupled sections together. By increasing the number of λ/4 coupled sections, a broadband response with minimal amplitude ripples can be achieved.¹ Crystal and Young have presented detailed design data for this type of device,² generating tabular data that provides the required even-mode impedance for each section to produce an equiripple frequency response.

One of the challenges in designing a broadband coupler is that the coupling required of the central elements in a multi-section configuration is always tighter than that of the overall coupler. Such tight coupling values are difficult or impossible to realize in microstrip.³ To resolve this problem, the authors employed a broadside stripline structure. Another challenge is the presence of discontinuities in the coupling coefficient function, corresponding to discontinuities in the dimensions of the coupler’s stripline transmission lines.⁴ This problem can be mitigated by tapering the stripline transmission lines to minimize discontinuities.

A number of reports have been published in recent years presenting new structures intended to increase bandwidth or decrease coupler size.^5-10 The present report will attempt to optimize broadband coupler performance by referring first to tables of coupler dimensions as starting points.² Following that, the LineCalc tool within the ADS software suite is used to obtain the dimensions for each coupler section. Then, the broadside stripline coupled-line elements in ADS are used to optimize these dimensions. The performance of the coupler design is simulated using the Momentum electromagnetic (EM) simulator within the ADS software suite. Finally, a full three-dimensional (3D) analysis is performed using HFSS.

This broadband coupler design is based on the interconnection of two symmetrical multisection couplers with a crossover in tandem. In this configuration, the coupled and direct ports of one multisection coupler must be connected to the isolated and input ports of another multisection coupler.¹¹ The broadside stripline structure, employing a crossover in each coupler, allows the coupled and direct outputs of each coupler to be fabricated on one side of the circuit, while the structure without the crossover and the coupled and direct ports are not fabricated on one side (Fig. 1 and Fig. 2).

As Fig. 3 shows, a 3-dB coupler can be realized by using two 8.34-dB couplers connected in tandem. In this configuration, 3 dB = 20log[sin(π/4)] and 8.34 dB = 20log[sin(π/8)]. Parameters C₁, C₂, and C_N are the coupling coefficients of each individual quarter-wavelength coupler stage, given by Eq. 1:

where Z_oe = even-mode impedance of each coupled line; Z_oo = odd-mode impedance of each coupled line; and Z₀ = 50 Ω.

Stripline was chosen as the transmission-line medium for the multisection coupler. The broadside coupling approach was also selected to realize the cascaded couplers (Fig. 4).

For a broadside stripline structure with δ = 0, the even-order and odd-order impedances are calculated by means of the equalities in Eq. 2¹²:

where: η₀ = the characteristic impedance of free space, which is equal to 120π.