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VHF/UHF antenna impedance matching
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The VHF/UHF antenna is no more or less immune from the need for impedance matching than lower-frequency antennas However, some methods are easier (coax baluns, delta match, etc) and others become either difficult or impossible An example of the latter case is the tuned LC impedance-matching network At 6 m, and even to some limited extent 2 m, inductor and capacitor LC networks can be used But above 2 m other methods are more reasonable We can, however, mimic the LC tuner by using stripline components, but that approach is not always suited to amateur needs The balun transformer makes an impedance transformation between balanced and unbalanced impedances Although both 1:1 and 4:1 impedance ratios are possible, the 4:1 ratio is most commonly used for VHF/UHF antenna work At lower frequencies it is easy to build broadband transformer baluns, but these become more of a problem at VHF and above For the VHF/UHF frequencies, a 4:1 impedance ratio coaxial balun (Fig 18-1A) is normally used Two sections of identical coaxial cable are needed One section (A) has a convenient length to reach between the antenna and the transmitter Its characteristic impedance is Zo The other section (B) is a half-wavelength long at the center of the frequency range of interest The physical length is found from L= where L is the cable length, in inches FMHz is the operating frequency, in megahertz V is the velocity factor of the coaxial cable The velocity factors of common coaxial cables are shown in the following table
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5904 V FMHz
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342 VHF/UHF transmitting and receiving antennas
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18-1A Coaxial 4:1 balun transformer
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Example 18-1 Calculate the physical length required of a 146-MHz 4:1 balun made of polyethylene foam coaxial cable Solution: 5904 V L FMHz
VHF/UHF antenna impedance matching 343 = = (5904) (080) 146 MHz 47232 = 324 in 146 MHz
A mechanical method of joining the coaxial cables is shown in Fig 18-1B In this example, three SO-239 coaxial receptacles are mounted on a metal plate This arrangement has the effect of shorting together the shields of the three ends of coaxial cable The center conductors are connected in the manner shown This method is used especially where a mounting bracket is available on the antenna The lengths of coaxial cable need PL-259 coax connectors installed in order to use this method The delta match gets its name from the fact that the structure of the matching element has the shape of the Greek letter delta, or a triangle Figure 18-2A shows the basic delta match scheme The matching element is attached to the driven element of the antenna (symmetrically, about the center point of the antenna) The width (A) of the delta match is given by 1416 L= in [182] FMHz While the height of the match (B) is 1776 L= in [183] FMHz The transmission line feeding the delta match is balanced line, such as parallel transmission line or twin lead The exact impedance is not terribly critical because the dimensions (especially A) can be adjusted to accommodate differences In general, however, either 450- or 600- line is used, although 300- line can also be used Figure 18-2B
Cable ends or solder lugs
Solder Solder
Mounting plate SO-239 connectors 18-1B Practical implementation of 4:1 balun using connectors
344 VHF/UHF transmitting and receiving antennas
L A D Center line D
Delta match
468 FMHz 118 FMHz 148 FMHz 2D
600-
line
18-2A Delta feed matching system
Center
Driven element Metal rods
BALUN
18-2B Practical VHF delta match
VHF/UHF antenna impedance matching 345 shows a method for using coaxial cable with the delta match The impedance is transformed in a 4:1 balun transformer (see Fig 18-1A) The elements of the delta match can be made from brass, copper, or aluminum tubing, or a bronze brazing rod bolted to the main radiator element A stub-matching system is shown in Fig 18-3 In this case, the impedance transformation is accomplished despite the half-wavelength shorted stub of transmission line The exact impedance of the line is not very critical, and is found from Zo 276 log10 2S d [184]
The matching stub section is made from metal elements such as tubing, wire, or rods (all three are practical at VHF/UHF frequencies) For a 3 16-in rod, the spacing is approximately 256 in to make a 450- transmission line A sliding short circuit is used to set the electrical length of the half-wave stub The stub is tapped at a distance
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