Friday, April 3, 2009

Smith Chart Views

As mentioned in the March 31 posting, I have used the Smith chart to check the calculation of impedance at the transmitter end of the 120' feedline. The starting point was the antenna's feed point impedance that is marked on the chart shown here (click on image to magnify). This calculated impedance is in the neighborhood of a dipole's impedance near resonance. The SWR circle shown is based on connecting with a feedline with characteristic impedance of 450 ohms, which is the reference point at the chart's center. The same results can be seen on a paper copy of the Smith chart when plotting with coordinates normalized to the 450-ohm characteristic impedance.

This second chart shows the location of impedance coordinates at the end of the 120' feedline, which represents 0.24 wavelength toward the generator (transmitter). The blue radial line (spoke) from the center is aligned with the 0.24 wavelength mark on the outer perimeter scale. The intersection of this line with the constant SWR circle gives the approximate impedance at the end of the feedline, as annotated in red. This value agrees, within the margin of positioning the mouse cursor, with the value calculated previously.

This final chart shows the range of impedance values (black dots along orange curve) that the program calculated when sweeping from 1.6 to 2.0 MHz. The 1.6 MHz point is identified with the green spoke line and its values are shown in the lower right-hand corner of the page.

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