Wireless electronic devices may include a transceiver, an antenna resonating element coupled to the transceiver via a transmission line path, transceiver and antenna impedance matching circuits, and other circuitry. The transceiver and the impedance matching circuits may be formed on a first substrate. The antenna resonating element may be formed using a second substrate. The antenna resonating element may be decoupled from the first substrate during testing. First and second sets of test points may be formed at first and second locations long the transmission line path. During testing, a test probe may mate with the first set of test points, whereas an impedance adjustment circuit that serves to electrically isolate the antenna impedance matching circuit from the transceiver may mate with the second set of test points. The impedance adjustment circuit need not be used if the antenna impedance matching circuit is decoupled from the transceiver during testing.