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Producing suitable bitmaps

atlc expects to find the transmission line's cross section to be found in a standard bitmap (.BMP or .bmp) file. There are several forms of bitmaps, some grayscale, some 8-bit colour (256 colours), some 16 bit colour, some 24-bit colour. Some bitmaps are compressed. atlc expects to see uncompressed 24-bit bitmaps. This may sound restrictive, but in practice most graphics software can save such files.

Since the form of the input file is critical, we will discuss this a little. 24-bit images have 8 bits per colour (8 for red, 8 for green and 8 for blue). Hence there are 256 shades of red, 256 shades of green and 256 shades of blue, giving a total of 256*256*256=16,777,216 possible colours. It follows that 3 bytes of data are needed to describe each pixel (ignoring compressed images which are not supported). Each bitmap has a small header of around 56 bytes, followed by 3 bytes for every pixel. The minimum possible length for a bitmap of x by y pixels is then 56+3*width*height (bytes). In practice, images are usually a little larger than this, as there is some padding. If your images are not at least this size, something is wrong! For a more detailed discussion of bitmap files (unnecessary for using atlc), see this HTML page I found on the web somewhere.

Colours in bitmap files are often written as red,green.blue as in 26,239,179 indicating the amount of red (26), the amount of green (239) and the amount of blue (179). Amounts vary from 0 (none of the colour) to 255 (the maximum possible amount of the colour. Often the colours are written in hexadecimal format as 0x1aefb3 or 0x1AEFB3. Such a colour will look like this.
The colour 0x1AEFB3 or 26,239,179

It is absolutely essential that you are able to produce bitmap images with exactly the colours atlc needs.

One conductor must be produced in pure red. i.e 255,0,0 or 0xFF0000. This red will look like the red square on the left. The one on the right is very slightly different, having a very small amount of blue, and so has the colour representation 255,0,1 or 0xFF0001. The colour on the left will be interpreted by atlc as one conductor, the one on the right will not. Hence it is essential to check the colours produced by your graphics package not only look about right, but are exactly right.
Pure red 0xFF0000 Re with a little blue 0xFF0001

Graphics packages such as Gimp (freely available for no cost on unix systems) will allow you to set a colour precisely.

You then need to draw an image of the cross section of the transmission line to be analysed. The scale can be anything you reasonably want, but should result in the largest dimension in your transmission line have 200 or more pixels allocated to it. Making the bitmap much smaller (say 32 pixels in one dimension) will results in fast but inaccurate results. Much larger bitmaps, say 1000x1000, will take a long time to compute. The bitmaps don't have to be square. You should aim to fill the whole of the bitmap with the relevant details, and not have a lot of unused space on the bitmap. For example, the image on the left below is fine, but the one on the right will spend a lot of time computing nothing of value.
Relevant data fills bitmap Wasted space = wasted computing time.

Predefined colours in atlc

The input file to atlc, which is a bitmap, must have the correct colours to indicate what parts of the image are conductors and dielectrics. Parts at ground (0 V) potential must be drawn green, those at +1 V must be drawn red and those at -1V must be drawn blue. Only red and green are noramlly used for conductors, with blue being used only on couplers. Vacuum dielectric must be drawn while. Many other colours have very specific meanings as shown below.

The following colours are predefined in atlc. These can be used, without the user specifying what they mean.

Predefined conductors

+1 V conductor 0 V conductor -1 V conductor
Red
live conductor
RGB=0xff0000
rgb=255,0,0
Green
ground conductor
RGB=0x00ff00
rgb=0,255,0
Blue
-1v conductor
RGB=0x0000ff
rgb=0,0,255
The following dielectrics are pre-defined.

Predefined dielectrics

Vacuum Air PTFE RT Duroid 5880 Polyethelene
White
er=1.0
Vacuum
RGB=0xffffff
RGB=255,255,255
Pink
er=1.0006
Vacuum
RGB=0xffcaca
RGB=255,202,202
Light Blue
er=2.1
PTFE
RGB=0x8235ef
RGB=130,52,255
Gray
er=2.2
RT duroid 5880
RGB=0x8e8e8e
RGB=142,142,142
Mauve
er=2.33
Polyethelene
RGB=0xff00ff
RGB=255,0,255
Polystyrene PVC Epoxy resin Fibreglass RT Duroid 6006
Yellow
er=2.5
Polystyrene
RGB=0xffff00
RGB=255,255,0
Sandy
er=3.3
PVC (at 1MHz)
RGB=0xefcc1a
RGB=239,203,27
Brown
er=3.335
Epoxy resin
RGB=0xbc7f60
RGB=188,127,96
Terquoise
er=4.8
Fibreglass PCB
RGB=0x1aefb3
RGB=26,239,179
Dark grey
er=6.15.
RT duroid 6006
RGB=0x696969
RGB=142,142,142
RT duroid 6010 Er = 100
Light gray
er=10.2
RT duroid 6010
RGB=0xdcdcdc
RGB=240,240,240
Dark Orange
er=100
Er of 100.0
RGB=0xd5a04d
RGB=213,160,77
Note, the permitivity of free space (vacuum) is by definition 1.0. Air is very close to 1, but depends on pressure and temperature. Although the difference in permittivity between air and vacuum is smaller than the errors in atlc, having another permittivity very close to 1.0 is useful, as for test purposes. The figures for the plastics given above are only approximate. The permittivity of plastics are usually frequency dependant and temperature dependant.

Entering the permittivity of other materials into atlc

If you know the exact value of your dielectric material, draw the dielectric in a new colour, but define the colour with a command line option. For example, if your transmission line has a dielectric of 2.42 somewhere, then you must
  1. Draw the image, using a different colour (say a golden colour with RGB=0xf9e77d or in decimal RGB=249,231,125)
  2. Run atlc with the following command line option.
    % atlc -d f9e77d=2.43
this sets the colour f9e77 to reresent a permittivity of 2.43.

atlc is written and supported by Dr. David Kirkby (G8WRB)

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