No, "tänka på" is a phrasal verb meaning "think about" or "think on".

Here's an excellent post by Lundgren8 about tycker and tänker: https://www.duolingo.com/comment/5577824

Jag har tänkt på dig

It means "think" on its own. So you could use it in phrases like "Hon sitter och tänker" - She is (sitting and) thinking.

But as soon as you want to add an object you need the preposition.

That works too (and it's an accepted answer). The Swedish sentence covers both.

That seems to be hard. For instance this dictionary gives them both the same definition: http://dictionary.cambridge.org/dictionary/english/think-of-about-sb-sth

On a more serious note, I understand that you meant 'in Swedish'. But since the difference isn't clear in English, the answer is that we translate depending on what is meant. So if you can replace 'think of' with 'mean', you can use mena. If you want to say something never crossed your mind, as opposed to not having thought about it seriously, you can say det har aldrig fallit mig in ('it's never occurred to me'). If you want to make it clear that you've pondered something as opposed to it just crossing your mind, you can use the verb fundera (på) instead. And so on, I suppose – there are probably more possibilities that didn't cross my mind now that I thought just briefly about this :)