Side grounded conductors dipped in a substrate of a microstrip line, as a tool of line characteristics control

Electrical design of on-board radio-electronic equipment is an important stage in spacecraft design. High charac- teristics of printed circuit boards (PCBs) are essential for miniature units that have reliability, speed, stability of elec- trophysical parameters, electromagnetic compatibility. In order to do that, new design and technological solutions are necessary, in particular transmission lines with stable characteristics of per-unit-length delay (τ) and wave impedance (Z). One of the main lines, realized on a PCB is a microstrip line (MSL). In multi-layer PCBs it is often used with poly- gons. However, their influence on the stability of characteristics is investigated insufficiently. The purpose of the work is to investigate the dependence of τ and Z of MSL on the distance between the side grounded conductors as they are dipped in a substrate. In the TALGAT software we built a geometric model of the line cross-section and calculated (using the method of moments) the matrices (3*3) of per-unit-length coefficients of electrostatic induction taking into account the dielectric as well as ignoring it. We calculated the values for the change of distance between side conductors (s), dipped in a sub- strate, for different values of the height of the side conductors (h1). We revealed that for large values of s (unlike small ones), approaching of the side conductors to the air-substrate boundary does not increase but it decreases the value of τ. When s = 0.38 mm, the change of the value of h1 in the whole range almost doesn't change the values of τ and, therefore zero sensitivity of τ to changes of h1 is possible. Thus we can obtain the required Z value in the range from 48 to 59 Ohms by changing the value of h1. These results are obtained for particular values of the parameters of the line. However it is easy to obtain similar dependencies for other values of parameters. The results can be used to design transmission lines with stable delay un- der control of the impedance value.