User's Manual

301UM0031_01 TD136 Enhanced Vehicle Detector User Manual Page: 15 of 29
5.2.2 Crosstalk
When two loop configurations are in close proximity, the magnetic fields of one can overlap and disturb
the field of the other. This phenomenon, known as crosstalk, can cause false detects and detector
lock-up.
Crosstalk between adjacent loops operating from different detector modules can be eliminated by:
1. Careful choice of operating frequency. The closer together the two loops, the further apart the
frequencies of operation must be.
2. Separation between adjacent loops. Where possible a minimum spacing of 2 metres between
loops should be adhered to.
3. Careful screening of feeder cables if they are routed together with other electric cables. The
screen must be earthed at the detector end only.
4. Running feeder cables in their own slots, separated by at least 300mm
For additional information on Crosstalk refer to the section “Crosstalk Prevention” in the
DU100 Diagnostic Unit User Manual Nortech Document No. 895UM0001.
5.2.3 Reinforcing
The existence of reinforced steel below the road surface has the effect of reducing the inductance,
and therefore the sensitivity, of the loop detection system. Hence, where reinforcing exists 2 turns
should be added to the normal loop, as referred to in section 5.4.
The spacing between the loop and steel reinforcing should be greater than 150 mm, although this is
not always practically possible. The slot depth should be kept as shallow as possible, taking care that
the feeder remains exposed after the sealing compound has been applied.
5.3 Loop and Feeder Specification
Extensive studies have been undertaken over the years by various agencies around the world in order
to ascertain the optimum loop installation materials.
As an insulated conductor is a prerequisite, PVC covered cable has been used for many years as a
first choice, but tests have shown, in fact, that this is unsuitable for long term installations. The PVC
tends to become porous with the result that adjacent loops become electrically coupled to one
another, with resultant crosstalk implications. Instability and susceptibility to electrical interference can
also result.
The insulation must withstand wear and abrasion from the shifting streets, moisture, and attack by
solvents and oils, as well as withstand the heat of high temperature sealants.
Silicone insulated cable has emerged as one of the preferred insulation materials. Other insulation
materials are rubber, thermoplastic, synthetic polymer and cross linked polyethylene.
Stranded loop wire is preferred over solid wire. Because of its mechanical characteristics, a stranded
wire is more likely to survive bending and stretching than a solid.
A heavy gauge conductor is definitely desirous in order to maintain the loop Q-factor. The loop and
feeder should preferably constitute a single length of insulated multi-stranded copper conductor, with
no joints and with the copper having a minimum cross section 1.5 mm2. The feeder is twisted to
minimize the effect of electrical noise.