ellipsoid with surf command?

obviously there are some complex numbers because of root in this code, but how can we plot ellipsoid using functions in the library?

[x,y] = meshgrid(1:0.5:10,1:20);
z = sqrt((3-x.^2-y.^2)/4);
surf(x,y,z)

[Walter Roberson]

z(imag(z)~=0) = nan;