a signal source. A "sense antenna" is used to resolve the two direction possibilities; the sense aerial is a non-directional antenna configured to have the same sensitivity as the loop aerial. By adding the steady signal from the sense aerial to the alternating signal from the loop signal as it rotates, there is now only one position as the loop rotates 360° at which there is zero current. This acts as a phase reference point, allowing the correct null point to be identified, removing the 180° ambiguity. A dipole antenna exhibits similar properties, as a small loop, although its null direction is not as "sharp". Yagi antenna for higher frequencies The Yagi-Uda antenna is familiar as the common VHF or UHF television aerial. The basis for the Yagi design is the combinations of multiple dipole elements, which include "reflector" and "director" dipole elements which act on radio waves similar to mirrors and lenses for light waves. The "reflector" is the longest dipole element and blocks nearly all the signal coming from behind it, hence a Yagi has no front vs. back directional ambiguity: The maximum signal only occurs when the narrowest end of the Yagi is aimed in the direction from which the radio waves are arriving; with a sufficient number of shorter "director" elements, a Yagi's maximum direction can be made to approach the sharpness of a small loop's null. Parabolic antennas for extremely high frequencies For much higher frequencies still, such as millimeter waves and microwaves, parabolic antennas or "dish" antennas can be used. Dish antennas are highly directional, with the parabolic shape directing received signals from a very narrow angle into a small receiving element mounted at the focus of the parabola. Electronic analysis of two antennas' signals More sophisticated techniques such as phased arrays are generally used for highly accurate direction finding systems. The modern systems are called goniometers by analogy to WW II directional circuits used to measure direction by comparing the differences in two or more matched reference antennas' received signals, used in old signals intelligence (SIGINT). A modern helicopter-mounted direction finding system was designed by ESL Incorporated for the U.S. Government as early as 1972. Operation Radio Direction Finding works by comparing the signal strength of a directional antenna pointing in different directions. At first, this system was used by land and marine-based radio operators, using a simple rotatable loop antenna linked to