Refresh Parent window from child window

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ASP.NET

Javascript

Hi, My parent page contains one DropDownList.

On button click from this page my child window get opened.
for this i am using

Response.Write("<script>window.open('Child_Page.aspx?Value=" + Convert.ToString(DropDownList.SelectedValue) +"','SITEID','MenuBar=No,Status=No,Toolbar=No,Width=980px,Height=500px,Scrollbars=Yes,resizable=yes');</script>");

On child pages for parent refreshing i am using

string str_Value= Request["Value"];
                Response.Write("<script language='javascript'> window.opener.location.href = window.opener.location.href+'&Value=" + str_Value+ "';window.close();</script>");

First time I got this value on Parent page in query string but second time it appears zero.

Please suggest me proper way for this.