systems use the comparison of phase or doppler techniques which are generally simpler to automate. Modern pseudo-Doppler direction finder systems consist of a number of small antennas fixed to a circular card, with all of the processing performed by software. Early British radar sets were also referred to as RDF, which was a deception tactic. However, the terminology was not inaccurate; the Chain Home systems used separate omnidirectional broadcasters and large RDF receivers to determine the location of the targets. Antennas Direction finding requires an antenna that is directional (more sensitive in certain directions than in others). Many antenna designs exhibit this property. For example, a Yagi antenna has quite pronounced directionality, so the source of a transmission can be determined simply by pointing it in the direction where the maximum signal level is obtained. However, to establish direction to great accuracy requires more sophisticated technique. Null finding with loop antennas A simple form of directional antenna is the loop aerial. This consists of an open loop of wire on an insulating frame, or a metal ring that forms the antenna's loop element itself; often the diameter of the loop is a tenth of a wavelength or smaller at the target frequency. Such an antenna will be least sensitive to signals that are perpendicular to its face and most responsive to those arriving edge-on. This is caused by the phase of the received signal: The difference in electrical phase along the rim of the loop at any instant causes a difference in the voltages induced on either side of the loop. Turning the plane of the loop to "face" the signal so that the arriving phases are identical around the entire rim will not induce any current flow in the loop. So simply turning the antenna to produce a minimum in the desired signal will establish two possible directions (front and back) from which the radio waves could be arriving. This is called a null in the signal, and it is used instead of the strongest signal direction, because small angular deflections of the loop aerial away from its null positions produce much more abrupt changes in received current than similar directional changes around the loop's strongest signal orientation. Since the null direction gives a clearer indication of the signal direction – the null is "sharper" than the max – with loop aerial the null direction is used to locate