The source code

    /*!
     * Ext JS Library 3.4.0
     * Copyright(c) 2006-2011 Sencha Inc.
     * licensing@sencha.com
     * http://www.sencha.com/license
     */
    /**
     * @class Ext.form.Radio
     * @extends Ext.form.Checkbox
     * Single radio field.  Same as Checkbox, but provided as a convenience for automatically setting the input type.
     * Radio grouping is handled automatically by the browser if you give each radio in a group the same name.
     * @constructor
     * Creates a new Radio
     * @param {Object} config Configuration options
     * @xtype radio
     */
    Ext.form.Radio = Ext.extend(Ext.form.Checkbox, {
        inputType: 'radio',

        /**
         * Overridden and disabled. The editor element does not support standard valid/invalid marking. @hide
         * @method
         */
        markInvalid : Ext.emptyFn,
        /**
         * Overridden and disabled. The editor element does not support standard valid/invalid marking. @hide
         * @method
         */
        clearInvalid : Ext.emptyFn,

        /**
         * If this radio is part of a group, it will return the selected value
         * @return {String}
         */
        getGroupValue : function(){
        	var p = this.el.up('form') || Ext.getBody();
            var c = p.child('input[name="'+this.el.dom.name+'"]:checked', true);
            return c ? c.value : null;
        },

        /**
         * Sets either the checked/unchecked status of this Radio, or, if a string value
         * is passed, checks a sibling Radio of the same name whose value is the value specified.
         * @param value {String/Boolean} Checked value, or the value of the sibling radio button to check.
         * @return {Ext.form.Field} this
         */
        setValue : function(v){
        	var checkEl,
                els,
                radio;
        	if (typeof v == 'boolean') {
                Ext.form.Radio.superclass.setValue.call(this, v);
            } else if (this.rendered) {
                checkEl = this.getCheckEl();
                radio = checkEl.child('input[name="' + this.el.dom.name + '"][value="' + v + '"]', true);
                if(radio){
                    Ext.getCmp(radio.id).setValue(true);
                }
            }
            if(this.rendered && this.checked){
                checkEl = checkEl || this.getCheckEl();
                els = this.getCheckEl().select('input[name="' + this.el.dom.name + '"]');
    			els.each(function(el){
    				if(el.dom.id != this.id){
    					Ext.getCmp(el.dom.id).setValue(false);
    				}
    			}, this);
            }
            return this;
        },

        // private
        getCheckEl: function(){
            if(this.inGroup){
                return this.el.up('.x-form-radio-group');
            }
            return this.el.up('form') || Ext.getBody();
        }
    });
    Ext.reg('radio', Ext.form.Radio);