The duplexer (a 4-port ferromagnetic circulator) provides isolation between receiver and transmitter. The City of Homestead operates two local circulator Homestead Trolley lines. Metrobus and the DC Circulator make scheduled stops near the Mall. FM-CW radars can be built with one antenna using either a circulator, or circular polarization. In addition, the Downtown Circulator, a proposed streetcar system, is in its planning stage. In practice the gain is limited by the backward "leakage" coupling between circulator ports. Washington, D.C.'s Circulator uses the A330 buses. One common circuit is a "reflection amplifier" which uses a circulator to separate the signals. Hartford's Downtown Area Shuttle (DASH) bus route is a free downtown circulator. Small AGT systems are also used as circulator or feeder systems within urban centers. In 1964, Mohr presented and experimentally demonstrated a circulator based on transmission lines and switches. The 72 Westview Circulator connects Memorial City with Spring Branch. This is analogous to the operation of an electronic circulator. Bowser responded by hailing her plan to make the DC Circulator free of charge. On August 23, 2017, the GLTC launched The Hopper, a free downtown circulator bus.