An isotropic Antenna is a theoretical antenna used as a reference for antenna gain. The antenna that has no gain in any direction is an isotropic antenna. One needs to know the feed point impedance of an antenna to match impedances for maximum power transfer from a feed line. The radiation resistance of an antenna is the value of a resistance that would dissipate the same amount of power as that radiated from an antenna. The total resistance of an antenna system is the radiation resistance plus ohmic resistance. Antenna efficiency is the term for the ratio of the radiation resistance of an antenna to the total resistance of the system. Antenna height and conductor length/diameter ratio, and location of nearby conductive objects are factors that determine the radiation resistance of an antenna. Antenna gain is the numerical ratio relating the radiated signal strength of an antenna in the direction of maximum radiation to that of a reference antenna. There are three questions in the pool asking to relate an antenna's gain to another antenna:
A folded dipole antenna is a dipole constructed from one wavelength of wire forming a very thin loop. Antenna efficiency calculated with the formula (radiation resistance / total resistance) x 100%. The efficiency of an HF quarter-wave grounded vertical antenna can be improved by installing a good radial system. The most important factor that determines ground losses for a ground-mounted vertical antenna operating in the 3-30 MHz range is soil conductivity. Antenna bandwidth is the frequency range over which an antenna satisfies a performance requirement. The orientation of its electric field (E Field) is determined by the free-space polarization of an antenna. In the antenna radiation pattern shown in Figure E9-1,
If a Yagi antenna is designed solely for maximum forward gain the front-to-back ratio decreases. If the boom of a Yagi antenna is lengthened and the elements are properly retuned, usually the gain increases. When a directional antenna is operated at different frequencies within the band for which it was designed the gain may exhibit significant variations. Assuming each antenna is driven by the same amount of power, there is no difference between the total amount of radiation emitted by a directional (gain) antenna compared with the total amount of radiation emitted from an isotropic antenna. It is just radiated in different directions. The approximate beamwidth of a directional antenna can be determined by noting the two points where the signal strength of the antenna is 3 dB less than maximum and compute the angular difference. The abbreviation NEC stands for Numerical Electromagnetics Code when applied to antenna modeling programs. A disadvantage of NEC-based antenna modeling programs is that the computing time increases as the number of wire segments is increased. A computer program using the technique called "Method of Moments" is commonly used for modeling antennas. The principle used by the "Method of Moments" analysis is modeling a wire as a series of segments, each having a distinct value of current. A disadvantage of decreasing the number of wire segments in an antenna model below the guideline of 10 segments per half-wavelength is that the computed feed-point impedance may be incorrect. All of the following answers are correct. The type of information that can be obtained by submitting the details of a proposed new antenna to a modeling program include:
The radiation pattern of two 1/4-wavelength vertical antennas spaced,
A basic rhombic antenna is bidirectional; four-sided, each side one or more wavelengths long; open at the end opposite the transmission line connection. The main advantages of a terminated rhombic antenna is wide frequency range, high gain and high front-to-back ratio. Disadvantages of a terminated rhombic antenna for the HF bands are the antenna requires a large physical area and 4 separate supports. The effect of a terminating resistor on a rhombic antenna is to change the radiation pattern from bidirectional to unidirectional. The type of antenna pattern over real ground shown in Figure E9-2 is elevation.
The far-field elevation pattern of a vertically polarized antenna mounted over seawater increases low-angle radiation compared to a rocky ground. The main effect of placing a vertical antenna over an imperfect ground is that it reduces low-angle radiation. The conductivity and dielectric constant of the soil in the area of the antenna strongly affects the shape of the far-field, low-angle elevation pattern of a vertically polarized antenna. For a Yagi with three elements mounted parallel to the ground, the electric field would be oriented horizontally. To achieve good performance at the desired frequency, a Beverage antenna should be one or more wavelengths long. When the operating frequency of a parabolic dish antenna is doubled, the gain increases 6 dB. One way to produce circular polarization when using linearly polarized antennas is to arrange two Yagis perpendicular to each other with the driven elements at the same point on the boom and fed 90 degrees out of phase. The beamwidth of an antenna decreases as the gain is increased. It is desirable for a ground-mounted satellite communications antenna system to be able to move in both azimuth and elevation in order to track the satellite as it orbits the earth. A loading coil is often used with an HF mobile antenna to cancel capacitive reactance. For a shortened vertical antenna, a loading coil be placed near the center of the vertical radiator to minimize losses and produce the most effective performance. An HF mobile antenna loading coil should have a high ratio of reactance to resistance to minimize losses. At the base feed-point of a fixed-length HF mobile antenna the resistance decreases and the capacitive reactance increases as the frequency of operation is lowered. The bandwidth of an antenna decreases as it is shortened through the use of loading coils. An advantage of using top loading in a shortened HF vertical antenna is improved radiation efficiency. An advantage of using a trapped antenna is that it may be used for multi-band operation. A disadvantage of using a multiband trapped antenna is that it might radiate harmonics. The approximate feed-point impedance at the center of a folded dipole antenna is 300 ohms. The type of conductor that would be best for minimizing losses in a station's RF ground system is a thin, flat copper strap several inches wide. A connection to 3 or 4 interconnected ground rods driven into the Earth would provide the best RF ground for your station. The delta matching system matches a high-impedance transmission line to a lower impedance antenna by connecting the line to the driven element in two places spaced a fraction of a wavelength each side of element center. The gamma match matches an unbalanced feed line to an antenna by feeding the driven element both at the center of the element and at a fraction of a wavelength to one side of center. The purpose of the series capacitor in a gamma-type antenna matching network is to compensate for the inductive reactance of the matching network. The Gamma match is an effective method of connecting a 50-ohm coaxial cable feed-line to a grounded tower so it can be used as a vertical antenna. To use a hairpin matching system, the driven element in a 3-element Yagi must be tuned with capacitive reactance. The equivalent lumped-constant network for a hairpin matching system on a 3- element Yagi is an L network. The parameter that best describes the interactions at the load end of a mismatched transmission line is the reflection coefficient. An SWR greater than 1:1 describes a mismatched transmission line. The stub match uses a short perpendicular section of transmission line connected to the feed line near the antenna. An effective way of matching a feed-line to a VHF or UHF antenna when the impedances of both the antenna and feed-line are unknown is to use the "universal stub" matching technique. An effective way to match an antenna with a 100-ohm terminal impedance to a 50-ohm coaxial cable feed-line is to insert a 1/4-wavelength piece of 75-ohm coaxial cable transmission line in series between the antenna terminals and the 50-ohm feed cable. The primary purpose of a "phasing line" when used with an antenna having multiple driven elements is to ensure that each driven element operates in concert with the others to create the desired antenna pattern. The purpose of a "Wilkinson divider" is to divide power equally among multiple loads while preventing changes in one load from disturbing power flow to the others. The physical length of a coaxial cable transmission line is shorter than its electrical length because electrical signals move more slowly in a coaxial cable than in air. Velocity factor is the term for the ratio of the actual speed at which a signal travels through a transmission line to the speed of light in a vacuum. Dielectric materials used in the line determine the velocity factor in a transmission line. The velocity factor of a transmission line is the velocity of the wave in the transmission line divided by the velocity of light in a vacuum. The typical velocity factor for a coaxial cable with solid polyethylene dielectric is 0.66. The formula for the electrical length of a transmission line is
A 450-ohm ladder line at 50 MHz, as compared to 0.195-inch diameter coaxial cable (such as RG-58) has lower loss. A transmission line presents the characteristics shown in the table to a generator when the line is either open or shorted at the far end,
All of the following answers are correct. The primary difference between foam-dielectric coaxial cable as opposed to solid-dielectric cable, assuming all other parameters are the same, include:
Impedance along transmission lines can be calculated using a Smith chart. The coordinate system used in a Smith chart is resistance circles and reactance arcs. Impedance and SWR values in transmission lines are often determined using a Smith chart. The two families of circles and arcs that make up a Smith chart are resistance and reactance. The chart shown in Figure E9-3 is a Smith chart. On the Smith chart shown in Figure E9-3,
The wavelength scales on a Smith chart are calibrated in fractions of transmission line electrical wavelength. The normalization process with regard to a Smith chart is the reassigning impedance values with regard to the prime center. The term that describes station output (including the transmitter, antenna and everything in between), when considering transmitter power and system gains and losses, is effective radiated power. There are three questions asking to compute the Effective Radiated Power (ERP) of transmitter given the feed line loss, duplexer/circulator loss and antenna gain. The losses and gains in dB can simply be added together to get the gain of the system,
The effective radiated power is then computed with the formula,
The triangulation method of direction finding involves using antenna headings from several different receiving stations to locate the signal source. The main drawback of a wire-loop antenna for direction finding is that it has a bidirectional pattern. The function of a sense antenna is to modify the pattern of a DF antenna array to provide a null in one direction. An antenna with a cardioid pattern is desirable for a direction-finding system because the response characteristics of the cardioid pattern can assist in determining the direction of the desired station. An RF attenuator is desirable in a receiver used for direction finding because it prevents receiver overload from extremely strong signals. A receiving loop antenna is one or more turns of wire wound in the shape of a large open coil. The output voltage of a receiving loop antenna can be increased by increasing either the number of wire turns in the loop or the area of the loop structure. An advantage of using a shielded loop antenna for direction finding is that it is electro-statically balanced against ground, giving better nulls. |