//jQuery
$('#Products').change(function(){
    var selection = $('#Products').val();
    $('.field').hide();
    switch(selection){
        case 0:
            $('#2Antennas').show();
            break;
        case 1:
            $('#4Antennas').show();
            break;
    }
});

//HTML
<select name = "Productx1" id = "Products">
		<option value="1">KKas1</option>
		<option value="2">Kkas2</option>
</select>

<div id = "2Antennas" class = "field">
Nav svarīgs saturs.
Ievades lauki.
</div>
<div id = "4Antennas" class = "field">
Nav svarīgs saturs.
Ievades lauki.
</div>