Correct. If you were to create an actual “access point” you will have the FCC and other agencies knocking on your door within minutes.
I would also think the widespread deployment of legit femtocells would provide a decent cover.
But yes, it is generally illegal. (Though this hasn't stopped OpenBTS from being developed)
Should I really read that as you had a legitimate cell radio device, and just attached an unauthorized antenna? Or was the radio itself experimental/unapproved?