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.
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.
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.
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 V000 C= 54.0683 pF/m L= 205.7862 nH/m Zo= N/A Ohms Zodd= 61.6931 Ohms Zeven= N/A Ohms v= 2.9979 e+08 m/s v_f= 1.0000 VERSION= 4.0.0
couplerxx.bmp Er= 1.0000 C= 54.0657 pF/m L= 205.7958 nH/m Zo= N/A Ohms Zodd= 61.6960 Ohms Zeven= N/A Ohms v= 2.99792e+08 m/s v_f= 1.0000 VERSION= 4.0.0
couplerxx.bmp Er= 1.0000 C= 36.6079 pF/m L= 303.9368 nH/m Zo= 74.9774 Ohms Zodd= 61.6960 Ohms Zeven= 91.1180 Ohms v= 2.99792e+08 m/s v_f= 1.0000 VERSION= 4.0.0
couplerxx.bmp Er= 1.0000 C= 35.5586 pF/m L= 312.9064 nH/m Zo= 76.0757 Ohms Zodd= 61.6960 Ohms Zeven= 93.8070 Ohms v= 2.99792e+08 m/s v_f= 1.0000 VERSION= 4.0.0
couplerxx.bmp Er= 1.0000 C= 35.4672 pF/m L= 313.7124 nH/m Zo= 76.1736 Ohms Zodd= 61.6960 Ohms Zeven= 94.0486 Ohms v= 2.99792e+08 m/s v_f= 1.0000 VERSION= 4.0.0
couplerxx.bmp Er= 1.0000 C= 35.4513 pF/m L= 313.8534 nH/m Zo= 76.1908 Ohms Zodd= 61.6960 Ohms Zeven= 94.0909 Ohms v= 2.99792e+08 m/s v_f= 1.0000 VERSION= 4.0.0
couplerxx.bmp Er= 1.0000 C= 35.4486 pF/m L= 313.8773 nH/m Zo= 76.1937 Ohms Zodd= 61.6960 Ohms Zeven= 94.0980 Ohms v= 2.99792e+08 m/s v_f= 1.0000 VERSION= 4.0.0
couplerxx.bmp Er= 1.0000 C= 35.4486 pF/m L= 313.8773 nH/m Zo= 76.1937 Ohms Zodd= 61.6960 Ohms Zeven= 94.0980 Ohms v= 2.99792e+08 m/s v_f= 1.0000 VERSION= 4.0.0
Note that atlc first computes the odd-mode impedance, and then on a second run computes the even-mode impedance. The characteristic impedance is Zo=sqrt(Zodd*Zeven), so this is also computed on the second run. The accuracy of the calculation of odd and even-mode impedances in couplers is discussed in the accuracy section
atlc is written and supported by Dr. David Kirkby (G8WRB)
It it issued under the GNU General Public License
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