307UM0200_01 May 2016
PD27x User Manual
Page 15 of 30
5.2.2
Crosstalk
When two loop configura
�
ons are in close proximity, the magne
�
c fields of one can overlap and disturb the field of
another loop. This phenomenon, known as crosstalk, can cause false detects and detector lock-up.
Should the loops be connected to the same dual channel detector crosstalk will not occur, due to the fact that sequential
polling of the loops takes place, resul
�
ng in only one loop being energised at a given
�
me.
Crosstalk between adjacent loops opera
�
ng from different detector modules can be eliminated by:
1.
Careful choice of opera
�
ng frequency. The closer together the two loops, the further apart the
frequencies of opera
�
on must be.
2.
Separa
�
on 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 electrical cables. The screen
must be earthed at the detector end only.
4.
Running feeder cables in their own slots, separated by at least 300 mm.
For addi
�
onal informa
�
on on
Crosstalk
refer to the sec
�
on “Crosstalk Preven
�
on” in the DU100 Diagnos
�
c Unit User
Manual Nortech Document No. 895UM0001.
For informa
�
on about resolving
Crosstalk
refer to the “Theory of Applica
�
on” sec
�
on in Diagnos
�
c Unit DU100 User
Manual 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
sensi
�
vity of the loop detec
�
on system. Where reinforcing exists 2 turns should be added to the normal loop, as referred
to in sec
�
on 5.4.
The ideal minimum spacing between the loop and the cable and steel reinforcing is 150mm, although this is not always
prac
�
cally possible. The slot depth should be kept as shallow as possible, taking care that no part of the loop or the
feeder remains exposed after the sealing compound has been applied.
5.3 Loop and Feeder Material Speci
�
ication
Extensive studies have been undertaken over the years by various agencies around the world in order to ascertain the
op
�
mum loop installa
�
on 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 installa
�
ons. The PVC tends to become porous with the result
that adjacent loops become electrically coupled to one another, with resultant crosstalk implications. Instability and
suscep
�
bility to electrical interference can also result.
The insula
�
on must withstand wear and abrasion from the shif
�
ng streets, moisture, a
�
ack by solvents and oils, as well
as withstand the heat of high temperature sealants.
Silicone insulated cable has emerged as one of the preferred insula
�
on materials. Other insula
�
on materials are rubber,
thermoplas
�
c, synthe
�
c polymer and cross linked polyethylene.
Stranded loop wire is preferred over solid wire. Because of its mechanical characteris
�
cs, a stranded wire is more likely
to survive bending and stretching than a solid wire.
A heavy gauge conductor is definitely desirous in order to maintain the loop Q-factor. The loop and feeder should
preferably cons
�
tute a single length of insulated multi-stranded copper conductor, with no joints and with the copper
having a minimum cross section 1.5 mm
2
. The feeder is twisted to minimise the effect of electrical noise.
Joints in the loop or feeder are not recommended. Where this is not possible, joints are to be soldered and terminated
in a waterproof junc
�
on box. This is extremely important for reliable detector performance. Other forms of joins such
as those available in kits, where the joint is properly sealed against moisture, are also permi
�
ed.
