Polar coordinate systems may be divided generally in two classes: linear
and logarithmic. In the linear coordinate system, the concentric circles are
equally spaced, and are graduated. Such a grid may be used to prepare a
linear plot of the power contained in the signal. For ease of comparison,
the equally spaced concentric circles may be replaced with appropriately
placed circles representing the decibel response, referenced to 0 dB at
the outer edge of the plot. In this kind of plot the minor lobes are
suppressed. Lobes with peaks more than 15 dB or so below the main lobe
disappear because of their small size. This grid enhances plots in which the
antenna has a high directivity and small minor lobes. The voltage of the
signal, rather than the power, can also be plotted on a linear coordinate
system. In this case, too, the directivity is enhanced and the minor lobes
suppressed, but not in the same degree as in the linear power grid.
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In the logarithmic polar coordinate system the concentric grid lines are
spaced periodically according to the logarithm of the voltage in the signal.
Different values may be used for the logarithmic constant of periodicity,
and this choice will have an effect on the appearance of the plotted
patterns. Generally the 0 dB reference for the outer edge of the chart is
used. With this type of grid, lobes that are 30 or 40 dB below the main
lobe are still distinguishable. The spacing between points at 0 dB and at -3
dB is greater than the spacing between -20 dB and -23 dB, which is
greater than the spacing between -50 dB and -53 dB. The spacing thus
correspond to the relative significance of such changes in antenna
performance.