Harmonic Radar Finds Hidden Electronics

For as long as small, hidden radio transmitters have existed, people have wanted a technology to detect them. One of the more effective ways to find hidden electronics is the nonlinear junction detector, which illuminates the area under investigation with high-frequency radio waves. Any P-N semiconductor junctions in the area will emit radio waves at harmonic frequencies of the original wave, due to their non-linear electronic response. If, however, you suspect that the electronics might be connected to a dangerous device, you’ll want a way to detect them from a distance. One solution is harmonic radar (also known as nonlinear radar), such as this phased-array system, which detects and localizes the harmonic response to a radio wave.

One basic problem is that semiconductor devices are very rarely connected to antennas optimized for the transmission of whatever harmonic you’re looking for, so the amount of electromagnetic radiation they emit is extremely low. To generate a detectable signal, a high-power transmitter and a very high-gain receiver are necessary. Since semiconductor junctions emit stronger lower harmonics, this system transmits in the 3-3.2 GHz range and only receives the 6-6.4 GHz second harmonic; to avoid false positives, the transmitter provides 28.8 decibels of self-generated harmonic suppression. To localize a stronger illumination signal to a particular point, both the transmit and receive channels use beam-steering antenna arrays.

In testing, the system was able to easily detect several cameras, an infrared sensor, a drone, a walkie-talkie, and a touch sensor, all while they were completely unpowered, at a range up to about ten meters. Concealing the devices in a desk drawer increased the ranging error, but only by about ten percent. Even in the worst-case scenario, when the system was detecting multiple devices in the same scene, the ranging error never got worse than about 0.7 meters, and the angular error was never worse than about one degree.

For a refresher on the principles of this technology, check out  nonlinear junction detectors. While the complexity of this system seems to put it beyond the reach of amateurs, some equally impressive homemade radar systems have been produced before.

FCC Recruiting 7 Field Agents – Electronics Engineers

The Federal Communications Commission (FCC) is looking for qualified applicants for Field Agents in seven Enforcement Bureau (EB) offices across the United States: Atlanta, GA; Boston, MA; Chicago, IL; Dallas, TX; New Orleans, LA; New York, NY, and Portland, OR. Incumbents will resolve Radio Frequency (RF) interference, educate users, and enforce regulations. The GS levels for this position have been expanded to GS 7, opening the opportunity for new college graduates. One year of work experience is not required for this position. Closing date is March 2, 2026.

From the FCC posting:

Performs and directs fieldwork in matters of importance to communications involving safety of life and property. Serves as a point of contact for FCC licensees including the US Government in matters of fixed and mobile radio direction-finding and interference resolution. Participates in unique enforcement and engineering projects that have regional or national applications.

Operates and understands all technical equipment typically used in the Field including RF spectrum analyzers, field strength meters, RF Field survey meters, and radio receivers. Maintains contacts with and assists other Federal agencies, foreign counterparts, and local law enforcement organizations concerning interaction and utilization of the radio spectrum for both authorized and unauthorized activities.

Initiates Official Notices of Violation, Warnings, Notices of Apparent Liability for Forfeiture, and other orders to radio operators and licensees, to bring unsatisfactory or violative conditions to their attention as a result of monitoring, investigations and inspections. Independently initiates correspondence or other communications with complainants and radio users concerning the enforcement functions of the office and region.

Participates in regional emergency planning meetings, serves as the local expert in emergency communications restoration, participates in FEMA conducted training exercises and is able to serve on ESF-2 task force and deployments as necessary. Participates and assists in planning of Bureau enforcement and compliance workshops for persons associated with the various industries and radio services the Commission regulates. 

The Salary is $57,736 to $158,322 per year per year depending on qualifications and experience.

See the official, complete announcement for GS-855-7/9/11/12/13 Electronics Engineer (Field Agent) on USAjobs.gov at www.usajobs.gov/job/857694500.

New Farsi Numbers Station Reported on 7910 kHz

A new shortwave numbers station is believed to be broadcasting in Farsi. 

Numbers stations–mysterious broadcasts that read sequences of numbers–have long been associated with intelligence agencies communicating with field operatives using unbreakable one-time pad encryption.

According to the report, this new signal first appeared around the time of the recent military strikes involving Iran and has been heard on 7910 kHz. One listener reported having heard it around 0215 UTC recently.

A new version of the popular VOACAP HF propagation forecasting tool has been released, featuring significant changes to the interface. In addition, there have been some stability and performance upgrades.   (This is the application on which HamClock's VOACAP routine is based.)

VOACAP Online for SWL is a free shortwave propagation prediction service designed specifically for shortwave listeners. Hams and short wave listeners have professional-grade tools previously reserved for broadcasters and engineers. Whether you’re planning your listening schedule or exploring propagation science, VOACAP Online for SWL delivers clarity, precision, and flexibility.

The video below highlights eleven changes that you need to know about. You can find the VOACAP web app at:

https://www.voacap.com/swl/

You can access the latest version of the help manual at:

https://www.voacap.com/2023/documents/VOACAP_Manual.pdf

And if you're a non-ham (or new ham for that matter) wondering what this radio "propagation thing" is all about, check out the following:

Rydberg Atoms Detect Clear Signals From a Handheld Radio

For the first time, a team of US researchers has used sensors containing highly excited Rydberg atoms to detect signals from an ordinary handheld radio. Through a careful approach to demodulating the incoming signals, Noah Schlossberger and colleagues at the National Institute of Standards and Technology (NIST) were able to recover audio encoded in multiple public radio channels, with promising implications for everyday uses in consumer electronics. The research has been published in Physical Review Applied.

In a Rydberg atom, a single electron is excited to an extremely high energy level, pushing it far from its host atom's nucleus. From a distance, these atoms resemble a single electron orbiting a positively charged ion.

When any atom is exposed to an external electric field, the positions of its electrons' energy levels shift through a process called the Stark effect. Yet in a Rydberg atom, the shift becomes far more pronounced, causing particularly striking changes in the spectral patterns produced when the atom is probed by a laser.

Untapped potential

This effect ultimately means that Rydberg atoms are ideally suited as electric-field sensors: a possibility the Rydberg Sensor project's group leader, Christopher Holloway, began to explore in 2009. After embarking on the project, Holloway's team soon realized that the possibilities were far more wide-ranging than they first anticipated.

"One of the more intriguing applications is atom-based receivers, where these Rydberg-atom sensors act like an antenna to detect the signal, and perform the demodulation and down conversion automatically," Holloway describes.

"In principle, these Rydberg receivers could eliminate a lot of the front-end devices and electronics when compared to conventional receivers."

So far, however, the possibilities of these atomic sensors have largely been explored within the confines of the lab—leaving the full scope of their potential real-world applications largely unexplored.