of a wavelength away. When this signal was mixed in, the opposite-phase signal from this aerial would strongly suppress the signal when the phase is in the direction of the sense aerial. This signal was sent into the brightness channel, or Z-axis, of the oscilloscope, causing the display to disappear when the signals were out of phase. By connecting the sense aerial to one of the loops, say the north/south channel, the display would be strongly suppressed when it was on the lower half of the display, indicating that the signal is somewhere to the north. At this point the only possible bearing is the north-east one. The signals received by the antennas are very small and at high frequency, so they are first individually amplified in two identical radio receivers. This requires the two receivers to be extremely well balanced so that one does not amplify more than the other and thereby change the output signal. For instance, if the amplifier on the north/south antenna has slightly more gain, the dot will not move along the 45 degree line, but perhaps the 30 degree line. To balance the two amplifiers, most set-ups included a "test loop" which generated a known directional test signal. For shipboard systems, the ship's superstructure presented a serious cause of interference, especially in phase, as the signals moved around the various metal obstructions. To address this, the ship was anchored while a second ship broadcast a test signal from about one mile away, and the resulting signals were recorded on a calibration sheet. The broadcast ship would then move to another location and the calibration would be repeated. The calibration was different for different wavelengths as well as directions; building a complete set of sheets for each ship required significant work. Naval units, notably the common HF4 set, included a rotating plastic plate with a line, the "cursor", used to help measure the angle. This could be difficult if the tips of the ellipse did not reach the edge of the display, or went off it. By aligning the cursor with the peaks at either end, this became simple. Hash marks on either side of the cursor allowed measurement of the width of the display, and use that to determine the amount of blurring. See also Operation RAFTER – remotely confirming that a superhet radio receiver is listening to a certain frequency Wullenweber - "Elephant