plotting functions but not for return values outside specified range

[n3mmr]

I'd like to do a polar plot of the logarithm of expressions, when the logarithm can attain values down to -infinity.

I want to plot only for those values that do not reach too large negative values.

And I want to use the included polar plot app.

A typical example is

R = 40 + 20*log(0.5+0.5*cos(theta))

where I'd like to plot only those values of R that are greater than 0, I e for thetas that keeps the logarithm part above -30 or so.

(The expression shown is the polar plot of an idealised cardioid microphone, plotted in dB relative to the response at 0 degrees, and is a typical example of the plots I want to do)

Is there any way to specify limits for what values to plot, w/o writing a rather complex function encapsulation?