Communications Engineering

Solving an EMC problem between a track safety system and rail locomotives

A Railtrack contractor approached JWD with a problem concerning a track warning system and particular locomotives. Sometimes, the locomotive triggered the warning equipment into a failsafe condition and the crews were losing faith in the system.

The problem was traced to high power LED spotlights fitted underneath the vehicle. Licencing conditions and frequency allocations were reviewed with OFCOM.

Searching for interference at Willesden Junction rail terminal

TV Surveying at wind farm sites

As part of planning law, a wind farm developer is required to carry out various surveys, including noise, bird and wild life habitat and interference to television and other radio services. JWD carries out such surveys, including both desk-based and pre- and post construction site surveys.

Common belief was that the digital switch over (DSO) would end the problems of wind farm interference to television. However, this has been found not to be the case. In a recent survey, there was serious interference from a small wind farm within 2 km of a village. It affected hundreds of viewers.

The interference was particularly bad in this case as the turbines were in a direct path to, and strongly illuminated by the TV transmitter, the turbines also being in the acceptance angle of the viewer's antennas. The signal scattered by the turbines was stronger than the direct signal due to geographical features.

When investigated, the periodic pixellation and drop out was found to be caused by a series of nulls sweeping across the OFDM spectrum changing quickly in time as the turbine blades rotated. This is known as "time varying multipath" and produces incorrectable errors in the TV demodulator. Plots of the spectrum are shown below, with and without the turbines turning:

Spectrum with turbines running - notches move in time

RF smog, health and safety and well-being

Several customers have requested surveys around their homes to ensure levels of RF radiation are below certain limits. Whilst there is little scientific evidence that humans are affected by any fields at normal levels (as might be found in a domestic environment e.g. from WiFi routers and mobile phones), a survey can reassure that there is no danger, or that fields are below limits.

This mast was worrying the owner of some luxury flats near Lake Como. I performed a survey which showed that there were no radio transmissions and that the mast had fallen into disuse.

Wireless car parking sensors

These devices were developed for a company after a research contract and are now in production. They have the advantage that no wired connection is required between them and their hub and they can be simply glued to the ground. They use a Zigbee-type network and run for several years on a lithium battery. The hub is solar powered and places information about occupied spaces onto the internet, so that the car-park owner can look at use, billing etc. The presence of the car is detected by RF and magnetic means and has been patented by the client.

Prototype of an early version of car park sensor

Puma gearbox health monitoring, homodyne backscatter detector

JWD designed a system which was installed in a Puma helicopter gearbox to monitor bearing noise using acoustic emission sensors. These devices pick up ultrasonic signals (10 kHz to 1 MHz) that might indicate bearing wear and crack formation.

Preliminary testing of the system at Cranfield, showing the planet carrier part of the gearbox

The system used a low frequency magnetically coupled homodyne detector with RF power scavenging to avoid the use of rotating joints within the gearbox assembly. The output of the receiver was sampled and stored, then analysed at a University with much experience in the field. The electronics had to withstand high temperatures, vibration levels and immersion in gearbox oil.

The unit was tested at the Airbus Helicopter plant in Marseille, under full load conditions on a test bench, driving the gearbox with 2.5MW of rotary power.

Miniature underwater radio

Printed circuit artwork and schematic produced in Proteus

This device was developed for a customer after a research project to select the best frequency. Pure water is quite transparent to RF energy, although the surface is highly reflective being of very high dielectric constant. However, water with dissolved salts and chlorine is difficult to penetrate as it is conductive. At low VHF frequencies, a depth of 3m could be obtained.

All receiver functionality is digital, including squelch, volume and channel control, synthesiser programming and LiPo battery management. The software runs on a small processor and was coded in assembler.