889-2106-03
Model U-1400 Operations Manual Rev Apr 2020 Page
39
of 39
8.2
THINGS TO KNOW ABOUT LOOPS
Always use a wire with cross-linked Polyethylene insulation (insulation type XLPE) for loop wire.
Typical sensing height is ⅔ of the shortest leg of a loop. Therefore, a 6΄ x 6΄ loop will have a detection height of 4΄.
The inductance of a conventional four-sided loop can be estimated using the formula:
L = P x (T
2
+ T) / 4
Where:
L = Loop Inductance in microhenries
P = Loop Perimeter in feet
T = Number of Turns of Wire.
Therefore, a 6΄ by 6΄ loop with 3 turns would have an inductance of:
L = (6 + 6 + 6 + 6) x (3
2
+ 3) / 4
L = 24 x (9 + 3) / 4
L = 24 x 12 / 4
L = 24 x 3
L = 72 microhenries.
The inductance of a Quadrupole
TM
loop can be estimated using the formula:
L = [P x (T
OL
2
+ T
OL
) / 4] + [CL x (T
CL
2
+ T
CL
) / 4]
Where:
L = Loop Inductance in microhenries
P = Loop Perimeter in feet
T
OL
= Number of Turns of Wire (Outer Legs)
T
CL
= Number of Turns of Wire (Center Legs)
CL = Length of Center Leg in feet.
T
CL
= Number of Turns of Wire (Center Leg)
Therefore, a 6΄ by 50΄ loop with a 2
-4-2 configuration would have an inductance of:
L = [(6 + 50 + 6 + 50) x (2
2
+ 2) / 4] + [50 x (4
2
+ 4) / 4]
L = [112 x (4 + 2) / 4] + [50 x (16 + 4) / 4]
L = (112 x 6 / 4) + (50 x 20 / 4)
L = (112 x 1.5) + (50 x 5)
L = 168 + 250
L = 418 microhenries.
Loop Feeder cable typically adds 0.22 microhenries of inductance per foot of cable.
Total inductance of loops connected in series:
L
TOTAL
= L
1
+ L
2
+ L
3
+ . . . + L
N
.
Total inductance of loops connected in parallel:
L
TOTAL
= 1 / [ (1 / L
1
) + (1 / L
2
) + (1 / L
3
) + . . . + (1 / L
N
) ].
