I will answear it via skype if you like.

this isn't kana, it's a kana symbol and the symbol of degree ゜added to it... in kana this round thing can only be added to one of 10 items: はひふへほ　　ハヒフヘホ with any other symbols -- it's meaningless

How about this page? http://www.nhk.or.jp/lesson/english/syllabary/katakana.html I hope this was helpful.

Domo arigatou gozaimasu. Boku wa wakarimasu bidakuon ima.

They are called 鼻濁音<びたくおん> or ガ行鼻音<がぎょうびおん>. カ゜［ŋa］≒［nga］ キ゜［ŋi］≒［ngi］ キ゜ュ［ŋjɯ］≒［ngjɯ］＝ngyu キ゜ョ［ŋjo］≒［ngjo］＝ngyo キ゜ェ［ŋje］≒［ngje］＝ngye ※This sound is not used in Japanese. キ゜ャ［ŋja］≒［ngja］＝ngya ク゜［ŋɯ］≒［ngɯ］ ケ゜［ŋe］≒［nge］ コ゜［ŋo］≒［ngo］ The next phrases are pronounced the following in standard(Tokyo) pronunciation. がっこうが<学校が>［gakko：ŋa］＝ガッコーカ゜ しんがっき<新学期>［ɕiŋŋakki］＝シンカ゜ッキ じゅうご<十五／15>［ʥɯ：go］＝ジューゴ じゅうごや<十五夜>［ʥɯ：ŋoja］＝ジューコ゜ヤ たんご<単語>［taŋŋo］＝タンコ゜ タンゴ［taŋgo］＝タンゴ 鼻濁音 https://ja.wikipedia.org/wiki/%E9%BC%BB%E6%BF%81%E9%9F%B3 ［ŋ］軟口蓋鼻音<なんこうがいびおん> https://ja.wikipedia.org/wiki/%E8%BB%9F%E5%8F%A3%E8%93%8B%E9%BC%BB%E9%9F%B3 https://en.wikipedia.org/wiki/Velar_nasal