FFT faulty results for high frequencies !

Hello all, I have sampled the 50Hz output current of an inverter with sampling frequency of 50 KHz for 1 minute. I am supposed to divide the time to 200ms packages(0.2s or 10 periods of the main signal)and do the FFT on each package. so it means that I have 10000 samples in each package (if not I zero pad or truncate ,that does not make a big difference). and I am supposed to extract the frequency spectrum up to 9 KHz. results are ok for low frequencies ,but I have wrong results (the values are the half of what for high frequency). could you help me to understand what am I doing wrong?!

may be some thing like the code below is hapenning to my fft. just change SF to 10000 and look how the results will be changed !

    in this code if you change the SF(sampling frequency) from 30000 to 10000, 
    the results for high frequncies will be distorted and disordered . why ?

    SF = 30000; %sampling frequency

    % signal 
    t = 0:1/SF:1-1/SF; %  sample points
    wave=15*sin(2*pi*1*t)+1*sin(2*pi*123*t)+2*sin(2*pi*203*t)+3*sin(2*pi*223*t)+4*sin(2*pi*331*t)+5*sin(2*pi*2812*t)+6*sin(2*pi*5752*t)+7*sin(2*pi*7993*t);

    wavefft = fft(wave);

    L=floor(size(wave,2)/2)+1; % removing the mirror side of spectrum
    MagSpec = abs(wavefft(1:L))/(SF/2);  %%removing the mirror side of the spectrum 
    and ranging the domain 

    plot(MagSpec);

[Ali Moghadam]

I have an Idea : it might be because of the number of samples. because I have 10000 samples in each package it is like that I have sampled with 10KHz. as FFT need the sampling fequency of at least 2 times of the highest frequency it causes the faulty results ! am I right ? is there still sth affecting my results ?

[Jacob Capra]

You are correct. That is called the Nyquist Condition , and it will affect your results by a factor of 2 in the frequency domain.

Your sampling frequency must be more than twice the highest frequency of the data being sampled.

Answer 1

[Jacob Capra]

I'm not sure if this constitutes an answer, seeing as I also commented about it, but now you may close this I guess.

You are correct, referring to your comment. That is called the Nyquist Condition , and it will affect your results by a factor of 2 in the frequency domain.

Your sampling frequency must be more than twice the highest frequency of the data being sampled.