The resonant longwire antenna is a standing wave antenna, because it is unterminated at the far end. A signal propagating from the feedpoint, toward the open end, will be reflected back toward the source when it hits the open end. The interference between the forward and reflected waves sets up stationary standing current and voltage waves along the wire.
A nonresonant longwire is terminated at the far end in a resistance equal to its characteristic impedance. Thus, the incident waves are absorbed by the resistor, rather than being reflected. Such an antenna is called a traveling wave antenna. Figure 9-8 shows a terminated longwire antenna.
The transmitter end is like the feed system for other longwire antennas, but the far end is grounded through a terminating resistor R1 that has a resistance R equal to the characteristic impedance Zo of the antenna (i.e., R = Zo). When the wire is 20 to 30 ft above the ground, Zo is about 500 to 600 Ω.
The radiation pattern for the terminated longwire is a unidirectional version of the multilobed pattern found on the unterminated longwires. The angles of the lobes vary with frequency, even though the pattern remains unidirectional.
The directivity of the antenna is partially specified by the angles of the main lobes. It is interesting to note that gain rises almost linearly with nλ, while the directivity function changes rapidly at shorter lengths (above three or four wavelengths the rate of change diminishes considerably).
Thus, when an antenna is cut for a certain low frequency, it will work at higher frequencies, but the directivity characteristic will be different at each end of the spectrum of interest.
A two-wavelength (2λ) pattern is shown in Fig. 9-7. There are four major lobes positioned at angles of ±36° from the longwire. There are also four minor lobes— the strongest of which is –5 dB down from the major lobes—at angles of ±75° from the longwire. Between all of the lobes, there are sharp nulls in which little reception is possible.
As the wire length is made longer, the angle of the main lobes pulls in tighter (i.e., toward the wire). As the lobes pull in closer to the wire, the number of minor lobes increases. At 5λ, there are still four main lobes, but they are at angles of ±22°from the wire. Also, the number of minor lobes increases to 16.
The minor lobes are located at ±47°, ±62°, ±72°, and ±83° with respect to the wire. The minor lobes tend to be –5 to –10 dB below the major lobes. When the longwire gets very much longer than 5λ, the four main lobes begin to converge along the length of the wire, and the antenna becomes bidirectional. This effect occurs at physical lengths greater than about 20λ.
In general, the following rules apply to longwire antennas:
• On each side of the antenna, there is at least one lobe, minor or major, for each half-wavelength of the wire element. For the overall element, there is one lobe for every quarter-wavelength.
• If there are an even number of lobes on either side of the antenna wire, then half of the total number of lobes are tilted backward, and half are tilted forward; symmetry is maintained.
• If there are an odd number of lobes on either side of the wire, then one lobe on either side will be perpendicular to the wire, with the other lobes distributed either side of the perpendicular lobe.
(source : Practical Antenna Handbook by Joseph J. Carr)
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