only first line of output gets printed to my text file

I have a text file which I read in and then I extract the data I require and try sending it to different new text file, but only the first line gets into the new text file.

    import csv
    f = open('C:\\Users\\c\\Documents\\DCX.txt')
    next(f)
    csv_f=csv.reader(f, delimiter='\t')
    for row in csv_f:
        if 'DCX3520E' in row[0] and 'NULL' not in row[6]:
            with open("out.txt","w") as f1:
                dcx = row[0] + " " + row[6]
                aa = dcx[9:]
                print(aa)
                f1.writelines(aa)