Abstract:
An antenna assembly has a lower break assembly and an upper break assembly. Both have a center fed transmission line within a conductive tube, and both are mutually connectable for connecting the transmission lines to each other at a junction. When connected, the lower break assembly and upper break assembly form one pole of a center-fed dipole radiator without significant signal loss through the junction.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. application Ser. No. 60/481,534 filed Oct. 21, 2003. 

   FIELD OF THE INVENTION 
   The invention relates to dipole antennas that can transmit and/or receive in one or more frequency bands and can be broken down for easy transport. 
   DESCRIPTION OF THE RELATED ART 
   It is known to isolate reception on a mobile antenna for vehicles in the 30–88 MHz range by a combination of coaxial cable at a lower end of the antenna and a dipole formed of a linear wire radiator at an upper end of the antenna. The length of such an antenna requires that it be broken down for easy transport. A mating connector at the point where the coaxial cable connects to the wire enables such a break, even though the feed point for the dipole is not at the break. In other words, the break occurs in one of the radiators of the dipole. 
   A similar structure is also known for NTDR (near term digital radio) antennas in the 225–450 MHz range. One problem has been noted at higher frequencies, however. Conventional point-of-contact connectors between the radiator and the leads from the antenna are not good RF conductors. An improvement for antenna performance at higher frequencies has been found with the use of N or coaxial connectors in place of conventional point-of-contact connectors. 
   Multiband antennas are known where traps isolate resonance in different frequency ranges, most commonly the AM, FM and CB frequency ranges. But it is also known for antennas with two isolated bands to transmit signals to and from the radiator along two separate leads, one for each band. Sometimes a multiplexer or filter circuit is needed to isolate signals if the separate leads are fed to a common point. 
   But problems remain in known mobile antennas with connectors between the radiator and the mount, or with connectors between lower and upper ends of an antenna that breaks in a radiator. For example, multiband antennas with three or more frequency ranges may utilize more leads or transmission lines than can reasonably fit within existing connector housings. Higher power antennas generate more heat than can safely be handled by existing connections. Connectors become abraded with repeated twisting of one part relative to another, as for example, the motion that occurs when one connects upper and lower sections of an antenna at a break. Solutions to these problems have heretofore proven illusive. 
   SUMMARY OF THE INVENTION 
   According to the invention, an antenna assembly comprises a lower break assembly having a center fed transmission line within a conductive tube forming a lower portion of a dipole radiator, and an upper break assembly having a center-fed transmission line within one or more conductive tubes, forming an upper portion of a dipole radiator. The lower break assembly and upper break assembly are mutually connectable for connecting the transmission lines to each other at a junction. Thus, the lower portion and upper portion, when joined, form one pole of a center-fed dipole radiator. Preferably, the feed point for the dipole radiator is disposed away from the junction, more particularly, away from the lower and upper portions. 
   In one embodiment, the lower portion comprises a conductive sleeve and the upper portion comprises a conductive cylinder. The conductive sleeve and the conductive cylinder form the one pole of the dipole radiator in the junction, when the junction is assembled. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a cross sectional view of a first embodiment of a multiband antenna according to the invention. 
       FIG. 2  is a cross sectional view of the mount assembly of  FIG. 1 . 
       FIG. 3  is a cross sectional view of the base mount subassembly of  FIGS. 1 and 2 . 
       FIG. 4  is a cross sectional view of the spring mount assembly of  FIGS. 1 and 2 . 
       FIG. 5  is a cross sectional view of the lower section assembly of the whip assembly according to the invention. 
       FIG. 6  is an enlarged cross-sectional view of the coupler assembly and the area labeled VI in  FIG. 5 . 
       FIG. 7  is an isometric view with parts broken away of the upper spring holder of  FIG. 4  and a first embodiment of the coupler assembly of  FIG. 5 . 
       FIG. 8  is an enlarged cross-sectional view of the lower break assembly and the area labeled VIII in  FIG. 5 . 
       FIG. 9  is a cross sectional view of the upper section assembly according to the invention. 
       FIG. 10  is an enlarged cross sectional view of the upper break assembly and the area labeled X in  FIG. 9 . 
       FIG. 11  is an isometric view with parts broken away of the lower break assembly of  FIG. 8  and a first embodiment of the upper break assembly of  FIG. 10 . 
       FIG. 12  is an enlarged cross section view of the junction and the area labeled XII in  FIG. 1 . 
       FIG. 13  is an elevational view of the upper element tube with conductive sleeves in the upper section assembly of  FIG. 9   
       FIG. 14  is a schematic view of an isolation circuit according to the invention. 
       FIG. 15  is a schematic and electrical view of the dipole for the first band. 
       FIG. 16  is a schematic and electrical view of the dipoles for the third band. 
       FIG. 17   a  is a schematic and electrical view of one embodiment of the dipole for the second band. 
       FIG. 17   b  is a schematic and electrical view of a second embodiment of the dipole for the second band. 
       FIG. 18  is a cross sectional view of a second embodiment of a multiband antenna according to the invention. 
       FIG. 19  is an exploded view of the mount assembly of  FIG. 18 . 
       FIG. 20  is an isometric view with parts broken away of the male and female connectors between the mount assembly and the whip assembly of  FIG. 18 . 
       FIG. 21  is a bottom view of the mount assembly of  FIG. 18 . 
       FIG. 22  is a schematic diagram of the electrical circuit of the antenna of  FIG. 18 . 
       FIG. 23  is a cross sectional view of a third embodiment of a multiband antenna according to the invention. 
       FIG. 24  is an enlarged cross sectional view of the area numbered XXIV in  FIG. 23 . 
   

   DETAILED DESCRIPTION 
   The invention is illustrated in one or more embodiments of a mobile antenna. Looking first at  FIGS. 1–4 , a multiband antenna  10  comprises a mount assembly  12  and a whip assembly  14 . The mount assembly  12  comprises a base mount subassembly  13  and a spring mount assembly  15 . The base mount subassembly  13  comprises a hollow base cover mount  16  with an annular mounting flange  18 , and a hollow, generally cylindrical, base support  20 , having a matching annular flange  22 . The annular flanges  18 ,  22  are disposed facing each other with a plurality of mounting holes  24  in registry. The base cover mount  16  is secured to the base support  20  by fasteners  25  spaced between the mounting holes  24 , and preferably sealed by a gasket  27  or similar seal. The base cover mount  16  and base support  20  thus form an interior chamber  28 . A reinforcement ring  26  (also having a plurality of mounting holes  24 ) is received over the base support  20  with the holes in registry. The mounting holes  24  are all sized so that mounting bolts (not shown) can be utilized to secure the mount assembly  12  to a vehicle. 
   In this embodiment, two connectors  34 ,  36  are attached to and extend from the base cover mount  16 . Two cable leads  30 ,  32  extend from the two connectors  34 ,  36  into the interior chamber  28  to eventually electrically connect to two transmission lines in the whip  14 . A base cover  38 , preferably made of aluminum or other highly conductive material, has a mount portion  40  and a stepped insert portion  42 , which is received in the open end of the base support  20 . The base cover  38  is secured to the base support  20  by conventional means. In the illustrated embodiment, the base cover  38  mounts two connectors  44 ,  46 . The exterior of the mount portion  40  has cooling fins to radiate heat that may build up within the chamber  28 . 
   Looking now more closely at  FIG. 3 , it will be seen that the interior chamber  28  houses a cable choke  48  with leads running from the connectors  44 ,  46 . The cable choke  48  is preferably mounted to the base cover  38  and comprises windings on a ferrite core to attenuate undesirable currents from the whip assembly  14 . Other acceptable forms for the cable choke  48  may include coiling the leads and mounting ferrite beads over the leads. Also, the ferrite core can be linear or toroidal, as dimensions within the interior chamber  28  permit. Cooling fins  47  on the base cover  38  help dissipate heat generated in the cable choke  48 . The interior chamber  28  can also house filters as needed. For example, in this embodiment, leads  49 ,  51  from the cable choke  48  extend first to a high pass filter  50 , and then to a low pass filter  52 , separated from each other by an RF shield  53 . The two connectors  34 ,  36  connect to the low pass filter  52  and to the high pass filter  50 , respectively, by way of the leads  32 ,  30 . 
   Looking now more closely at  FIG. 4 , the spring mount assembly  15  comprises a lower spring holder  54 , a barrel spring  56 , and an upper spring holder  58 . The lower spring holder  54  comprises a hollow, generally cylindrical, body portion  60  that has an annular flange  62  at one end, centered on the longitudinal axis of the body portion. The annular flange  62  has several apertures at its periphery by which it is securely mounted to the mount portion  40  of the base cover  38 . The body portion  60  is secured within a lower end of the barrel spring  56 . Importantly, the interior chamber  28 , including, preferably, all connections leading to the interior chamber, is sealed against moisture. Thus, for example, a seal  59  can be provided between the annular flange  62  and the body portion  40  of the base cover. 
   The upper spring holder  58  comprises a lower body portion  64 , a hex flange  66 , and an upper body portion  68 . A recessed cavity  70  is defined in the upper body portion. In this embodiment two male coax connectors  72 ,  74  are mounted to the upper body portion  68  within the cavity  70 . Flexible leads  73 ,  75  extend, respectively, from the connectors  72 ,  74  through the lower body portion  64 . The leads are long enough to extend through the interior of the barrel spring  56  to connectors  76 ,  78  that are adapted to connect to the connectors  44 ,  46 , respectively. The leads  73 ,  75  will accommodate any flexion of the barrel spring  56  while maintaining secure connections at both ends. The upper body portion  68  is externally threaded at  77 . 
   Looking now briefly at  FIG. 7 , a keyway  80  is provided within the cavity  70  in the wall of the upper body portion  68 . In this embodiment, the keyway  80  takes the form of a chordal wall, thereby defining, roughly, a “D” shape to the cavity  70 . Other forms of keyways are possible, such as a channels or slots. 
   Turning now again to  FIG. 1  and  FIGS. 5–10 , it will be seen that the whip assembly  14  comprises a lower section assembly  90  and an upper section assembly  92 , separable from each other at a junction  94 . The lower section assembly  90  comprises at one end a coupler assembly  96  (adapted to connect to the mount assembly  12 ), an intermediate tubular section  98 , and, at the other end, a lower break assembly  100 . The intermediate tubular section  98  comprises a dielectric housing  102 , preferably fiberglass, into which is nested a conductive sleeve  104 , preferably aluminum. Several spaced ribs  106  within the conductive sleeve  104  provide strength and rigidity, and also provide support for two coaxial leads or transmission lines  108 ,  110 , and maintain them centered within the conductive sleeve. If the transmission lines  108 ,  110  do not remain centered, the performance of the antenna is adversely affected. 
   In this embodiment as shown in  FIG. 6 , the coupler assembly  96  comprises an insert  126  having an annular flange  128  with a keyed extension  130  on one side of the flange, and an externally threaded portion  132  on the other side of the flange. An annular securing channel  135  is located adjacent the threaded portion, away from the annular flange  128 . The keyed extension  130  surrounds a pair of female connectors  136 ,  138 , which are positioned to be in registry with and to matingly connect to the male connectors  70 ,  72 . The female connectors  136 ,  138  are also permanently connected, respectively, to the coaxial leads  108 ,  110 , respectively. Preferably, the keyed extension  130  has a key  131  comprising a flat wall so as to be “D” shaped to nest within the “D” shaped cavity  70 . 
   An internally threaded lock nut  140  is loosely disposed over the annular flange  128  to enclose the keyed extension  130 . A conductive hex ferrule  142 , having a hex nut  144 , an externally threaded portion  146 , and an extension  148 , is disposed over the insert  126  with the hex nut  144  threaded onto the externally threaded portion  132  of the insert  126 . Preferably, the hex ferrule  142  can be further secured to the insert  126  by set screws  150  extending through the hex nut  144  into the securing channel  135 . The extension  148  of the hex ferrule  142  preferably has a flat  152  adapted to support a high power impedance matching circuit  154 . 
   A tube reinforcement  155  is fixed within the end of the conductive sleeve  104  and is further secured to the hex ferrule  142 . The tube reinforcement  155  not only reinforces the end of the intermediate tubular section  98 , but it also provides additional structure to hold the high power impedance matching circuit  154 . A conductive coupler  156  surrounds the dielectric lower housing  102 , and threads onto the externally threaded portion  146  of the hex ferrule  142 . 
   It can be seen that the coupler assembly  96  mounts to the upper spring holder  58  to secure the whip assembly  14  to the mount assembly  12 . This occurs by inserting the keyed extension  130  into the cavity  70 . Since it is keyed, it will insert only one way, with the key adjacent the keyway  80 . This ensures that the connectors  136 ,  138  are aligned, respectively, with the connectors  72 ,  74 . As the respective connectors are connected, the lock nut is threaded onto the external thread  77  of the upper body portion  68  until secured tight. Preferably, one or more seals  158  will prevent migration of moisture to the electrical connections within the cavity  70 . 
   The high power impedance matching circuit  154  is needed to maintain an effective balance of current distribution and impedances in the conductive elements of the antenna. In this way, it assists the cable choke  48 . This is especially needed where the antenna is broadband, i.e., tuned to optimally receive and/or transmit in a wide frequency range. The high power impedance matching circuit  154  preferably comprises at least one resistor and one capacitor connected in series between the conductive flat  152  of the hex ferrule  142  and the conductive sleeve  104 . It may be that in some applications capacitance alone will suffice, which normally improves gain. But in some cases, resistance is needed to obtain matching impedance at a lower end of the desired frequency range. Where resistance is helpful, the resistance and capacitance can be in parallel. In this embodiment, preferably, a high power impedance matching circuit  154  is disposed on opposite sides of the intermediate tubular section  98 . A natural consequence of the high power impedance matching circuit  154 , especially at high power, is that it generates heat and therefore must dissipate power. When the antenna  10  is used in a high power situation, for example on the order of 300 watts, the mount assembly  12  effectively becomes an integral heat sink. Having a high power impedance matching circuit  154  on opposite sides of the intermediate tubular section  98  assists in dissipating heat around the mount assembly  12 , and enables smaller, less costly components to handle the currents at higher powers. As well, the conductive coupler  156  not only strengthens the bottom of the whip assembly  14 , but it adds capacitance to affect current distribution, and it increases the area serving as a heat sink. 
   As shown more clearly in  FIGS. 5 and 8 , the lower break assembly  100  is disposed at the end of the intermediate tubular section  98  away from the coupler assembly  96 . It comprises a conductive cylinder  160 , preferably aluminum, with a cable sleeve  162  closing one end and a connector mount  164  near the other end. The connector mount  164  is externally threaded and supports a male connector  166  that is electrically connected to a break cable  168  that runs from the connector  166  through the cable sleeve  162  to a male coax connector  170 . The exterior wall  172  of the conductive cylinder  160  is preferably knurled and dimensioned to be press fit within the dielectric lower housing  102 , with the connector mount  164  protruding therefrom. An adapter  173 , having an external threaded portion  175  roughly the same diameter as the dielectric lower housing  102  can be mounted to the connector mount  164 . The adapter  173  defines a cavity  167  at the end of the connector mount  164 , in which the male connector  166  is disposed. An interior wall of the adapter  173  has a keyway  169 , preferably a chordal wall similar to the structure in the coupler assembly  96 . 
   The conductive sleeve  104  in the intermediate tubular section  98  terminates at a point spaced from the lower break assembly  100 . The two coaxial leads  108 ,  110  extend beyond the end of the conductive sleeve  104 . The lead  108  has a female coax connector (not shown in  FIG. 8 ) that mates directly with the male coax connector  170  on the break cable  168 . The other lead  110  connects to a line transformer such as balun  176 . The balun  176 , in turn, connects to the conductive sleeve  104  and to the conductive cylinder  160  of the lower break assembly  100  and can act within a given frequency range as a feed point  178 . In this embodiment, it functions as the center feed point  178  of the dipole radiator for the lower frequency band of 30–88 MHz. 
   Turning now to the upper section assembly  92 , shown best in  FIGS. 9–17 , it can be seen that the upper section assembly  92  comprises an upper break assembly  180  and a top section  182 . As shown more closely in  FIG. 10 , the upper break assembly  180  comprises a conductive cylinder  184 , preferably aluminum, with a cable sleeve  186  at one end and a connector mount  188  at the other end. The connector mount  188  supports a female connector  192  that is electrically connected to a break cable  194  that runs from the female connector  192  through the cable sleeve  186  to a male coax connector  196 . The connector mount  188  has a key  189  that is preferably a chordal surface on the mount so it has a “D” shape, complementary in size to be received within the cavity  167  in the lower break assembly  100 . 
   The conductive cylinder  184  at the connector mount  188  has an external flange  190 . A lock nut  200 , having an internal annular shoulder  202  at one end and an internal thread  204  intermediate the annular shoulder  202  and the other end, slides over the conductive cylinder  184  until the internal shoulder  202  bears against the external flange  190 . The exterior wall  206  of the conductive cylinder  184  is preferably knurled and dimensioned to be press fit within a dielectric upper housing  208 . 
   The junction  94  in the whip assembly  14  is provided when the lower break assembly  100  is attached to the upper break assembly  180 . This occurs simply and easily by inserting the connector mount  188  into the cavity  167  with the key  189  bearing against the keyway  169 , mating the male connector  166  on the upper break assembly  180  to the female connector  192  of the lower break assembly  100 , and then threading the internal threads  204  of the lock nut  200  of the upper break assembly  180  onto the external threaded portion  175  of the adapter  173  on the lower break assembly  100 . The resultant junction  94  of the combined lower break assembly  100  and upper break assembly  180  is not only strong, but effectively becomes one pole of a dipole radiator. The conductive sleeve  104  and conductive cylinder  184  are electrically connected via the balun  176  and function together as an electrical radiator, fed by the coaxial transmission line  110 . Preferably, the length of the junction  94  is sufficient to provide a portion of a dipole in a predetermined frequency band. For an application in the range of 108–175 MHz, the length can be about 19 inches. If necessary to achieve this length, one or more extensions  191  of the conductive portions can be provided at either the lower break assembly  100  and/or, as shown in  FIG. 9 , at the upper break assembly  180 . 
   Looking now at  FIGS. 9 and 13 , the top section  182  comprises the dielectric upper housing  208  that completely encloses a non-conductive upper element tube  210  having a proximal end  212 , a distal end  214 , and a plurality of slots, preferably four,  216 ,  218 ,  220 , and  222  spaced from each other intermediate the proximal and distal ends. Conductive sleeves  224 ,  226 ,  228 ,  230 , and  232 , spaced from each other, are provided between the slots, as well as between the slots and the proximal and distal ends. The conductive sleeves can be metal foil, preferably wrapped around the upper element tube  210 . Interior of the upper element tube  210  are a plurality of cable sleeves  234  adapted to support one or more cables extending through the interior of the upper element tube and maintain them centered within the tube. 
   Looking now also at  FIGS. 14–17 , a first cable  240 , supported by cable sleeves  234 , extends out of the proximal end  212  to a connector  242 . A ferrite toroid  236  surrounds the first cable  240  between the connector  242  and the proximal end  212 , and functions as a cable choke. The connector  242  connects to the connector  196  of the upper break assembly  92 . A lead  244  runs from the first cable  240  to the conductive cylinder  184  (or extension  191  as the case may be) and to the conductive sleeve  224  where it can function as a feed point  245  in a given frequency range. The first cable  240  preferably has a rated impedance of 50 Ohms. 
   The first cable  240  extends in the other direction to a feed point  246  where it connects to a second cable  248  and a third cable  250 . The second and third cables  248 ,  250  are preferably identical in impedance and length, each having a rated impedance of 93 Ohms. The second cable  248  extends to the fourth slot  222  where it is electrically connected to the fourth  230  and fifth  232  conductive sleeves at a 1 st  dipole feed point  252 . The third cable  250  extends back parallel with the first cable  240  to the first slot  216  where it is electrically connected to the first  224  and second  226  conductive sleeves at a 2 nd  dipole feed point  254 . 
   An isolation circuit  256  is provided at slot  216 , electrically connected between conductive sleeve  224  and conductive sleeve  226 . Another isolation circuit  258  is provided at slot  218 , electrically connected between conductive sleeve  226  and conductive sleeve  228 . Another isolation circuit  260  is provided at slot  220 , electrically connected between conductive sleeve  228  and conductive sleeve  230 . And yet another isolation circuit  262  is provided at slot  222 , electrically connected between conductive sleeve  230  and conductive sleeve  232 . Each isolation circuit  256 ,  258 ,  260 , and  262  is preferably an LC parallel circuit with series capacitor, as shown in  FIG. 14 . Each isolation circuit  256 ,  258 ,  260 , and  262  functions to isolate a higher frequency band from a lower frequency band, with the values of inductance and capacitance being selected for the midrange of a given frequency band. An end cap  264  is provided at the end of the dielectric upper housing  208  to enclose the interior and protect it from atmospheric elements. 
   It will be apparent that the foregoing structure provides a multiband antenna with multiple dipoles, capable of effectively receiving at least three frequency bands. Say, for example, one wanted to receive or transmit signals in a first band of 30–88 MHz, a second band of 108–175 MHz, and a third band of 225–450 MHz. The relatively low frequency first band is resonant in the dipole radiator defined by the conductive sleeve  104  on the one hand, and the dipole connector  94  and top section  182 , with the feed point for the first band being the feed point  178 , all as shown in  FIG. 15 . The relatively high frequency third band is resonant in the stacked dual dipoles of the top section  182 , the 1 st  dipole comprising conductive sleeves  230  and  232  with feed point  252 , and the 2 nd  dipole comprising conductive sleeves  224  and  226  with feed point  254 , all as shown in  FIG. 16 . 
   The relatively mid range second frequency band can be resonant in a dipole that spans the junction  94 , as shown in  FIG. 17A , or in a dipole wholly located in the top section  182 , as shown in  FIG. 17B . In the first alternative, the dipole radiator is defined by the junction or dipole connector  94  on the one hand, and the conductive sleeves  224  and  226  on the other hand, with the feed point being the feed point  245 . In this case, the isolation circuit  256  is transparent in the second frequency band. In the second alternative, the dipole radiator is defined by the conductive sleeves  224  and  226  on the one hand, and the conductive sleeves  228  and  230  on the other hand, with the feed point being the feed point  246  at the junction of the first  240 , second  248  and third  250  cables. 
   In either the dual dipole situation for the third band or the single dipole situation for the second band where the dipole is located entirely in the upper section assembly, it has been found that adding a resonant circuit  252  such as, for example, a capacitor and an inductor in series, electrically connected between the conductive cylinder  184  and the conductive sleeve  224  at the feed point  245  helps gain in both bands. 
   It has also been found that if the same values are used for the isolation circuits  256 ,  258 ,  260 , and  262 , interactions among the first cable  240  and the conductive sleeves  224 ,  226 ,  228 ,  230 , and  232  generate current distribution problems in the first (low frequency) band. Rather than selecting values for each isolation circuit to resonate at the midrange of the first band (e.g., 56 MHz), a solution has been found in selecting values so that each isolation circuit will resonate at a graduated step within the first band. For example, isolation circuit  252  can be made to resonate at 70 MHz, isolation circuit  256  to resonate at 60 MHz, isolation circuit  258  to resonate at 50 MHz, and isolation circuit  260  to resonate at 40 MHz. All isolation circuits referred to herein can be as shown in  FIG. 14  or they can be any effective equivalent circuit, such as coaxial stubs. 
   It will be apparent in the illustrated embodiment that while dipoles are provided to resonate at three frequency bands, only two ports are provided to carry signals from the antenna: connectors  34  and  36  in the base cover mount. Signals in the first band (relatively low frequency) will always be conducted through the connector  34  by way of the cable  110  that communicates with the dipole at the feed point  178 . Signals in the third band (relatively high frequency) will always be conducted through the connector  36  by way of the cables  108  and  240  that communicate with the dual dipoles at the feed points  252  and  254 . Signals in the second band (mid range frequency) will be communicated through either of the connectors  34 ,  36 , depending upon the dipole chosen. Providing isolation circuits that turn on and off at given frequencies will enable the second band to be communicated through either connector  34  or  36 . 
   A second embodiment of a multiband antenna  300  according to the invention is shown in  FIGS. 18–24 . The antenna  300  comprises a mount assembly  302  and a whip assembly  304 . The mount assembly  302  comprises a base housing  306  with an annular mounting flange  308 , a base connector  310 , a spring plate  312 , a barrel spring  314 , and an upper spring holder  316 . The base housing  306  in this embodiment is conventional, adapted to mount to a vehicle (not shown) by bolts through apertures in the annular mounting flange  308 . 
   Looking now at  FIGS. 19–21 , the base connector  310  comprises a hollow cylindrical body portion  318  that is covered at one end by a plate  320  centered on the longitudinal axis  322  of the body portion. The plate  320  has several apertures  324  at its periphery and the base connector  310  has three receptacles  326 . The receptacles  326  are sealed against moisture. 
   The spring plate  312  is fixedly mounted to the spring  314  and bolted to the base connector plate  310 , and has a central aperture  332  through which the connectors  326  are accessible. The interior of the spring  314  surrounds the central aperture  332 . 
   At the upper end  334  of the spring  314  is the upper spring holder  316  nested within the spring  314  and comprising a lower body portion  338  that is received within the spring  314 , a hex flange  340 , and an upper body portion  342 . The lower and upper body portions  338 ,  342  are hollow, separated by a wall at the hex flange  340 . Three apertures extend through the wall, each aperture having a female coax connector  348  mounted therein. A key  350  in the form of a pin projects from the cylindrical wall of the upper body portion  342 . The upper body portion  342  is externally threaded. A cable  352  is connected to each female coax connector  348  in the upper spring holder  316  and extends through the hollow lower portion  338 , through the interior of the spring  314  to the spring plate  312  where each connector terminates in a female coax connector. Before the spring plate  312  is bolted to the base connector plate  310 , each female coax connector is secured to a corresponding male coax connector  326  on the base connector plate  310 . Leads connected to the male coax connectors  326  in the base connector plate  310  run through the base housing  306  to electrical circuitry. 
   Looking again at  FIG. 18 , the whip assembly  304  comprises a lower physical portion  360  and an upper physical portion  362 . The lower  360  and upper  362  physical portions are integral, but they can be separable in a manner hereinafter described. The lower physical portion  360  carries a lower electrical element  366  and the upper physical portion  362  carries an upper electrical element  368 . The lower electrical element  366  and upper electrical element  368  are together adapted to receive signals in the 30–175 MHz range. The upper electrical element comprises a set of dipoles that are adapted to receive frequencies in the 225–450 MHz range and 500–1000 MHz, respectively, through two separate coaxial transmission lines. 
   It will be understood that the physical structure of the electrical elements  366 ,  368  is similar to that in the first embodiment above, i.e., one or more transmission lines centered within a dielectric tube, wrapped with a conductive sleeve of copper or aluminum, all encased by a fiberglass housing. The lower electrical element  366  thus comprises a conductive sleeve  372  and three transmission lines  383 ,  384 , and  385 . The upper electrical element  368  comprises five conductive sleeves  396 ,  397 ,  398 ,  400 , and  402 , with one or two of the transmission lines  384 ,  385  centered therein. The transmission line  383  is a coaxial cable servicing the 30–175 MHz range. The transmission lines  384 ,  385  are also coaxial cables servicing the 225–450 MHz and 500–1000 MHz ranges, respectively. All of the transmission lines  383 ,  384 , and  385  are centered within the conductive sleeves  372 ,  396 ,  397 ,  398 ,  400 , and  402  by spacers  392 . 
   At a lower end of the lower physical portion  360  is a male connector assembly  370 . The male connector assembly  370  electrically connected to the conductive sleeve  372 . The male connector assembly  370  comprises an elongated body portion  374  that is sized to be received by friction fit within one of the dielectric tube or the fiberglass housing, and a cylindrical portion  376  separated from the elongated body portion  374  by an annular flange  378 . The cylindrical portion  376  is sized to fit within the upper body portion  342  of the upper spring holder  316  at the upper end of the spring  314 . An internally threaded coupling nut  380  is received over the annular flange  378 , and is sized to thread securely on to the externally threaded upper body portion  342  of the upper spring holder  316 . Within the cylindrical portion  376  are three male coax connectors  382 , one or more of which is connected to the coaxial transmission line  383  that runs through the elongated body portion  374  and into the conductive sleeve  372 . 
   The external wall of the cylindrical portion  376  has a keyway  386  that extends from the annular flange  378  to the distal end of the cylindrical portion  376 . The keyway  386  is adapted to interact with the key  350  on the upper body portion  342  of the upper spring holder  316 , and is so located that the male and female coax connectors  348 ,  382  will be in registry when the cylindrical portion  376  is received within the upper body portion  342 . It will be apparent that when the cylindrical portion  376  of the male connector assembly  370  is received within the upper body portion  342  of the upper spring holder  316 , the coupling nut  380  can be threaded on to the external threads of the upper body of the upper spring holder to securely attach the two together. In this manner, the whip assembly  304  is secured to the mount assembly  302 . The key  350  and keyway  386  enable the connection to be accomplished under any condition so that all electrical leads are properly aligned and connected. 
   The key  350  and keyway  386  can take many different forms. For example, the key can be a knob or protrusion of any shape extending from the cylindrical wall of the upper body portion  342 , so long as it is complementary in shape to the keyway  386 . Thus, for example, the key  350  and keyway  386  can take the form of a chordal wall on the upper body portion and a “D” shaped cylindrical portion  376 , as in the first embodiment of the antenna. 
   Looking now more closely at  FIG. 22 , near the upper end of the lower physical portion  360  of the whip assembly  304  there is a transition from the lower electrical element  366  to the upper electrical element  368 . The transition is from the balanced load of the lower electrical element  366  and upper electrical element  368  to the unbalanced impedance of the 30–175 MHz coaxial transmission line  383 . This transition is accomplished by a balun  394 , a transformer that effectively carries the load between the coaxial transmission line  383  and the lower  366  and upper  368  electrical elements. In the upper electrical element  368 , further along the whip assembly  304 , the conductive sleeves  397 ,  398 ,  400 , and  402  form a series of dipole antennas  404 ,  406 . Each dipole antenna  404 ,  406  comprises a pair of conductive sleeves electrically connected to each other at a feed point. The coaxial transmission lines  384 ,  385  extend concentrically within the dipole antennas to the respective feed points. At the balun  394 , there is a connection between the transmission line  383  and the conductive sleeves  372 ,  396 . The coaxial transmission line  384  feeds the lower and upper electrical elements in the frequency range 30–175 MHz. The dipole antennas  404 ,  406  are tuned to resonate in the frequency ranges of 225–450 MHz and 500–1000 MHz, respectively. 
   Looking now at  FIGS. 22–24 , a modification of the second embodiment of a multiband antenna according to the invention will effectively receive signals in all three separate frequency bands, including a broadband frequency range of 500–2500 MHz. In this modification, signals in each frequency range are channeled through one of the three ports in the connector between the whip assembly and the mount assembly, as before. The first frequency range at 30–175 MHz is received by the lower electrical element  366  and upper electrical elements  368 . The second frequency range at 225–450 MHz is received by the single dipole  404  of the upper electrical element  368 . The broadband high frequency range at 500–2500 MHz is received by what is effectively an open sleeve dipole  422  on the upper dipole antenna  406  near the upper end  424  of the whip assembly  304 . This is effectively accomplished by providing a metal sleeve  425  on the outside of the fiberglass sleeve  390  and a dielectric spacer  426  of the whip assembly  304  at the feed point of the top dipole  406  of the upper electrical element  368 . 
   It may be necessary for transportation and storage purposes to enable the antenna  300  to be broken down further. If that is needed, a break such as that described above for the first embodiment can be provided between the lower physical portion  360  and the upper physical portion  362 . The break will be keyed as described above to ensure alignment of the two transmission lines  384   385  of the upper electrical element  368 . 
   While the invention has been specifically described in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation, and the scope of the appended claims should be construed as broadly as the prior art will permit.