Matlab Custom Eq in Curve Fitting Toolbox

I have a data set I'm trying to fit to the equation

However, the resulting plot does not follow the data trend at all.

ft = fittype( '(((a^2)*cosd(x)-sqrt(a^2-(sind(x)).^2))./((a^2)*cosd(x)+sqrt(a^2-(sind(x)).^2))).^2', 'independent', 'x', 'dependent', 'y' );
opts = fitoptions( 'Method', 'NonlinearLeastSquares' );
opts.Lower = 1;
opts.StartPoint = .9;
opts.Upper = 5;

The resulting fit looks like this:

How can I get the fit to accurately follow the data line?

Thanks!