I don't think this is workable.
In general, a detector has to be on the same scale as the wavelength. You can make it somewhat smaller, but not, say 1/1000 the wavelength. Broadcast VHF TV uses a wavelength of several feet. UHF gets down below a foot. 3 GHz gets down to about 3 inches.
So at that frequency the sense is more akin to hearing.
At the other end, broadcast A.M. radio a station on 1000 KHz has a 300 meter wave length. To get efficient transmission, a quarter wave dipole antenna has to be 75 meters long. Yes you can make antennas for pickup at this band that are under a meter.
General observation: It's hard to make a sensor that will pick up more than a few octaves of anything. Human ear is good for about 8 octaves. It's just barely an octave for light. Multiband radios switch in different components for different bands. There's some shared circuitry.
Decryption on the fly: You are postulating that the aliens either have built in quantum computers or that they can factor the products of 100 digit prime numbers in their head in real time.
All the Terrans would need to do is change encryption keys faster than the aliens can think. Or use longer keys. Or spread around one time pads. Or broadcast random noise. Or broadcast endless streams of cookie recipes, and only the 7th letter of the 3rd line of instructions is meaningful.
In addition most of the communication right now is NOT on EM, but is on fiber-optic cable.
It's also multiplexed. A cell tower get's its feed either by fiberoptic cable or by microwave relay. The microwave antennas look like snare drums mounted on the mast. They typically are directional to a degree or two, and have a range of 10 to 30 miles. A link may carry many conversations, each encrypted differently, mixed up in some combination of time and frequency multiplexing.
Military communication may use frequency hopping. Not only encrypted, but several hundred times a second, it switches channels. If both radios switch at the same time, no problem.
Another technique for secure communication is to broadcast very low level signals on multiple channels. The receiver adds the signals. The noise on the average cancels out, the signal adds.