CATEGORY 3 DESIGN

VERTICAL DISPLCEMENT

Cat3 loop wiring is quite different from cats 1&2 because we need to install more cable in a precise pattern to overcome all the losses due to metalwork. Loop wires are almost always installed at floor level in most cases and allows the use of flat copper foil which can be laid under a carpet. Alternatively, the arrays can be positioned at ceiling height and the cables laid within plastic conduit.

ASPECT RATIO

Aspect Ratio for a Cat3 design is always = 1.

ARRAY WIRING DESIGN

For a category 3 project, the layout of the loop cables is by neccesity, more complicated as we have to overcome lots of metalwork losses. This section explains how to work out the layout of the two arrays. It is important to follow the layout carefully to achieve adequate and uniform magnetic field strengths.

First of all, take a look at the wiring layout diagram. Note how there are two seperate loops (different colours) arranged in an overlapping fashion. Each individual loop is further arranged into a number of side by side sections with a uniform width (w). The gap (g) between each of these is 0.56 * w. The second array simply overlaps the first symetrically.

The first design point to note is that the width of each section (w) is between 2 and 3.5 meters. The length of each section is only limited by the wire impedance which influences the loop current and hence choice of amplifier.

The second point to note is that whilst the two arrays need not have the same number of sections, it keeps everything simpler if there are - no need to worry over different cable length / impedance issues.

Selecting the Number of Sections Within an Array

Each section as stated earlier needs to be between 2 and 3.5 meters in width. Selecting a smaller dimension gives more predictable results when there is a lot of metalwork present. Obviously the number of Sections required is proportional to the dimensions of the room and to keep things simple, each array will have an equal number of sections. The last point to note before calculating the number of sections is that the second array overlaps the first array in a precise way. Each section of the second array overlaps symmetrically the GAP of the first array. Take a look at the diagram drawn for a two section loop layout to understand this.

Divide the shortest loop side dimension by one of the following values such that the answer is between 2 and 4:

÷by	1.78	3.34	4.9	6.46	8.02	9.58
No of Sections	1	2	3	4	5	6

EXAMPLE - Suppose your loop was to cover an area 15m x 11m. The shortest side is 11m. Dividing by 1.78 gives 6.18 which is too high (aim between 2 and 4), however dividing by 4.9 gives 3.29m for the section width (w) which is ideal. You could divide by 4.9 to give a section width 2.25m with 3 sections per array but unless there is a huge amount of metalwork, this would be uneccesary. Using a 2 section array the gap (g) between sections (0.56*w) would be: 0.56*3.29 = 1.84m. We can use this information to work out a full layout plan starting from a datum point adjacent the longest side.

Refer to the diagram to follow this.

Assume point A is the datum at 0m - Table for a 2 section loop array layout

Point	A	B	C	D	E	F	G	H	Gap
Distance (m)	0	0.78*w	w	1.56*w	1.78*w	2.34*w	2.56*w	3.34*w	0.56*w
Example (m)	0	2.57	3.29	5.14	5.86	7.71	8.43	11.00	1.84

It is vital to follow the physical layout of the wiring - notice it does NOT go straight up and down the room but crosses at one end. Notice that NONE of the loop wiring connects together electrically at any point within the area covered. When wiring an array, use two different colours of wire if possible and finish the first array before starting the second. Pieces of colored insulating tape can make the job MUCH easier - Points A, C, D and G for example would use green tape and B, E, F and H blue. If you write on the tape An(ear) at one end and Af(ar) to correspond at the other end, you can then follow the pairs of letters on the diagram - easy. Finally, the loop amplifiers can be fitted at any point around the area to be covered - just intercept the nearest section and divert the two ends to the loop amplifiers. Don't forget to twist the cables together to eliminate any unwanted stray magnetic fields. Back to design example page.

An example of a much larger 4 section array can be seen here.

So we need to take four things on to the next section:

1. Size of area to be covered.
2. Loop wire height relative to the floor.
3. Aspect Ratio figure.
4. Array Diagram.

Now we can go to the final design phase - equipment choice.

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