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Conclusion 201 the antenna, and an antenna tuner This method of feed is common on AM broadcast antennas (which are usually perhaps always verticals) Although you would think that the sloping feed wire would distort the pattern, that is not the case The distortion of the pattern, if any, is minimal, hence it can be neglected The gamma feed system is shown in Fig 7-15B This method is commonly used by amateurs to feed Yagi beam antennas, so it is quite familiar in the amateur radio world The feed system consists of a variable capacitor to tune the system, and a matching rod that parallels the antenna radiator element It is important that the rod not be anywhere near a quarter-wavelength, or it would become a vertical antenna in its own right, and in fact would resemble the so-called J-pole antenna A review of the gamma match is given in Chap 12 The omega feed (shown in Fig 7-15C) is similar to the gamma match except that a shunt capacitor is used
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The vertical antenna is a viable alternative for many situations, especially where real estate is at a premium Contrary to popular opinion, the vertical antenna works well when installed properly and when due consideration has been given to matters such as the grounding and angle of radiation desired
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202 Vertically polarized HF antennas
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C2 C1
CHAPTER
Multiband and tunable-wire antennas
MOST COMMUNICATIONS OPERATORS REQUIRE MORE THAN ONE BAND, AND THAT MAKES
the antenna problem exactly that a problem to be solved Amateur radio, commercial, and military operators are especially likely to need either multiple antennas for different bands, or a multiband antenna that operates on any number of different bands This situation is especially likely on the high-frequency (HF) bands from 35 to 297 MHz Another problem regards the tunability of an antenna Some amateur bands are very wide (several hundred kilohertz), and that causes any antenna to be highly variable from one end of the band to another It is typical for amateurs to design an antenna for the portion of the band that they use most often, and then tolerate a high VSWR at the other frequencies Unfortunately, when you see an antenna that seems to offer a low VSWR over such a wide range, it is almost certain that some problem exists that reduces the Q, and the antenna efficiency, to broaden the response However, it is possible to tune an antenna for a wide band It is also possible (now that amateurs have new HF bands) to use a single antenna between them, and then tune the difference out For example, designing a single antenna for 21/24 MHz, 14/18 MHz, or 7/10 MHz should prove possible In this chapter we will take a look at both problems: the multiband and the tunable antenna
Multiband antennas
Although a triband Yagi or quad beam antenna will undoubtedly work better than a wire antenna (when installed correctly!), the low-budget amateur operator need not lament any supposed inability to get out on wire antennas To quote an old saying: Better is the enemy of good enough Or, to put it in terms of Carr s law: If it s good enough then don t waste a lot of energy fretting over making it better unless you really want to make it a lot better
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204 Multiband and tunable-wire antennas
Trap dipoles
Perhaps the most common form of multiband wire antenna is the trap dipole shown in Fig 8-1A In this type of antenna, one (or more) pairs of parallel resonant traps are placed in series with the quarter-wavelength elements of the dipole The purpose of the traps is to block their own resonant frequency, while passing all other frequencies In the example of Fig 8-1A, a 10-m trap isolates the first 8 ft or so (quarterwavelength on 10 m) so that the antenna resonates on that band A 40-m or 15-m signal, on the other hand, passes through the traps and uses the whole length of the antenna (Note: A half-wavelength 40-m dipole works as a 3/2-wavelength antenna on 15 m) The overall length of the trap dipole will be a little less than the natural nontrap length for the lowest frequency of operation At the low frequencies, the traps add a little inductance to the circuit so that the resonant point is lower than the natural resonant frequency In general, most trap dipoles are just a few percent shorter than nontrap dipoles at the same band The actual amount of shortening depends upon the values of the components in the traps, so consult the data for each trap purchased Where more than one pair of traps is used in the antenna, make sure they are of the same brand and are intended to work together Another solution to this problem is shown in Fig 8-1B This type of antenna actually has two or more half-wavelength dipoles fed from the same transmission line In this illustration, a total of three dipoles are fed from the same 75- transmission line There is no theoretical limit to how many dipoles can be accommodated, although there is certainly a practical limit For one thing, there is a mechanical limit to how many wires are supportable (or desirable) hanging from any given support There is also an electrical limit, although it is less defined Having a lot of dipoles increases the possibility of radiating harmonics and other spurious emissions from your transmitter The 75- coaxial cable is connected to the center feedpoint of the multidipole either directly or through a 1:1 balun transformer, as shown in Fig 8-1B Each an-
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