Plotting Transmission Loss with for loop

Hi,

So I am trying to plot the following script but all I am getting is an empty plot with an x-axis going from 2999 to 3000.1.. It should be a sinusoidal wave due to the sine squared at the end of the equation. At least I think so..

Any advice?

Thanks for the help.

Answer 1

[Star Strider]

The ‘f’ variable will be the scalar last value at the end of the loop. What you likely intended was:

s1 = 0.05;
s2 = 0.15;
L = 0.18;
c = 343;
n = 3000;
fv = 1:n;

for f = 1:numel(fv)
    TL(f) = 10*log10(1+.25*(((s2/s1)-(s1/s2))^2)*sin((2*pi*f*L)/c)*sin((2*pi*f*L)/c));
end

plot(fv,TL)

This will produce the plot you anticipated, with ‘fv’ and ‘TL’ now both being vectors. .

[Articat]

I see, thank-you!

So previsouly I had it only as TL being a vector? Or was the key here the "numel" function?

[Star Strider]

My pleasure!

‘TL’ is a vector, however ‘f’ is a scalar in every loop iteration, and plotting a vector against a scalar plots one point only. The plot function plots lines between points, not points themselves, unless you use a marker, so the plot appeared blank.

The key was defining ‘fv’ as a vector, and then using ‘f’ as elements of the vector in the loop. The numel call returned the number of elements in ‘fv’ (similar functions are length and size), so ‘f’ as an index counter also worked as a variable here. A more rigorous approach would be:

for f = 1:numel(fv)
    TL(f) = 10*log10(1+.25*(((s2/s1)-(s1/s2))^2)*sin((2*pi*fv(f)*L)/c)*sin((2*pi*fv(f)*L)/c));
end

that would allow ‘fv’ to be any vector of values, not only integers.