D.A. Abaev, A.B. Danilov, V.I. Tishchenko
The article reports that, for the directional couplers intended for measuring low power in H-waveguide and having stripline with a die-lectric substrate as a secondary channel, limitations on the effective dielectric permittivity values are established for the purpose of diminishing the frequency dependence of the coupling.
H-waveguide and a stripline having a dielectric layer (substrate) are coupled by means of slots in a side-wall of the waveguide that are located equidistantly and inclined at an angle of 45 degrees. The coupling system is symmetrical.
Two designs of the stripline are considered. The first one contains a round in the cross-section inner conductor on the suspended substrate. The second design is characterized by a flat inner conductor on the suspended dielectric substrate. Teflon was choosen as a di-electric material.
Effective dielectric permittivity values in the stripline are put down in the dependence on boundary wavelengths in the waveguide and in the stripline over the operating frequency range with a frequency span ratio equal to 3.
Two cases are considered. The firs variant: values of effective dielectric constants are chosen so that it is possible to exclude practically the influence of phase velocities difference in the transmission lines on the frequency deviation of coupling characteristic. The second variant: the effective dielectric constant has such a value that it is possible to compensate frequency dependences of couplings of coupling slots at the expense of a difference of phase velocities. Thus, a 0.52 dB- deviation of coupling-versus-frequency characteristic of directional coupler is achieved over the operating band with frequency span ratio equal to 3. The own directivity of the system is not decreased.
The calculation of directional couplers performed with the help electro-dynamic modeling system HFSS. The article is meaning for specialists.