The countermeasure available before frequency hopping was to use frequencies outside the enemy’s expected monitoring range, or to change frequencies manually before each engagement. Neither approach was reliable. A sophisticated enemy with broadband monitoring equipment could detect any single-frequency transmission quickly, and the time required to change frequencies on both the transmitter and receiver manually was too long to be practical in combat.
The Lamarr-Antheil proposal addressed this by making the frequency change too rapid for a jammer to follow. If the transmitter and receiver hop to a new frequency many times per second, a jammer broadcasting on any single frequency can interfere with only that fraction of a second before the system has already moved on. The jamming signal is there; the useful signal is not. The same principle applies to interception: monitoring the signal requires knowing the frequency sequence in advance, which requires knowing the strip design.
This is what the patent calls a “secret communication system” — secret not because it is classified but because its frequency sequence is unknown to the adversary. The word “secret” in the title is the same word used in “secret ballot”: not necessarily classified, but confidential, known only to those authorized to know it.
The engineering required to jam this system would be, in principle, a broadband jammer that broadcast across all possible frequencies simultaneously at power levels sufficient to drown out the signal on each frequency. Such a jammer is enormously expensive in energy terms and requires equipment that, in 1941, was impractical to put on a submarine or a surface vessel. The frequency-hopping system did not make jamming impossible; it made jamming practically very difficult with available technology.