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Gain in antennas is provided by directivity In other words, by taking the power radiated by the antenna, and projecting it into a limited direction, we obtain the appearance of higher radiated power In fact, the effective radiated power (ERP) of the antenna is merely its feedpoint power multiplied by its gain Although most antenna patterns are shown in the horizontal dimension (as viewed from above), it is also possible to obtain gain by compressing the vertical aspect In this manner it is possible to have a vertical antenna that produces gain Figure 18-6 shows a collinear gain antenna, with vertical polarization and a horizontally omnidirectional pattern Incidentally, when mounted horizontally the pattern becomes bidirectional The collinear antenna shown in Fig 18-6 is basically a pair of stacked collinear arrays Each array consists of a quarter-wavelength section A and a half-wavelength section C separated by a quarter-wavelength phase reversing stub B The phase reversal stub preserves in-phase excitation for the outer element (referenced to the inner element) The feedpoint is between the two elements of the array (ie, between the A sections) The coaxial-cable impedance is transformed by a 4:1 balun transformer (see Fig 18-1A) Alternatively, 300- twin lead can be used for the transmission line If this alternative is used, then the use of UHF shielded twin lead is highly recommended If the transmitter lacks the balanced output needed to feed twin lead, then use a balun at the input end of the twin lead (ie, right at the transmitter)
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The Yagi beam antenna is a highly directional gain antenna, and is used both in HF and VHF/UHF systems The antenna is relatively easy to build at VHF/UHF In fact, it is easier than for HF systems The basic Yagi was covered in Chap 12, so we will only show examples of practical VHF devices A 6-m Yagi antenna is shown in Fig 18-7 This particular antenna is a four-element model The reflector and directors can be mounted directly to a metallic boom, because they are merely parasitic The driven element, however, must be insulated from the metal boom The driven element shown in Fig 18-7 is a folded dipole While this is common practice at VHF, because it tends to broadband the antenna, it is not strictly necessary The dimensions of the driven element are found from Eq 184 Set the equation equal to 300 , select the diameter of the tubing from commercially available sources, and then calculate the spacing Example 18-2 Calculate the spacing of a 300- folded dipole when 3 4-in tubing is used in its construction
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350 VHF/UHF transmitting and receiving antennas
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B 1 in 195 in 19 in A VHF balun 4:1 A 19 in 195 in 1 in B Coax to XMTR 18-6 Vertical collinear antenna
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VHF/UHF antenna examples 351
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Director No 2
1448 cm
267 m
Director No 1
1169 cm
279 m Driven element 4:1 VHF balun
1168 cm Coax to XMTR
Reflector 295 m
18-7 Six-meter beam antenna
Solution: Zo = 276 log10 300 300 276 108 10108 Using 075-in pipe results in 10108 1202 so S 45 in 2S 075 267S 276 log10 log10 log10 2S d 2S d 2S d 2S d 2S d
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F E D C B A
30 cm
30 cm
305 cm
31 cm
50 cm
Driven element
Director no 3
Director no 2
Director no 4
Director no 1
2 2 Wood
Coax (see text)
Mast Tie wire Solder A:102 cm B:98 cm C:915 cm D:90 cm E:89 cm F:88 cm 2 2 Boom
Solder
18-8 Two-meter vertical beam
Two-meter yagi
Figure 18-8 shows the construction details for a six-element 2-meter Yagi beam antenna This antenna is built using a 2 2-in wooden boom and elements made of either brass or copper rod Threaded brass rod is particularly useful, but not strictly necessary The job of securing the elements (other than the driven element) is easier when threaded rod is used, because it allows a pair of hex nuts, one on either side of the 2 2-in boom, to be used to secure the element Nonthreaded elements can be secured with RTV sealing a press-fit Alternatively, tie wires (see inset to Fig 18-8) can be used to secure the rods A hole is drilled through the 2 2 to admit the rod or tubing The element is secured by wrapping a tie wire around the rod on either side of the 2 2, and then soldering it in place The tie wire is no 14 to no 10 solid wire Mounting of the antenna is accomplished by using a mast secured to the boom with an appropriate clamp One alternative is to use an end-flange clamp, such as
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