I would like to type the diameter symbol ⌀ and some other special characters in Kubuntu. I tried in Kmail, Korganizer, LibreOffice Writer. The only place that a keystroke combination worked was in this text entry box in Firefox. I found a tutorial on the Compose Key here:
and I set my compose key to Scroll Lock. But for Unicode entry it turns out the Compose key is not used.
I found the Unicode code for the diameter symbol (2300) here:
I found the method of Unicode Composition here:
Press Shift+Ctrl+U, release U, enter the hexadecimal (0123456789abcdef) Unicode character code point, then release Shift+Ctrl. An underlined u followed by the number will be displayed as you type.
I use English and Thai keyboards.