Experimental and theoretical studies of the properties and performance of electromagnetic loops and feeder lines used in road vehicle detections systems were carried out to determine causes of erratic behaviour of loops, and to improve their performance.
Basic concepts and terminology of loops are introduced and the effects of objects in their proximity are studied. The magnetic flux density distributions near rectangular and quadrupole loops are analysed, assuming the loops to be in free space or away from other objects that may affect their magnetic fields. Magnetic flux density distribution for a quadrupole is compared with published measured spatial sensitivity for the same type of loop. The comparison indicates a strong correlation between spatial sensitivity of a loop, defined as the relative change in loop inductance caused by an object's presence, and its magnetic flux density distribution. From these findings, optimum loop sizes that give maximum sensitivity to conducting sheets are determined using horizontal plane sheets to simulate a vehicle's underside such as of a car or truck, and vertical conducting sheets to model motorcycles and bicycles.
The effects of various types of feeders (or trasmission lines) on the sensitivity of the loop-feeder system are defined and an analytical expression for the sensitivity is derived. A relationship exists between the value of the loop reactance and the feeder characteristic impedance that reduces the adverse effects of the feeder length.
The effects of conducting screens (slabs, steel reinforcing in concrete, etc) on loop performance are also investigated both analytically and experimentally using models. Again, the results show excellent agreement in that the sensitivity of a loop is reduced dramatically by the presence of conducting objects, although ferromagnetic materials do not seem to have such a great effect as non-ferromagnetic materials.