Use the following to redirect to login page in DotNetNuke:
If Not Request.IsAuthenticated Then
Response.Redirect(Globals.NavigateURL(PortalSettings.LoginTabId, True, PortalSettings, “Login”, “returnurl=” & Server.UrlEncode(Request.Url.ToString)))
End If
Courtesy: http://blog.pnbconsulting.com.au/index.php/dotnetnuke-redirect-after-login/