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VHF/UHF antenna examples 353 is sometimes used to support pole lamps, etc The mast should be attached to the boom at the center of gravity, which is also known as the balance point If you try to balance the antenna in one hand unsupported, there is one (and only one) point at which it is balanced (and won t fall) Attach the mast hardware at, or near, this point in order to prevent normal gravitational torques from tearing the mounting apart The antenna is fed with coaxial cable at the center of the driven element Ordinarily, either a matching section of coax, or a gamma match, will be needed because the effect of parasitic elements on the driven element feedpoint impedance is to reduce it
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8 -wavelength 2-m antenna
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The 5 8-wavelength antenna (Fig 18-9) is popular on 2 m for mobile operation because it is easy to construct, and it provides a small amount of gain relative to a dipole The radiator element is 5 8-wavelength, so its physical length is found from: L 7380 in FMHz 
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The 5 8-wavelength antenna is not a good match to any of the common forms of coaxial cables Either a matching section of cable, or an inductor match, is normally used In Fig 18-9 an inductor match is used The matching coil consists of 2 to 3 turns of no 12 wire, wound over a 1 2-in OD form, 1 2-in long The radiator element can be tubing, brazing rod, or a length of heavy piano wire Alternatively, for low-power systems, it can be a telescoping antenna that is bought as a replacement for portable radios or televisions These antennas have the advantage of being adjustable to resonance without the need for cutting
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The J-pole antenna is another popular form of vertical on the VHF bands It can be used at almost any frequency, although the example shown in Fig 18-10 is for 2 m The antenna radiator is 3 4-wavelength long, so its dimension is found from 8838 L in  FMHz and the quarter-wavelength matching section length from L= 2952 in FMHz
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Taken together the matching section and the radiator form a parallel transmission line with a characteristic impedance that is 4 times the coaxial cable impedance If 50- coax is used, and the elements are made from 05 in OD pipe, then a spacing of 15 in will yield an impedance of about 200 Impedance matching is accomplished by a gamma match consisting of a 25-pF variable capacitor, connected by a clamp to the radiator, about 6 in (experiment with placement) above the base
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354 VHF/UHF transmitting and receiving antennas
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8-wavelength 2-m antenna
#12 Wire 2 to 3 turns 05 in dia 05 in long
5 ; LFT 8 LIN
615 FMHz 7380 FMHz
VHF/UHF antenna examples 355
/4 Matching section
C D B
D SO-239 18-10 J-pole antenna
356 VHF/UHF transmitting and receiving antennas
The groundplane antenna is a vertical radiator situated above an artificial RF ground consisting of quarter-wavelength radiators Groundplane antennas can be either 1 4-wavelength or 5 8-wavelength (although for the latter case impedance matching is needed see the previous example) Figure 18-11 shows how to construct an extremely simple groundplane antenna for 2 m and above The construction is too lightweight for 6-m antennas (in general), because the element lengths on 6-m antennas are long enough to make their weight too great for this type of construction The base of the antenna is a single SO-239 chassis-type coaxial connector Be sure to use the type that requires four small machine screws to hold it to the chassis, and not the single-nut variety The radiator element is a piece of 3 16-in or 4-mm brass tubing This tubing can be bought at hobby stores that sell airplanes and other models The sizes quoted just happen to fit over the center pin of an SO-239 with only a slight tap from a lightweight hammer and I do mean slight tap If the inside of the tubing and the connector pin are pretinned with solder, then sweat soldering the joint will make a good electrical connection that is resistant to weathering Cover the joint with clear lacquer spray for added protection The radials are also made of tubing Alternatively, rods can also be used for this purpose At least four radials are needed for a proper antenna (only one is shown in Fig 18-11) This number is optimum because they are attached to the SO-239 mounting holes, and there are only four holes Flatten one end of the radial, and drill a small hole in the center of the flattened area Mount the radial to the SO-239 using small hardware (4-40, etc) The SO-239 can be attached to a metal L bracket While it is easy to fabricate such a bracket, it is also possible to buy suitable brackets in any well-equipped hardware store While shopping at one do-it-yourself type of store, I found several reasonable candidate brackets The bracket is attached to a length of 2 2-in lumber that serves as the mast