TABLE OF CONTENTS Radial stubs are typically found in bias networks of active microstrip circuits such as amplifiers, oscillators, and frequency multipliers. Unlike low impedance uniform microstrip stubs illustrated in Figure 6.99, radial stubs provide a well-defined point for radial wave excitation due to their narrow coupling aperture. As was already pointed out, the range of usable characteristic impedances with uniform line stubs is severely limited. The limitation on low characteristic impedance becomes particularly troublesome at high operating frequencies when the stub width approaches one-half wavelength. Because the main and shunted line represent an asymmetric excitation problem, this would support the emergence of the first higher-order HE 1( H [10]) mode in the stub. As was stated in Section 6.2.2, the operating frequency usually should not exceed one-third of the respective cutoff frequency f c [10] in order to avoid any severe intervention of higher-order modes. At higher frequencies, when the width of the stub becomes a substantial fraction of a wavelength of the stimulating quasi-TEM mode propagating along the main line, the electrical field in the coupling aperture of the stub becomes less uniform (TEM-like). This lowers the launching efficiency of the fundamental mode and allows for growing influence of the first higher-order parasitic mode on the circuit performance. Unlike with radial stubs the "point" of contact is poorly defined at higher frequencies (Figure 6.100). In many cases, this kind of problem can be avoided if radial line stubs are used as shunt elements. Radial...
