Alternatively, you can subtract the most negative dB value from the rest, then scale the radius grid lines --
Perhaps something like this --
a = 0:0.01:2*pi;
r = 1 + sin(a);
r = mag2db(abs(r));
[rmin,rmax] = bounds(r)
rs = r - rmin;
figure
polarplot(a, rs)
Ax = gca;
rtix = Ax.RTick;
rtixs = rescale(rtix, rmin, rmax);
Ax.RTickLabel = compose('%.1f dB',rtixs);
% get(Ax)
.
