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480 Mobile, emergency, portable, and marine antennas you must limit antenna size to 8 or 9 ft; in the amateur radio bands the 8-ft antenna is most popular because it is resonant on 10 m At all lower frequencies, the 8-ft whip becomes capacitive, and therefore requires an equal inductive reactance to cancel the capacitive reactance of the antenna Figure 25-1 shows three basic configurations of coil-loaded HF antennas for frequencies lower than the natural resonant frequency of the antenna In each case, the antennas are series fed with coaxial cable from the base; point A is connected to the coaxial cable center conductor, and point B is connected to the shield, and the car body (which serves as ground) The system shown in Fig 25-1A is base-loaded Although convenient, there is some evidence that the current distribution is less than optimum The version shown in Fig 25-1B is center-loaded, and results in an improved current distribution This configuration is probably the most common among commercially available HF mobile antennas, although the coil is often located slightly above the center point Finally, we see the top-loaded coil system in Fig 25-1B In all three cases, the point is to use the inductance of the coil to cancel the capacitive reactance of the antenna A modified version of the coil-loaded HF mobile antenna is shown in Fig 25-2A In this configuration the loading inductance is divided between two individual coils, L1 and L2 Coil L1 is adjustable with respect to the antenna, while L2 is fixed; coil L2 is tapped, however, in order to match the impedance of the antenna to the charac-
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25-1 Loaded mobile antennas
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Mobile HF antennas 481
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25-2A Practical mobile antenna for HF bands below 10 m; L1 resonates, L2 matches impedance
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teristic impedance of the coaxial-cable transmission line When tuning this antenna, two instruments are needed: a field strength meter and a VSWR meter The field strength meter gives a relative indication of the amount of power radiated from the antenna, while the VSWR meter helps determine the state of the impedance match A pair of relative field strength meter projects are discussed later in this chapter Variations on the theme are shown in Figs 25-2B and 25-2C A helical whip is shown in Fig 25-2B In this configuration, the inductor is distributed along the length of a fiberglass antenna rod The conductor is a wire that is helically wound on the fiberglass shaft An adjustable tip sets the antenna to resonance The length of this tip can be set by a local field strength meter Fixed versions of this type of antenna are very popular on the Citizens Band where they may be as short as 30 in, or as long as 48 in There are also some amateur radio commercial antennas based on this concept The other variant is shown in Fig 25-2C In this case, the lower end of the radiator consists of a metal tube topped with a loading coil An adjustable shaft at the top end is used to tune the antenna to resonance This form of antenna is popular among commercial makers of HF mobile antennas, such as the Hustler The fixed shaft tends to be universal for all bands, while the coil and adjustable shaft form a separate resonator for each band Multiband operation of this form of antenna can be accomplished by using a bracket such as shown in Fig 25-2D Although early versions of this scheme were homebrewed, several manufacturers currently make factory-built versions The idea in Fig 25-2D is to mount two or more resonators to a common fixed shaft
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482 Mobile, emergency, portable, and marine antennas
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Adjustable tip Adjustable shaft Loading coil
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Helically wound wire embedded in fiberglass
A B A B 25-2C Top-loaded mobile antenna common in amateur radio
25-2B Helically wound mobile antenna
Resonator #2 Resonator #1
25-2D Multiband toploaded antenna Resonator #3
Bracket
Fixed shaft
Mobile HF antennas 483 A common problem with all coil-loaded mobile antennas is that they tend to be very high Q antennas In other words, they are very sharply tuned The VSWR tends to rise rapidly as the operating frequency departs from the frequency to which the antenna is tuned; as little as 25-kHz change of operating frequency will detune the antenna significantly Although an antenna tuner at the output of the transmitter will reduce the VSWR to a point that allows the transmitter to operate, that type of tuner is merely a line flattener that does not fully address the problem The actual problem is that the antenna is not resonant The efficiency of the antenna drops off rapidly as the frequency changes The only cure for this problem is to readjust the resonator s adjustable shaft as the band segment is changed (not merely the band, but the band segment) Unfortunately, this solution requires tools, and the operator must get out of the vehicle in order to do a good job of retuning Another solution is to use a motor-driven variable inductor for the loading coil Several manufacturers offer both base-loaded and center-loaded coils that are either motor-driven or relay-selectable to permit frequency changing from the operator s seat
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