The Smith chart Z nomogram plots impedances as their corresponding reflection coefficients, and can help match an antenna to the impedance of a transmitter. All values are normalised to the resistance assigned to prime centre. A resistance axis runs through the centre, whereas complex reflection coefficients are intersections between resistance circles and reac-
tance arcs, with inductive (+) impedances above and capacitive (-) below the axis.
>> Smith 3.1 software
Advanced user interface integrated network analyser.
A basic antenna matching network consists of an inductance and a capacitance in L configuration. If the inductance is in series and the capacitance in parallel with the antenna, the network also acts as a low-pass filter. The impedance transformation ratio is limited by circuit losses, primarily the Q of the inductor coil. Several L sections may be cascaded to achieve a higher impedance transformation ratio or greater bandwidth with less reflected sideband power.
>> L network matching
L network matching using an interactive Smith chart applet.
If the matching network and feeder correspond to an integer multiple of quarter wavelengths on the transmitter frequency, the impedance deviations will be symmetrical around the centre of the Smith chart, and should present a zero degree cusp. If the phase is not as required, a reactance added in series with the antenna to form a T section with the matching network can rotate the cusp, with additional compensation in the matching network for the change in load.
>> Antenna case studies
L network with serial inductance for symmetry.
The maximum reflection coefficient for a DRM transmit-
ter can be drawn on the Smith chart as a circle around prime center. Even with a symmet-
rical match, the limit will often be exceeded on existing antennas for medium and long waves. Therefore total system Q must be reduced by slope correction or loading, or the matching network sideband return be absorbed in the same impedance across a complementary serial circuit with identical Q.
>> Evaluating bandwidth