angular ng-href read from scope not working

Question

here is my downloadable link : download.xxxxxx.com/yyyyy.json

for company policy reason, I use xxxxxx and yyyyy to illustrate domain and json file name. but I have tested on my end, this is a downloadable link.

HTML

<input type="button" class="btn-round btn-primary" ng-href="winlink" value="Download">

Controller

$scope.winlink =  'http://download.xxxxxx.com'+ '/' + $scope.name;

the problem here is ng-href doesn't work at all. When I click the button, nothing happens.