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3. How is the vehicle detected?
When a vehicle enters or crosses the loop, the body and frame provide a conductive path for the magnetic field. This produces a loading effect, which in turn causes the loop inductance to decrease. The decreased inductance causes the resonant frequency to increase from its nominal value. If the frequency change exceeds the threshold set by the sensitivity setting, the detector module will output a detect signal.

There is a common misconception that an inductive loop requires a mass of metal or ferrous material for detection. Placing a single wire around the perimeter of the loop and shorting the ends together will quickly disprove this misconception. The single wire forming a shorted turn provides a current path for the magnetic field; thus causing a loading effect similar to that of a vehicle. The shorted turn effect of the single wire coil in the proximity of the loop acts much like a shorted turn secondary of a transformer.

6. Does increasing the number of turns in the loop increase the sensitivity of the loop?
NO. Increasing the turns does not increase the sensitivity of the loop. It can improve the efficiency of the loop system (loop inductance + lead-in inductance), if the lead-in length is over 400 feet. The amount of inductance change a vehicle can cause in a loop is determined by the following factors:

Amount of Change » Vehicle Size
Caused by Vehicle (Loop Size) x (Vehicle Height)

The above formula indicates the following:

1. Increasing the loop size will decrease the amount of change caused by the vehicle.
Example: If a vehicle causes a 1.0% change on a 6'x6' loop, then the same vehicle will cause a 0.5% change when over one of two 6'x6' loops connected in series.
2. A smaller vehicle will cause less change. A small motorcycle causes approximately 1% to 2% of the change caused by standard automobiles.
3. The higher the vehicle is from the road (loop) surface, the smaller the inductance change.

8. How deep should the loop wire be installed?
The deeper the wires are below the road surface the more they are protected from road surface wear and the elements. The top wire should be a minimum of 1 inch below the road surface.

Nonconductive materials such as concrete and asphalt will not influence the loop fields. Installing the loop one inch deeper (e.g. 3" depth instead of 2" depth) would have the same result as raising the vehicle one inch above the pavement surface.

To reduce stress and abrasion of the loop wire the 90° corners should be cut at a 45° angle; core drilled (1.5" diameter); or at a minimum, the sharp inner corners should be rounded with a chisel.

9. What type of wire should be used for the loop?
Number 16 or 20 AWG stranded wire can be used. The wire gauge is not critical to proper operation of the loop detector. The wire should maintain its integrity under the pavement stress. Since asphalt is more flexible than concrete, it is recommended that a heavier gauge wire be used for loop installations in asphalt.

The main consideration in selecting a wire for loop installations is the type of insulation. Cross-linked polyethylene (XLPE) insulation rated at 600 volts is highly recommended over PVC insulation. Under similar conditions, XLPE insulation will absorb approximately one percent of the moisture absorbed by PVC. When insulation absorbs moisture, loop drift occurs, which if great enough, can cause false detections. XLPE also has higher resistance to abrasion, heat, oils, and gasoline.

After insulation, and any time there appears to be a loop related problem, the loop should be tested. Use a MegOhm Meter to test the integrity of the loop / lead-in wire insulation. Readings of 100MO or less indicate possible insulation damage. Use a Multimeter to check the total resistance of the loop / lead-in combination. Total loop / lead-in resistance should never exceed 4 Ohms.

10. How far from a gate should the loop be installed?
As the length of the sides of the loop that parallel the gate increases, the inductance change caused by the gate also increases. The graph shows the inductance change for different distances between the gate and the loop for different sized loops.

The closer the loop is to a gate, the more influence the gate has on the loop! Hence, the detector sensitivity must be set lower to ensure the gate will not cause the detector to generate an output when the gate closes.

The following rule should be observed: The longer the loop, the greater the spacing must be between the gate & the loop!

The inductance change at two feet is one third of the change at one foot. At four feet, the effects of the gate on the loop are minimal.