Knowing this, it's not surprising then that the ciphertext also is cyclic over 14 characters when encrypting the same character repeatedly:
AZYXWVUTSRQPOBAZYXWVUTSRQPOBAZYXWVUTSRQPOBAZYXWVUT...
--->
AZYXWVUTSRQPOB
AZYXWVUTSRQPOB
AZYXWVUTSRQPOB
AZYXWVUT...
Eek.