General Description. The R-X Noise Bridge contains a wideband noise generator and an r-f impedance bridge. Two arms of the bridge are driven equally by the noise generator through a broadband ferrite transformer. A third leg of the bridge has a calibrated variable resistor R and a calibrated variable capacitor C in series. The antenna or other "Unknown" circuit to be measured is connected as the fourth leg of the bridge. A short-wave receiver is used as the detector.