a system for reading from and for writing data into electronic tags which may be associated with objects which are moving with respect to an interrogator , or may be located in fixed positions in the path of a moving interrogator . the interrogator , which is stationary if the tag is moving , sends a continuous rf signal to the remote tag . the tag backscatter - modulates the received rf signal with data temporarily or permanently stored in the tag , including , for example , data associated with the object to which the tag is attached , for example , its identity or contents . the system of this invention uses a very efficient encoding technique for the data which is backscatter - modulated by the tag and received by the interrogator . the backscatter - modulated signals are made up of signals of first and second frequencies f 1 and 2f 1 , respectively , where the second frequency 2f 1 is twice the first frequency f 1 , and where one of the two binary bits one and zero is represented by one - half period of a signal of the first frequency followed by one period of the second frequency , and the other of the two binary bits is represented by one period of a signal of the second frequency followed by one - half period of a signal of the first frequency . the signals also contain frame markers made up of five periods of frequency 2f 1 followed by one - half period of frequency f 1 .