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The toroidal transformer forms a broadbanded means for matching antenna impedance to the transmission line, or matching the transmission line to the transmitter The other matching methods (shown thus far) are frequency-sensitive, and must be readjusted whenever the operating frequency is changed even a small amount Although this problem is of no great concern to fixed-frequency radio stations, it is of critical importance to stations that operate on a variety of frequencies Figure 24-16A shows a trifilar transformer that provides a 1:1 impedance ratio, but it will transform an unbalanced transmission line (eg, coaxial cable) to a
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476 Impedance-matching in antenna systems balanced signal required to feed a dipole antenna Although it provides no impedance transformation, it does tend to balance the feed currents in the two halves of the antenna This fact makes it possible to obtain a more accurate figure-8 dipole radiation pattern in the horizontal plane Many station owners make it standard practice to use a balun at the antenna feedpoint The balun shown in Fig 24-16B is designed to provide the unbalanced to balanced transformation, while also providing a 4:1 impedance ratio Thus, a 300 folded dipole feedpoint impedance will be transformed to 75 unbalanced This type of balun is often included inside antenna tuners, including all three models shown in Figs 24-8A and 248B A variable (or at least settable) broadbanded transformer is shown in Fig 24-16C In this case the output winding is tapped, and the operator selects the correct tap needed to provide the desired impedance ratio The usual turns ratio criterion applies Another multiple-impedance transformer is shown in Fig 24-16D In this case, the operator can select impedance transformation ratios of 15:1, 4:1, 9:1 or 16:1 A commercial version of this type of transformer is shown in Fig 24-16E It is manufactured by Palomar Engineers, and is intended for vertical antenna feeding in the HF bands It will, however, work well on antennas other than simple verticals A top view of the vertical antenna feeding transformer is in Fig 24-16F
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24-16C Tapped BALUN R1
16:1 9:1 4:1 15:1 R2
24-16D Multi-impedance BALUN
24-16E Vertical antenna feeding transformer
24-16F Top view of Fig 24-16E
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CHAPTER
Mobile, emergency, portable, and marine antennas
MOBILE OPERATION OF RADIO COMMUNICATIONS EQUIPMENT DATES BACK TO ONLY A
little later than base station operation From its earliest times, radio buffs have attempted to place radio communications equipment in vehicles Unfortunately, twoway radio was not terribly practical until the 1930s, when the earliest applications were amateur radio and police radio (which used frequencies in the 17- to 20-MHz region) Over the years, the land mobile and amateur radio mobile operation has moved progressively higher in frequency because of certain practical considerations The higher the frequency, for example, the shorter the wavelength; and, therefore, the shorter a full-size antenna On the 11-m Citizens Band, for example, a quarterwavelength whip antenna is 102 in long, and for the 10-m amateur band only 96 in long At VHF frequencies, antennas become even shorter As a result, much mobile activity takes place in the VHF and UHF region The amateur 144-, 220-, and 440-MHz bands are popular because of several factors, not the least of which is the ease of making 1 4- and 5 8-wavelength antennas Because low-cost commercial antennas are available for these frequencies, however, we will not examine such antennas in this chapter Rather, we will concentrate on high-frequency antennas
Mobile HF antennas
High-frequency (HF) mobile operation requires substantially different antennas than VHF or UHF Quarter-wavelength antennas are feasible only on the 11- and 10-m bands, with some argument in favor of 13 m as well But by the time the frequency drops to the 21-MHz (15-m) band, the antenna size must be approximately 11 ft long, and that is too long for practical mobile operation Because of practical considerations,
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