MICROWAVES AND THE HAVANA SYNDROME
The National Academy of Sciences recently released its conclusions about what sickened dozens of American Embassy diplomats in Cuba, a phenomenon dubbed the Havana Syndrome. Though the panel reached no definitive conclusion, it found pulsed RF (a.k.a. directed microwave energy) was the most likely cause. Panel members could not rule out the possibility that the whole episode was a case of mass hysteria but considered the idea unlikely.
The directed microwave energy theory rests on what’s called the microwave auditory effect. When a human head absorbs a pulse of RF energy, a rise in temperature causes tissue inside the head to expand slightly. The expansion launches a pressure wave that propagates throughout the skull to the inner ear, potentially causing clicking or buzzing sounds. Fortunately, the temperature rise is tiny (microdegrees) and the pressure wave is far too weak to injure tissue unless the microwave power density is huge.
Critics have pooh-poohed the directed energy conclusion, claiming a microwave generator big enough to cause tissue damage would stick out like an NBA center at a jockey convention. Fortunately, there is open research on directed energy effects. So sufficiently interested individuals can do a little research and draw their own conclusions.
One paper in this area published in Frontiers of Neurology points out experimenters were able to kill rats by exposing them to 2.45 GHz microwaves with a field intensity of 1,000 W/cm2. There’s no data in the open literature on the threshold of microwave power that causes human brain damage, but the researchers suggest a minimum intensity of 1 W/cm2 impinging on the human head–using 50 μsec pulses on a 7 kHz repetition rate–might be a good place to start.
It has also been reported that the wife of a member of the Cuban embassy staff once looked outside her home after hearing disturbing sounds and had seen a van speeding away. The implication is that the microwave generator was small enough to fi t in the van. Also, the incident provides a means for making a ballpark estimate of range; we might say 150 ft from the street to the woman’s home would be a reasonable guess.
So here’s how the calculation shakes out: We want to know the minimum output power of a 2.45 GHz microwave generator able to produce at least 1 W/ cm2 inside a house 150 ft away, through at least one wall. (For simplicity, we’ll assume the van was transparent to EM radiation.) We might also assume the transmitter is teamed with a parabolic antenna. A 6-ft-diameter parabolic antenna, which should fi t in a van, can add about 30 dB of gain at 2.45 GHz.
Using these parameters in a back-of-the-envelop calculation will lead you to conclude that the 2.45-GHz transmitter must put out at least 2 MW to get the job done. Today, a 2-MW transmitter in the 2.45-GHz range is about the size of a laundry basket and weighs about 150 lb. So it can certainly fi t in a van, along with a power supply and modulation source, while leaving room for a human operator.
If this scenario really did unfold as we theorize, we doubt the frequency used was smack in the middle of the WiFi band as in our example. (Though perhaps that doesn’t matter in Cuba.) But big microwave generators put in place to hassle diplomats seem to be at least theoretically feasible.
Of course, there is one question this exercise doesn’t answer: Given all the controversy over pulsed microwaves, why not just stick spectrum analyzers in the homes and offices of U.S. diplomats? If this simple solution was mentioned in the NAS report, it certainly isn’t getting any press coverage.
Leland Teschler
Executive Editor
Filed Under: DIGITAL ISSUES • DESIGN WORLD, DIGITAL ISSUES • EE WORLD
