Er, OK?
Now I am wondering and guessing.
530 Khz is a standard South American AM transmission frequency right?
Good transmission characteristics day or night, yet still in the lower end of a band heavily used for "talk radio" stations, right?
250 yards?
point to point telemetry?

It sounds like an ideal way to measure the way the windows in an office building vibrate.

Check 530 Khz ultrasonic, not radio. No cross talk, very directional.

And yes, some of the IR telemetry was window vibrations. In fact, we had to site acquire using a red beam to before we could switch. You gotta switch because the bright red dot is a giveaway.