Banana Skins #358-364

358. Interference increasing in the aircraft bands

Very little has happened to Section 21 recently other than a reduction of the limits in the receiver band for certain test categories and the banning of circularly polarised antennas, with both horizontal and vertical testing being required above 25MHz. However, more significant changes are proposed for the “F” revision.

The reduction of limits was required because of the increasing interference occurring, in aircraft operation, in the aircraft receiver bands. One theory proposed by the author is that this problem has been caused by the increasing use of absorber lined chambers without an accompanying reduction in the test limits. Because the RF absorber damps out the resonances of the chamber the radiated emissions can appear lower at some frequencies. In an experiment, emissions from a simulated EUT were measured in an unlined chamber as previously allowed by early versions of DO160, and the same chamber partially lined with RF absorber. A reduction in peak emissions of up to 26dB was observed in the semi-anechoic chamber.

(Extracted from: “To DO160E and Beyond” by Dr Nigel Carter of Qinetiq, EMC-UK Conference, Newbury, October 11-12 2005, pp 127-130.)

359. EMI suspected of causing electrical meltdown

Question: I have a Rover 827 Si, bought new in Spain in 186. When I travelled to the UK recently, it suffered a major electrical meltdown and was rendered immobile. It has done little more than 40,000 miles and is otherwise in first‑class condition. Is there are alternative to scrapping it?

Reply: Don’t despair yet. It’s possible that, while parked, the car encountered electronic interference, possibly from an illegal short-wave radio, or from police or ambulance transmitters, to which its immobiliser system was vulnerable. 

(“Start Wreck”, in ‘Honest John’s’ motoring questions column, The Daily Telegraph Motoring Section, 18 Feb 2006, page 10.)

360. Document shredder interferes with set-top box

Operating my personal document shredder crashes my digital TV set-top box, although it is 5 metres away. Toggling the on/off button on the set-top box restores normal operation. 

(Sent in by Peter Cryer, 2nd February 2006.)

361. Lack of good PCB EMC design delays product launch 

The day after attending your course on Advanced PCB Design for EMC, during which you emphasised the exponential relationship between cost of modification and the date of market introduction, I went back to my OATS and tested some more customers’ products. One of them emphasised the above point – EMC testing was the last thing this manufacturer thought of, and a 16dB over Class B ‘surprise’ was the result. 

Adding ferrites to the cables made no difference as most of the noise was radiating directly off the PCB. The company has had to engage an EMC consultant to try to fix their 8 layer board, and the product shipping has been postponed. 

(Sent in by Bruce Holdsworth, Sydney, Australia.)

362. When is a dozen ferrites too many? 

This job started out just the same as any. With the client present I set up the EUT on the test table, warmed up the analyser and began the test. Almost immediately I could tell the emissions from the EUT were going to exceed the 40dBuV Class A 10 metre limits. A quick look at the other frequency ranges up to 1GHz confirmed this. The emissions were over the limit everywhere. Ok I said to the client, lets take a look inside and see if we can come up with a solution. Upon opening the fairly large cabinet I was astounded to see at least a dozen clip-on ferrites randomly attached to cables. I asked why so many? The reply was “We had them back at the workshop so I just put them in”. 

I suggested that we should remove the ferrites and take a baseline reading and start work from there. With both hands now full of ferrites, we headed back inside. Although the EUT was still failing Class A, a quick measurement showed the emission levels had not changed! I asked the client to take ONE ferrite with him and to go out to the EUT, open the cabinet, stand to the side and to carefully touch the cables inside with out disturbing their position. (Obviously this method is suitable for low voltage equipment only) while I watched the analyser. Sure enough, as soon as he placed his hand on the offending cable the analyser readings dropped. “That’s it” I called to him to clip the ferrite on that cable and the rest is history. The offending cable was a noisy RS485 cable and ONE clip-on ferrite cured the entire problem.

(Another anecdote sent in by Bruce Holdsworth, Sydney, Australia.)

363. Can interference from passenger electronic devices make aircraft unsafe? – Part 1

More and more passengers are bringing cellphones, PDAs, laptops, DVD players, and game machines on board aircraft. All of these items emit radiation and have the potential to interfere with aircraft instrumentation. More and more passengers, however, do not believe that using portable electronic devices presents a risk to passenger safety. We, on the other hand, have had our doubts that such use was safe. Over the course of three months in late 2003, we investigated the possibility that portable electronic devices interfere with a plane’s safety instruments by measuring the RF spectrum inside commercial aircraft cabins. What we found was disturbing. Passengers are using cellphones, on the average, at least once per flight, contrary to FCC and FAA regulations, and sometimes during the critical flight phases of takeoff and landing. 

Regulations already permit a wide variety of other portable electronic devices (PEDs) – from game machines to laptops with Wi-Fi cards, to be used in the air today. Yet our research has found that these items can interrupt the normal operation of key cockpit instruments, especially Global Positioning System (GPS) receivers, which are increasingly vital to safe landings. Two different studies by NASA further support the idea that passenger’s electronic devices dangerously produce interference in a way that reduces the safety margins for critical avionics systems.

There is no smoking gun to this story: there is no definitive instance of an air accident known to have been caused by a passenger’s use of an electronic device. The data support a conclusion that continued use of portable RF-emitting devices such as cellphones will, in all likelihood, someday cause an accident by interfering with critical cockpit instruments such as GPS receivers.

The study found that intermodulation between some cellular phones caused emissions in the frequency bands used by an aircraft’s GPS and distance-measuring equipment. The report identified other combinations of common passenger transmitters that could potentially produce intermodulation effects in aircraft communication and navigation RF bands.

GPS-certified landing approaches are now widely used in general aviation. Though most airliners presently use instrument landing systems, use of GPS technology will increase significantly over the next few years. There are three times as many GPS-certified approaches as instrument landing system approaches in the United States. 

In March 2003, acting on a number of reports from general aviation pilots that Samsung SPH-N300 cellphones had caused their GPS receivers to lose satellite lock, NASA issued a technical memorandum that described emissions from this popular phone. It reported that there were emissions in the GPS band capable of causing interference. Disturbingly, though, they were low enough to comply with FCC emissions standards. 

In one telling incident, a flight crew stated that a 30-degree navigation error was immediately corrected after a passenger turned off a DVD player and that the error reoccurred when the curious crew asked the passenger to switch on the player again.  Game electronics and laptops were the culprits in other reports in which the crew verified in the same way that a particular PED caused erratic navigation indications.

(The above are some paragraphs selected from: “Unsafe at any airspeed? Cellphones and other electronics are more of a risk than you think”, Bill Strauss, M Granger Morgan, Jay Apt and Daniel D Stancil, IEEE Spectrum, March 2006, pp 38-43. The IEEE paper includes many references for further information.) (Editor – the USA’s 2001 DOT/Volpe report – see Banana Skins 223, 227 and 230 – said that interference, either intentional or unintentional, could deny GPS access, so I am totally amazed that GPS is permitted to be relied upon for aircraft landings! Any comments from the CAA?.) 

364. Can interference from passenger electronic devices make aircraft unsafe? – Part 2

While flying home from a house-hunting trip in 1981 in a turboprop Short Bros. 360, I began feverishly working out possible mortgage payments on a cheap credit card calculator. Soon the stewardess was walking down the aisle asking if anyone had anything “electrical” they were using. I replied that I had a calculator, that was electronic but not electrical – that is, it had no motors or anything. She borrowed my calculator and took it to the cockpit. 

She returned in a few minutes and admonished me with words like, “Please don’t use that anymore, because when you press the keys it makes the needles in the cockpit swing around.” Needless to say, I complied. As pointed out in “Plane Talk about Cellphones” [Spectral Lines, March], much of the data on signals interference in aircraft is informal and hearsay – but this is my anecdote; I witnessed it myself. I vote for keeping the ban on cellphones.

(Letter from Chris Jones responding to an editorial about the article mentioned in Banana Skin No. 363 above, in “Forum” in the IEEE Spectrum, May 2006, page 4.) 


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: https://www.emcstandards.co.uk/emi-stories, 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.

About The Author

Keith Armstrong

After working as an electronic designer, then project manager and design department manager, Keith started Cherry Clough Consultants in 1990 to help companies reduce financial risks and project timescales through the use of proven good EMC engineering practices. Over the last 20 years, Keith has presented many papers, demonstrations, and training courses on good EMC engineering techniques and on EMC for Functional Safety, worldwide, and also written very many articles on these topics. He chairs the IET’s Working Group on EMC for Functional Safety, and is the UK Government’s appointed expert to the IEC committees working on 61000-1-2 (EMC & Functional Safety), 60601-1-2 (EMC for Medical Devices), and 61000-6-7 (Generic standard on EMC & Functional Safety).

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