Try this
x = linspace(20,100000);
y = 20*log(1./(1+(2*(3.14)*(x)*(102*10^-9)*(985.1)).^2).^0.5);
plot(x,y)
Read about element-wise opetators in MATLAB: https://www.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html
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