Philco Multiple Aerial Switch, Philco Parts Catalogue 1938

This little device is a simple current sensitive relay that senses the flow of AC current when the associated radio is turned on. A relay inside then connects the antenna feed line to the set's antenna and ground terminals. When the set is turned off, the relay drops out and disconnects the set from antenna and ground. It solves the two radio, one antenna problem. Each set can be automatically connected to the antenna. Only one set at a time could be used at a time, since local oscillator interaction would occur when two or more sets were connected to a single antenna.

Method of connecting multiple aerial switches to radio sets.
This pictorial diagram shows how two radios would be connected in a home or in a dealer's showroom for demonstration purposes. The device is plugged into the wall outlet and the radio is plugged into the switch box. Next, the output ANT/GND leads from the switch box are connected to the radio's terminals. Then the balanced feeders from the antenna are connected to each set of ANT/GND terminals on the switch box, or boxes.