The source code

    /**
     * @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;
        },

        // private
        onClick : function(){
        	if(this.el.dom.checked != this.checked){
    			var els = this.getCheckEl().select('input[name=' + this.el.dom.name + ']');
    			els.each(function(el){
    				if(el.dom.id == this.id){
    					this.setValue(true);
    				}else{
    					Ext.getCmp(el.dom.id).setValue(false);
    				}
    			}, this);
    		}
        },

        /**
         * 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){
        	if (typeof v == 'boolean') {
                Ext.form.Radio.superclass.setValue.call(this, v);
            } else {
                var r = this.getCheckEl().child('input[name=' + this.el.dom.name + '][value=' + v + ']', true);
                if(r){
                    Ext.getCmp(r.id).setValue(true);
                }
            }
            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);