Basic Password Generator - How to get different rand(number) when is called

The output now is HHHHHHHHHHHHHHHHHHHH. What I would like the program to do is randomNum to have a different value each time it goes through the loop. so it would be like AacCEe... And goes on.

#include <stdio.h>
#include <iostream>
#include <cstdlib>
#include <time.h>


using namespace std;

int main(){

    srand(time(0));

    cout << "You'r PW is: \t" << endl;
    char abc [] {'A', 'a', 'B' ,'b', 'C', 'c', 'D', 'd', 'E', 'e', 'F', 'f', 'G', 'g', 'H', 'h', 'I', 'i', 'J', 'j', 'K', 'k'};
    int randomNum =  rand() % 22;

    for(int i = 0; i <20; i++){
        cout << abc[randomNum];
    }
}