%andreas98, assumes r=r80, computes source at r=r98
function dfdr=fdrop_an(u,r)
z=fdrop_sm(u,10);
c1=z*10;
c2=c1*37.5^1.8;
c3=c2*100^5.2;
dfdr=c1./r;
ii=find(r<=10);
dfdr(ii)=z;
ii=find(r>=37.5);
dfdr(ii)=c2./r(ii).^2.8;
ii=find(r>=100);
dfdr(ii)=c3./r(ii).^8;
ro=(r/.518).^(1/.976);
dfdr=3.5*dfdr*.506./ro.^.024;%factor 3.5 to agree with monahan, other factor is 
%dr80/dr98

