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The basic half-wavelength dipole antenna is a very good performer, especially when cost is a factor The dipole yields relatively good performance for practically no in-
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170 High-frequency dipole and other doublet antennas vestment A standard half-wavelength dipole offers a bidirectional figure-8 pattern on its basic band (ie, where the length is a half-wavelength), and a four-lobe cloverleaf pattern at frequencies for which the physical length is 3 /2 Thus, a 40-m halfwavelength dipole produces a bidirectional pattern on 40 m, and a four-lobe cloverleaf pattern on 15 m The dipole is not easily multibanded without resorting to traps (covered in Chap 8) One can, however, tie several dipoles to the same center insulator or balun transformer Figure 6-24 shows three dipoles cut for different bands, operating from a common feedline and balun transformer: A1 A2, B1 B2, and C1 C2 Each of these antennas is a half-wavelength (ie, Lfeet = 468/FMHz) There are two points to keep in mind when building this antenna First, try to keep the ends spread a bit apart, and second, make sure that none of the antennas is cut as a half-wavelength for a band for which another is 3 /2 For example, if you make A1 A2 cut for 40 m, then don t cut any of the other three for 15 m If you do, the feedpoint impedance and the radiation pattern will be affected
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The longwire antenna is an end-fed wire more than 2 long It provides considerable gain over a dipole, especially when a very long length can be accommodated Although 75- to 80-m, or even 40-m longwires are a bit difficult to erect at most locations, they are well within reason at the upper end of the HF spectrum Low-VHF
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EI EI
Rope
A1 Rope EI B1 C1
C1 or 1:1 BALUN B2 C2
A2 EI Rope
EI Rope
EI Rope
Coax to receiver
6-24 Multiband dipole antenna
The counterpoise longwire 171 band operation is also practical Indeed, I know one fellow who lived in far southwest Virginia as a teenager, and he was able to get his family television reception for very low cost by using a TV longwire (channel 6) on top of his mountain There are some problems with longwires that are not often mentioned Two problems seem to insinuate themselves into the process First, the Zepp feed is a bit cumbersome (not everyone is enamored of parallel transmission line) Second, how do you go about actually grounding that termination resistor If it is above ground, then the wire to ground is long, and definitely not at ground potential for RF If you want to avoid both the straight Zepp feed system employed by most such antennas, as well as the resistor-grounding problem, then you might want to consider the counterpoise longwire antennas shown in Fig 6-25
4 Rope EI 4:1 balun transformer
4 R1 EI Rope
Radiator
Coax to transmitter EI R R1 end insulator rope termination resistor (noninductive)
6-25A Longwire with /4 radials used as counterpoise ground at feedpoint and terminating resistor Wire 2 long
4 Radial Rope EI
T1 4:1 balun transformer
Rope
EI R 75coaxial cable to receiver
end insulator rope support
6-25B Balun transformer feed for longwire antenna
172 High-frequency dipole and other doublet antennas A counterpoise ground is a structure that acts like a ground, but is actually electrically floating above real ground (and it is not connected to ground) A groundplane of radials is sometimes used as a counterpoise ground for vertical antennas that are mounted above actual earth ground In fact, these antennas are often called ground plane verticals In those antennas, the array of four (or more) radials from the shield of the coaxial cable are used as an artificial, or counterpoise, ground system In the counterpoise longwire of Fig 6-25A, there are two counterpoise grounds (although, for one reason or another, you might elect to use either, but not both) One counterpoise is at the feedpoint, where it connects to the cold side of the transmission line The parallel line is then routed to an antenna tuning unit (ATU), and from there to the transmitter The other counterpoise is from the cold end of the termination resistor to the support insulator This second counterpoise makes it possible to eliminate the earth ground connection, and all the problems that it might entail, especially in the higher end of the HF spectrum, where the wire to ground is of substantial length compared with 1 of the operating frequency A slightly different scheme used to adapt the antenna to coaxial cable is shown in Fig 6-25B In this case, the longwire is a resonant type (nonterminated) Normally, one would expect to find this antenna fed with 450- parallel transmission line But with a /4 radial acting as a counterpoise, a 4:1 balun transformer can be used to effect a reasonable match to 75- coaxial cable The radial is connected to the side of the balun that is also connected to the coaxial cable shield, and the other side of the balun is connected to the radiator element
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