On July 1, 2020, the Extra Class question pool is being updated, making this post obsolete. See the corresponding post from my 2020 version of No Nonsense Extra Class License Study Guide.
E9H – Receiving Antennas: radio direction finding antennas; Beverage Antennas; specialized receiving antennas; longwire receiving antennas
Many antennas, such as the Beverage antenna, are designed to be receiving antennas only. When constructing a Beverage antenna, one of the factors that should be included in the design is that it should be one or more wavelengths long to achieve good performance at the desired frequency. (E9H01)
While directionality is a desirable feature for a receiving antenna, gain often is not, especially on the low bands. The reason for this is that atmospheric noise is so high that gain over a dipole is not important for low band (160 meter and 80 meter) receiving antennas. (E9H02)
Direction finding antennas
Direction finding is an activity that’s both fun and useful. One of the ways that it’s useful is to hunt down noise sources. It can also be used to hunt down stations causing harmful interference.
A variety of directional antennas are used in direction finding, including the shielded loop antenna. A receiving loop antenna consists of one or more turns of wire wound in the shape of a large open coil. (E9H09) The output voltage of a multi-turn receiving loop antenna be increased by increasing either the number of wire turns in the loop or the area of the loop structure or both. (E9H10)
An advantage of using a shielded loop antenna for direction finding is that it is electro-statically balanced against ground, giving better nulls. (E9H04) The main drawback of a wire-loop antenna for direction finding is that it has a bidirectional pattern. (E9H05)
Sometimes a sense antenna is used with a direction finding antenna. The function of a sense antenna is that it modifies the pattern of a DF antenna array to provide a null in one direction. (E9H08)
Another way to obtain a null in only one direction is to build an antenna array with a cardioid pattern. One way to do this is to build an array with two dipoles fed in quadrature. A very sharp single null is a characteristic of a cardioid-pattern antenna is useful for direction finding. (E9H11)
Another accessory that is often used in direction finding is an attenuator. It is advisable to use an RF attenuator on a receiver being used for direction finding because it prevents receiver overload which could make it difficult to determine peaks or nulls. (E9H07)
If more than one operator can be mobilized for a direction-finding operation, they could use the triangulation method for finding a noise source or the source of a radio signal. When using the triangulation method of direction finding, antenna headings from several different receiving locations are used to locate the signal source. (E9H06)