Many antennas, such as the Beverage antenna, are designed to be receiving antennas only. When constructing a Beverage antennas, make sure that it is one or more wavelengths long to achieve good performance at the desired frequency.
The Beverage antenna is a directional antenna, but it has no gain when compared to a dipole. 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.
QUESTION: When constructing a Beverage antenna, which of the following factors should be included in the design to achieve good performance at the desired frequency? (E9H01)
ANSWER: It should be one or more wavelengths long
QUESTION: Which is generally true for low band (160 meter and 80 meter) receiving antennas? (E9H02)
ANSWER: Atmospheric noise is so high that gain over a dipole is not important
Another type of low-noise receiving antenna is the pennant antenna. They are called this because they consist of a triangular loop that looks like a pennant. They are smaller than Beverages, and some designs are even rotatable.
QUESTION: What is a Pennant antenna? (E9H09)
ANSWER: A small, vertically oriented receiving antenna consisting of a triangular loop
The ratio of forward gain to the average gain over the hemisphere in which a directional antenna is pointing is a good measure of how well a receiving antenna works. This is called the receiving directivity factor (RDF). Pennant antennas have an RDF of about 7.5 dB, which a one-wavelength Beverage antenna has an RDF of about 8.6 dB.
QUESTION: What is Receiving Directivity Factor (RDF)? (E9H03)
ANSWER: Forward gain compared to average gain over the entire hemisphere
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 receiving loop antennas. The main drawback of a wire-loop antenna for direction finding is that it has a bidirectional pattern. Another disadvantage of a receiving loop antenna is that it can have a low output, but the output voltage of a multi-turn receiving loop antenna can be increased by increasing either the number of wire turns in the loop or the area of the loop.
QUESTION: What is the main drawback of a small wire-loop antenna for direction finding? (E9H05)
ANSWER: It has a bidirectional pattern
QUESTION: How can the output voltage of a multiple-turn receiving loop antenna be increased? (E9H10)
ANSWER: By increasing the number of turns and/or the area
Another way to improve the performance of a loop antenna is to place a grounded electrostatic shield around the loop. This improves the nulls by eliminating unbalanced capacitive couple to the antenna’s surroundings.
QUESTION: What is an advantage of placing a grounded electrostatic shield around a small loop direction-finding antenna? (E9H04)
ANSWER: It eliminates unbalanced capacitive coupling to the surroundings, improving the nulls
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 direction-finding (DF) antenna array to provide a null in one direction.
QUESTION: What is the function of a sense antenna? (E9H08)
ANSWER: It modifies the pattern of a DF antenna array to provide a null in one direction
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. An antenna with a cardioid pattern has a very sharp single null that makes it useful for direction finding.
QUESTION: What feature of a cardioid pattern antenna makes it useful for direction finding? (E9H11)
ANSWER: A very sharp single null
An accessory that is often used in direction finding is an attenuator. Attenuation is useful when direction finding because it prevents receiver overload which reduces the sharpness of the nulls, and thereby, the accuracy of the directional antenna.
QUESTION: Why is RF attenuation used when direction-finding? (E9H07)
ANSWER: To prevent receiver overload which reduces pattern nulls
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 triangulating a signal source, operators use antenna headings from several different receiving locations to locate it.
QUESTION: What is the triangulation method of direction finding? (E9H06)
ANSWER: Antenna headings from several different receiving locations are used to locate the signal source