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There is a running controversy regarding how the antenna compares with other beam antennas, particularly the Yagi Some experts claim that the cubical quad has a gain of about 15 to 2 dB higher than a Yagi (with a comparable boom length between the two elements) In addition, some experts claim that the quad has a lower angle of radiation Most experts agree that the quad seems to work better at low heights above the earth s surface, but the difference disappears at heights greater than a half-wavelength The quad can be used as either a single-element antenna or in the form of a beam Figure 12-9 shows a pair of elements spaced 013 to 022 wavelengths apart One element is the driven element, and it is connected to the coaxial-cable feedline directly The other element is a reflector, so it is a bit longer than the driven element A tuning stub is used to adjust the reflector loop to resonance Because the wire is arranged into a square loop, one wavelength long, the actual length varies from the naturally resonant length by about 3 percent The driven
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element is about 3 percent longer than the natural resonant point The overall lengths of the wire elements are 1 Driven element: L 1005 ft FMHz 1030 ft FMHz 975 FMHz ft [125]
2 Reflector:
[126]
3 Director:
[127]
One method for the construction of the quad beam antenna is shown in Fig 12-10 This particular scheme uses a 12 12-in wooden plate at the center, bamboo (or fiberglass) spreaders, and a wooden (or metal) boom The construction must be heavy-duty in order to survive wind loads For this reason, it is probably a better solution to buy a quad kit consisting of the spreaders and the center structural element
Inverted bobtail curtain (Thorne array) 267 More than one band can be installed on a single set of spreaders The size of the spreaders is set by the lowest band of operation, so higher frequency bands can be accommodated with shorter loops on the same set of spreaders This quad antenna is an example of a multielement, large loop antenna Additional information on large loops, but not in a beam antenna array, is found in Chap 14
Inverted bobtail curtain (Thorne array)
The bobtail curtain antenna is a fixed array consisting of three individual quarterwavelength elements spaced a half-wavelength apart, and fed from the top by a
Boom 12-10 Quad construction
Mast
268 Directional beam antennas shorting element or wire The inverted bobtail curtain, or Thorne array, consists of an upside down bobtail curtain as shown in Fig 12-11 The radiator elements are each a quarter-wavelength long Their lengths are found from L 246 ft FMHz [128]
The lengths of spacing between the elements are exactly twice above the value, or L 492 FMHz ft [129]
The antenna is fed at the base of the center element, through a parallel resonant tuner The capacitor is a 100- to 200-pF transmitting variable, while the inductor is set to resonate at the band desired (with the capacitor at half to three-quarters full capacitance) A loop or link coupling scheme connects the tuner to the transmission line An alternate feed method (Fig 12-12) worked out by the late J H Thorne (K4NFU/5), feeds the end elements from the shield of the coaxial cable, and the center element of the array is fed from the center conductor of the coaxial cable A coaxial impedance-matching section is used between the cable transmitter and the antenna feedpoint
12-11 Inverted bobtail curtain
Inverted bobtail curtain (Thorne array) 269
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Matching section (75 )
Coax to XTMR (52 ) 12-12 Thorne array
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CHAPTER
Antennas for shortwave reception
ALTHOUGH MOST OF THIS BOOK ADDRESSES ANTENNAS FOR TRANSMITTING, THERE IS A certain body of material that pertains purely to receiving antennas This material also needs to be addressed, and that is the function of this chapter There are two readers in mind for this chapter First, and foremost, is the shortwave listener (SWL) Second, however, is the amateur radio operator who wants to either use a separate receiving antenna on the main station receiver or use an ancillary receiver (common among DXers)
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