Re: why
have you read the book regarding these topics or mearly asking after attending any lecture? these are very well available in any antenna book.
Any way, λ/2 length is used because it is the shortest length by which high radiation could be obtained for the particular freq, bcoz the in an antenna current distribution will not be uniform, only time varying field can give rise to radiation. the electrons get accelerated for quater of the cycle and then suddenly chages its direction and moves in the opp direction, the same action is performed for quater cycle (i.e. λ/2 length) thats why the minimum length required is λ/2.
i couldnt answer all your question- short of time. you better go through books...