the source of income is the situs of the income-producing service.
The repeated use [in the statute] of the words within and
without the United States denotes a concept of some physical
presence, some tangible and visible activity. If income is produced
by the transmission of electromagnetic waves that cover
a radius of several thousand miles, free of control or regulation
by the sender from the moment of generation, the source of that
income is the act of transmission. All of respondent's broadcasting
facilities were situated without the United States. and all of
the services it rendered in connection with its business were performed
in Mexico.