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Banana Skins – January 2019 (#142-146)

142.  Intermodulation of two broadcast transmitters interferes with automatic garage doors

We have news from down-under about things that bump and grind in the night…

Australia’s ABC TV and Sydney’s new FM radio station ‘Nova 96.9’ have unwittingly joined forces to meddle with automatic garage doors. VK2WI reports that hundreds of radio-controlled garage doors across Sydney have been overloaded by the ABC and Nova transmissions and some are refusing to open and close. In several cases the doors have developed a life of their own, randomly opening and closing at all hours. The reason is that Nova broadcasts on 96.9MHz and the ABC TV sound signal is on 69.75MHz.

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A Dash of Maxwell’s: A Maxwell’s Equations Primer – Part Two

Maxwell’s Equations are eloquently simple yet excruciatingly complex. Their first statement by James Clerk Maxwell in 1864 heralded the beginning of the age of radio and, one could argue, the age of modern electronics.

When the two signals mix in an overloaded door receiver, the result is a 27.15MHz signal, which passes straight through most door receivers which are tuned to 27.145MHz, only 5kHz different. This causes erratic behaviour, dependent upon signal content, and the doors open and close in sympathy. Perhaps the designer has sleepless nights, too!

(From Graham Eckersall G4HFG / W4HFG 6th July 01, who got it from the ‘News’ section of the RSGB website,

143.  Radio transmitters interfere with photographic cameras

Subject: Bizarre Solution… What you describe is a common problem to commercial-radio technical folk. Proximity to active radio-transmitting antennas can cause really wild things to happen to electronic circuits such as the metering portions of your 645 Pro. (It’s interesting to note that even some very well-designed and expensive electronic test equipment can be rendered essentially useless by these strong radio-frequency fields.)

The wiring within your camera (it may be nothing more than a centimetre or so of printed-circuit land on a fiberglass circuit board) acts as an antenna. The signal it picks up can then be rectified (changed from a radio signal to a small D.C. voltage) and this voltage can, in turn, add to or subtract from the small voltage generated or controlled by the light-sensing elements in your camera metering system. I would guess that if it adds, you get underexposure; if it subtracts, you get overexposure. It will depend upon exactly how the rectification occurs in the camera circuit.

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A parallel case of this type of interference occurs when an automobile with a radio-controlled door lock is located near a high-powered radio or television transmitting tower. Often, it’s impossible to unlock the car using the small, keychain-type device because of the same type of effect. In some cases, a particular program signal will trigger the car alarm! The tops of some of those California hills are loaded with radio transmitters and can contain very strong radio-frequency (RF) fields. It would be my guess that the metal housings of some cameras would shield the internal electronics from RF effects.

Of course, a plastic housing offers little such shielding. At best, even a metal-bodied camera would be subject to “radio interference.” It can be pretty squirrely stuff! Radio signals and light are both forms of electromagnetic radiation and the inverse square law works for both. The best cure, therefore, is increasing the distance between your camera and the transmitters. As a radio engineer, I’ve learned long ago that you just can’t do certain things close in. Hope this simple explanation helps.

(David Mehall, 22 Aug 1996, from an email thread on photography.)

144.  Video surveillance system interferes with car central locking and security

Gun Wharf, a leisure centre in Portsmouth, opened in Easter 2001. It had an underground car park, and the car park had a video surveillance system. It seems that electromagnetic emissions from the video system would often interfere with car central-locking and security systems – locking the cars as soon as they were unlocked, or just not allowing them to be unlocked at all. Many people had to leave their cars in the car park and take taxis home.

(From Anne Cameron, Alenia Marconi Systems, 6th July 01)

145.  Electromagnetic weapons could be used for Information Warfare

A relatively recent (1997) definition of Information Warfare given by the Ministry of Defence is: “The deliberate and systematic attack on critical information activities to exploit information, deny services to the authorised user, modify and corrupt data.” The issues involved reach well beyond the realm of military warfare, extending to e-business, e-commerce, e-finance and e-government. The power, water and food distribution systems, the emergency services, air traffic control systems, the banking sector and the financial markets, to name but a few, are all dependant on networked digital systems for effective communication and control.

It is a sobering reflection that ‘the most advances society in the world is really only four meals away from anarchy, and if you could attack a society through its computers to cause the breakdown of the mechanisms, the infrastructure, which cause it to run, you will bring about mass deaths.’

Line of sight devices. Two distinct classes of line-of-sight devices have been described. The first is a form of low-energy radio-frequency (LERF) jammer, which can be used to temporarily disrupt digital electronic circuits at close range (of the order of metres). Since any cable or circuit component in an electronic system is in principle an unintended antenna, capable of both transmitting and receiving at its characteristic frequency, a low-energy wide spectrum RF field will contain with high probability frequencies matching the resonant frequencies of critical circuit components.

If this is the case then the system would go into ‘random output mode’; its behaviour would be impossible to predict, but could range from single recoverable processing error to total loss of the RAM contents. A parts list and circuit design for such a low-tech device was posted on an Internet bulletin board in 1995 and described at InfoWarCon in 1996. The device was subsequently built and tested in the UK to check the veracity of its design.

The second class of device is the high-energy radio-frequency (HERF) gun or non-nuclear electromagnetic pulse (NN-EMP) cannon, which can permanently damage digital circuits at longer ranges (or the order of a kilometre) by blasting them with a pulse of microwave energy in the Gigahertz frequency range. The MOS chips are effectively ‘fried’ by this process. HERF technology is high-tech and remains the subject of classified military research.

However, the unclassified technology had been reviewed and discussed in detail. In order to protect (or ‘harden’) systems against RF attack they need to be entirely enclosed in a Faraday cage, ideally including the electrical power feeds and communications links, since these can act as antennas for the RF field or EM transients.

Eavesdropping and surveillance. Since a cable or circuit component can act as a transmitting antenna, unshielded computers and networks are liable to leak compromising RF emanations that are a potential source of intelligence. Passive intelligence gathering from unshielded systems (ElInt) has been given the name TEMPEST (transient electromagnetic pulse emanation standard, see Reference 16, note 3) while emanations specifically arising from the CRT screens of VDUs are known as van Eck radiation after the scientist who was able to demonstrate remote reconstruction of the screen contents using low-tech equipment.

It has been pointed out that malicious software could be used to infiltrate a target system, obtain critical information, and encode it in the system’s Tempest emanations in order to broadcast it back to the attacker.

Summary and prospects. The threats to, vulnerabilities of, and impacts on critical national infrastructures (CNI) are real and capable of assessment, although in practice this is a complex and challenging task, the more so since information warfare possesses several characteristics of that are not shared by conventional warfare: it is global (there are no borders); it is precise (surgical strikes are possible); it is un-proportionate (the cost of attack is much less than the cost of defence).

Some of the accounts of information warfare carried by the media have been exaggerated or are inaccurate, but this must not deflect us from addressing the crucial issues of defining, developing, and deploying critical infrastructure protection (CIP) policies and strategies.

(Extracted from “Information Warfare: battles in cyberspace”, by Richard E Overill, IEE Computing and Control Engineering Journal, June 2001, pp125 –
128, There was a lot about hackers and such, which has been omitted from this extract.)

146.  Case #22804: Lead boxes – Good for Kryptonite, bad for CRT monitors

A prestigious New York Hotel had upgraded their check-in and reservation computers to modern PCs with colour CRT monitors. One unit’s screen, in the managers office, was nearly impossible to read due to a wavy image. Magnetic fields at up to 60 milligauss were found at the monitor location and were coming from a power company electrical vault under the sidewalk outside. Some monitors can be disturbed by as little as 10 milligauss.

Due to some bad advice, the hotel had an aluminium box built (didn’t work), then a lead box (didn’t work either but at least now the monitor was protected from Superman’s X-ray vision). Eventually they came to us and our standard five-sided ‘ImageGuard’ CRT monitor enclosure did the trick.

(From the Journal of Magnetic Shield Case Studies, an in-house advertising medium published by the Magnetic Shield Corporation,

The regular “Banana Skins” column was published in the EMC Journal, starting in January 1998. Alan E. Hutley, a prominent member of the electronics community, distinguished publisher of the EMC Journal, founder of the EMCIA EMC Industry Association and the EMCUK Exhibition & Conference, has graciously given his permission for In Compliance to republish this reader-favorite column.

The Banana Skin columns were compiled by Keith Armstrong, of Cherry Clough Consultants Ltd, from items he found in various publications, and anecdotes and links sent in by the many fans of the column. All of the EMC Journal columns are available at:, indexed both by application and type of EM disturbance, and new ones have recently begun being added. Keith has also given his permission for these stories to be shared through In Compliance as a service to the worldwide EMC community.

We are proud to carry on the tradition of sharing Banana Skins for the purpose of promoting education for EMI/EMC engineers.

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