The receiver’s antenna jack connects to the RF summer output and the external speaker connects to the audio input on the direction finder. Series 5900/6000/6100 contain loudspeakers for monitoring the signal. Series 7000 has an audio output jack for connecting a loud speaker.  Note that some receivers use an isolated external speaker output that contains a DC voltage. In our Series 5900/6000/6100 the audio input is single ended, so you will need to provide transformer or capacitive isolation.  Our series 7000 direction finders are capacitively isolated so no additional hardware is required.