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Hearing Microwaves - It's All in Your Head
Kirt's Cogitations™ #306

RF Cafe University"Factoids," "Kirt's Cogitations," and "Tech Topics Smorgasbord" are all manifestations of my rantings on various subjects relevant (usually) to the overall RF Cafe theme. All may be accessed on these pages:

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Cuba embassy sonic attack timeline (Science magazine) - RF CafeHearing Microwaves in Your Head (Kirt's Cogitation #306) - RF CafeLate in 2016, news outlets began reporting on American and Canadian diplomats stationed at their respective embassies in Cuba complaining of dizziness, nausea, headaches, ringing in the ears, and other seemingly sound-related illnesses (click on timeline at right). Similar reports have come out of China as well. At the time, doctors and scientists investigating the phenomenon thought maybe some sort of weaponized sonic beam was being directed at the personnel. Certain people were affected while others nearby experienced no such phenomena, leading researchers to believe that the presumably sonic beams were highly concentrated and directional. Some of the targeted personal were diagnosed as having suffered mild traumatic brain injury with likely damage to the central nervous system.

Now, headlines are saying that the "sounds" might not have been sonic in nature at all, but could have been caused by microwaves*. I almost couldn't believe it when I read, "In particular, a growing number of analysts cite an eerie phenomenon known as the Frey effect, named after Allan H. Frey, an American scientist. Long ago, he found that microwaves can trick the brain into perceiving what seem to be ordinary sounds." "Zounds," I said to myself, that is exactly what happened to the NCO who was assigned as my trainer in the U.S. Air Force at Robins AFB, Georgia, in the early 1980s. I have told the story many times over the years, but here it is again now that it is very timely and relevant.

Members of our radar shop assembling the MPN-14 (one of two we maintained). In the picture are Chuck Powell, Ron Dames, Don Hicks, Linda(?), Ed Nugent, Allen Coker, Scott Breen(?) and Supreme Commander TSgt. "Red" Worthington. I welcome feedback from anyone that wants to correct my identifications. (circa 1979-82)

AN/MPN-14 Mobile Radar "Landing Approach Central"

Airport Surveillance Radar (ASR)

Frequency: 2780-2820 MHz

PRF: 1100 Hz

PW: 0.7 μs (?)

Peak Power: 750 kW (pk)

Precision Approach Radar (PAR)

Frequency: 9000-9160 MHz

PRF: 5500 Hz

Peak Power: 45 kW (pk)

MIL-HDBK-162A, 15 December 1965

After completing Air Traffic Control Radar Repairman (AFSC 303x1) technical school at Keesler AFB, Mississippi, in the fall of 1979, I headed to Robins AFB as my permanent duty station. The next phase of learning about the equipment was OJT (on-the-job- training). OJT consisted of a combination of hands-on instruction by an experienced radar technician (called a "trainer"), and a self-study curriculum of printed course material known as Career Development Courses (CDC's). CDC's were tailored to the specific type of equipment on which you would be working as well as containing general knowledge.

Along with troubleshooting and repairing problems with the S-band airport surveillance radar (ASR, aka "search"), the X-band precision approach radar (PAR), rapcon (radar approach control) equipment, IFF synthetic radar, VHF and UHF radios, video mappers, and other systems, we were responsible for performing periodic maintenance and alignment procedures to keep everything running per specifications. One of those tasks was verifying that the circular polarization mechanism (used to mitigate rain echoes) in the search radar antenna feedhorn was functioning properly. Doing so involved climbing onto the roof of the radar trailer and removing the weatherproof cover at the end of the feedhorn to measure the positions of the polarizer.

As you might expect, there are safety switches provided to help prevent personnel from being swept off the roof if someone didn't know anyone was up there and, wondering why the antenna wasn't turning, flipped the switch to get it going. A safety kill switch for the antenna was located on the outside of the trailer at the top of the ladder used to access the roof. As my trainer and I ascended the ladder, he explained the switch's purpose and admonished me to always remember to place the switch in the "OFF" position prior to climbing on the roof, which he did. He was a by-the-book guy who took his responsibilities seriously. I duly noted the advice. We set our bag of tools near the antenna and dutifully opened the alignment manual to the circular polarizer section. The next step of my learning about radars was about to begin.

Sgt. Trainer, who was experienced and quite adept at this radar maintenance thing (and a very nice guy), proceeded to demonstrate the proper procedure for removing the feedhorn cover. That went well. Next, after a short verbal tutorial on how the polarizer worked and what he was about to do, he moved his face in front of the feedhorn to see inside, then very quickly jumped back out of the way. "Whoa, did you hear that?," he asked me. "Hear what?" I responded. "That loud noise," said he. After recovering from the surprise, he began work again. Once more he snapped his head away from in front of the feedhorn. "Yow, there it is again," he remarked in an obviously shaken voice. After composing himself, he climbed back down the access ladder and went into the equipment trailer and discovered that the high voltage to the transmitter was turned on, even though a warning sign had been placed near the emergency shutoff switch on the transmitter rack. There was no physical lockout mechanism, and evidently "we" neglected to actually turn the HV off, since nobody entered the trailer while we were working on the antenna. The poor guy worried for a long time whether he would eventually develop brain cancer from it. 750 kW peak, 0.7 μs wide, at a 1100 pps rate is a lot of noggin fry time even for less than a second. I learned a valuable lesson that day, having to this day never "heard" microwaves in my lifetime.

In retrospect, that episode, evidently, was an example of the "Frey effect" which, according to the paper linked to above, was first identified in 1961.

* See New York Times article, "Microwave Weapons Are Prime Suspect in Ills of U.S. Embassy Workers," dated September 2, 2018.


--- I would like to get ahold of the MPN-13 or MPN-14 radar system technical orders (TO's) with schematics and/or periodic maintenance in order to scan and post some of the pages. If you are able to provide copies of even a few of the pages - especially the transmitter and receiver circuit schematics, please contact me at kirtrfc (@) aol (.) com.



Posted September 4, 2018

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