Abstract:
A wide bandwidth high-power T network tuner obtains its wide bandwidth operation by connecting a fixed capacitor between the input and ground, with the tuner using large, series connected variable capacitors for high power so as to solve the minimum capacitance problem on the higher frequencies, with the input and output matching capacitors being independent one from the other.

Description:
FIELD OF THE INVENTION 
   This invention relates to antenna tuners and more particularly to a wide bandwidth high-power T network tuner. 
   BACKGROUND OF THE INVENTION 
   Antenna tuners or transmatches have been used in the past to match the output impedance of a transmitter with the impedance of an antenna. Antenna tuners come in a wide variety of forms, the most popular being either the L network or the T network tuners. 
   It is noted that traditional T network tuners have three controls, namely the inductor tap control, the input matching capacitor control and the output matching capacitor control. In an effort to simplify the tuning of T network tuners, MFJ Enterprises provided a so-called differential T tuner in which a ganged capacitor assembly incorporated a dual capacitor having a single shaft with two sets of stator plates to either side of the shaft such that as the rotor blades engaged one set of stators, the rotor blades associated with the other set of stators disengaged. 
   In order to accommodate a wide range of antenna impedances, especially for the 1.8 MHz 160 meter band, the differential T tuner required large valued input and output matching capacitance as well as a large inductor. It can be shown mathematically that for the same load and input power the differential T tuner requires larger capacitors than the traditional T network tuner for the same efficiency. Because the capacitors had to have large values, their minimum capacitance was very high when one attempted to disengage the rotators from the associated stators. Furthermore, the minimum capacitance is made even higher because the ganged capacitor is a single assembly with the edges of the two stators directly parallel to each other and in close proximity. 
   The inability to reduce the minimum capacitance by disengaging the rotators and stators made high impedance antenna matching impossible, especially on the 10-meter band. Thus, for the differential T tuner it was not possible to provide a broadbanded response. The problem, therefore, was how to increase the high impedance matching range for the 10-meter band for the differential T tuner. This was solved by placing a fixed capacitor across the input, the net effect of which was to lower the minimum capacitance associated with the transmitter. 
   This technique was, however, not applied to the traditional three-control T network tuners because traditional T network tuners did not need large capacitors as they only had to operate at relatively low power. The result of that these tuners used only low value capacitors, on the order of 250 picofarads, to provide relative broadband performance. 
   When using a 250-picofarad capacitor, which is a relatively small capacitor, the minimum capacitance is low when the rotor blades are completely disengaged from the stators. The construction of these two independent and separately mounted capacitors are such that the rotor and stator plate edges of one variable capacitor while parallel to the rotor and stator plates of the other capacitor are nonetheless staggered relative to each other instead of directly across from each other in the differential T tuner capacitor assembly thus reducing the minimum capacitance. Also, since the traditional T tuner uses two independent and separate capacitors connected in series, the minimum capacitance is reduced by half. Thus matching on the 10-meter band was not a problem. As a result, the traditional low-power T network tuners had a good high-impedance matching range even on 10 meters and there was no need to place a capacitor across the input to extend the high-impedance matching range of the tuner. 
   However, due to periods of low sunspot activity prevalent in the 1980s, higher and higher powers were required in order to be able to communicate using ionospheric skip. Low sunspot activity was responsible for poor communication not only on the 20-meter bands and higher, but also on the lower 40-, 80- and 160-meter bands. Thus, with low sunspot activity even marginal communication could not take place unless one doubled or tripled the output power of the transmitter. This meant that transmitters exceeding 1,000 watts and often in foreign countries exceeding 5,000 watts were used in an attempt to counter the poor propagation. 
   The problem with such high power outputs for the transmitters was that the power ratings of the tuners needed to move up dramatically. One therefore needed to build very high-power antenna tuners. The problem in so doing was that at 160 meters, namely at 1.8 MHz, matching to low impedances required exceptionally large input matching capacitors. For instance, to match a low impedance of 12.5 ohms, one needed to markedly increase the value of the output matching capacitor to reduce losses. In many instances the value of the capacitors had to either double or quadruple. 
   While the use of extremely large capacitors solved the low impedance matching problem at 160 meters, the capacitors were so large that the associated minimum capacitance was unacceptable on 10 meters. 
   The net result is that for high-power T network tuners, in order to reduce losses at 160 meters, one would not tune high impedances at 10 meters. Since these tuners could not be used on 10 meters, people tended to use the lower power tuners, which they regularly burned out because of the high losses at 160 meters. 
   By way of further background, to reduce tuner losses at 160 meters, one had to provide relatively large capacitors of up to 1,000 picofarads. Minimum capacitance of such input matching capacitors was well above 100 picofarads or about 10 percent of its maximum capacitance, totally unsatisfactory for high impedance matching at the higher frequencies. Thus, while moderate power antenna tuners were available that could barely meet the legal limit of 1500 watts by using capacitors that went up to 300 picofarads, these tuners were not suitable for 160 meters when matching low impedance loads due to excessive losses. 
   Especially for marine applications and for foreign and commercial applications, transmitters having outputs between 2500 and 5000 watts are regularly used. For any particular band it was not a problem to provide a specialized antenna tuner for the band. However, it became difficult to provide broadband coverage, for instance, between 160 meters and 10 meters for such high-power applications. 
   Note that such high-power applications include radio teletype and other types of commercial transmitting applications. 
   SUMMARY OF INVENTION 
   It has been found that in high-power tuners one can use very large capacitors and reduce the associated minimum capacitance to provide broadband coverage. As part of the subject invention, by providing a fixed capacitance of between 100-200 picofarads between the tuner&#39;s input terminal and ground, one can effectively reduce the minimum capacitance when the variable input matching capacitor is completely open. By reducing the effective minimum capacitance, one can provide a wideband high-power tuner that can match high impedances at the higher frequencies while at the same time reducing tuner losses at the lower frequencies due to the use of the oversized capacitors. 
   Prior T network tuners used 250-picofarad capacitors and were able to tune both 10 meters and 160 meters. However, with high power requirements the traditional T network tuner required oversized variable capacitors that solved the problem of loss reduction at 160 meters but at the same time created the problem of minimum capacitance at 10 meters. Thus the new demand for higher power antenna tuners could not be met by the traditional T network tuner. 
   It was found that one could use the large capacitors to reduce losses at 160 meters and still solve the minimum capacitance problem with the use of a fixed capacitor. 
   In so doing, it was not immediately obvious that one could adapt the technology used in the prior differential T network tuner because of the difference in physical configuration and location of the variable capacitors and due to the fact that the differential capacitor tuner inherently has a high minimum capacitance, whereas the traditional T network tuner does not. 
   As to the physical configuration differences in the differential capacitor tuner, the ganged capacitors interact due to the positioning of the fixed edges of the stator plates aligned one on top of the other that caused the failure of the tuner to match high antenna impedances on 10 meters. The traditional T network tuners have no interaction because the capacitors are not ganged together, the stator edges are not aligned, and because the minimum capacitance is halved due to the fact that the minimum capacitance of each capacitor is connected in series because of the series connection of the capacitors. 
   Secondly, as to minimum capacitance, the differential T capacitor assembly inherently has more minimum capacitance than the two separate capacitors used in the traditional T network tuner for two reasons. 
   This is because of the construction of the differential T capacitor. In the differential capacitor tuner the stator plate edges of each section are directly opposite, i.e., one on top of the other with opposed edges aligned with each other and facing each other. Note that the opposed stator edges are parallel to each other and in close proximity. This means that the stator plates for the two capacitors are aligned and on diametrically opposite sides of the rotor. 
   The construction of the normal capacitors used in the traditional T network tuner is such that while the stator and rotor plate edges are parallel when fully unmeshed they are not directly opposite from each other but are staggered or laterally offset from each other, for less interaction. This results in less capacitance. 
   Second, as mentioned above, since the traditional T network tuner uses two separate independent capacitors connected in series, the minimum capacitance is also connected in series. This reduces the minimum series capacitance by half. 
   Moreover, it can be shown mathematically that for the same load and input power the differential T tuner requires larger capacitors than the traditional T network tuner for the same efficiency. This results in higher minimum capacitance for the same efficiency and reduces the matching range on the higher bands especially on 10 meters. 
   To say the same thing in a different way, a differential T tuner having the same value capacitor as a traditional T network tuner will not handle as much power as the traditional T network tuner before the capacitors arc due to voltage breakdown. 
   Thus there are many differences between the differential and traditional T network tuners, so that one could not necessarily copy from one to the other. 
   Note, the most desirable quality of a differential T tuner over a traditional T network tuner is that it is quick and easy to use. There are only two knobs to adjust and for a given load there is only one unique setting of the capacitor and inductor. For the traditional T network tuner there are three knobs to adjust and there are an infinite number of settings for the capacitors and inductor for a given load so it is harder and slower to use but is much more versatile and can handle much more power. 
   Additionally, the action of the differential T tuner was found to be highly unusual in that for a fixed setting for the differential capacitors it was found that there were two inductor settings that were efficient in matching different antenna loads. On the other hand, this is not generally thought to be a property of traditional T tuners for which only one load can be matched by the variable inductor for a given capacitor setting. 
   Thus because of the unusual properties, the different physical configurations and the series connection of the traditional T network tuner, it could not be predicted that the fixed capacitor at the input of the differential T tuner could be used elsewhere. 
   It was found that, by providing a fixed capacitor at the tuner input of a traditional T network tuner in which the input matching capacitor and the output matching capacitor are completely decoupled and independent, good performance at 10 meters could be achieved when using large high power capacitors. 
   In summary, a wide bandwidth high-power T network tuner obtains its wide bandwidth operation by connecting a fixed capacitor between the input and ground, with the tuner using large, series connected variable capacitors for high power so as to solve the minimum capacitance problem on the higher frequencies, with the input and output matching capacitors being independent one from the other. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the subject invention will be better understood in connection with a Detailed Description, in conjunction with the Drawings, of which: 
       FIG. 1  is a schematic diagram of a prior art differential T tuner employing a number of shunt capacitances on the input thereto; 
       FIG. 2  is a schematic diagram of the subject traditional T tuner with a shunt capacitor across the input to improve the matching performance to high impedances so as to counteract inability to reduce the capacitance of the input capacitor; 
       FIG. 3  is a schematic diagram illustrating the effect of the shunt capacitor to effectively provide a series capacitor in series with the input tuning capacitor; and, 
       FIG. 4  is a graph showing the effect of using a fixed shunt capacitor across the input to increase the matchable load resistance that can be matched with the traditional T Tuner. 
   

   DETAILED DESCRIPTION 
   Referring now to  FIG. 1 , a schematic diagram of a differential T tuner employing multiple shunt capacitances across the input to the tuner is illustrated. Here the differential T tuner includes a differential capacitor  10 , which includes a variable input section  12  and a variable output section  14 . The differential nature of the tuner is such that as one moves the rotatable plates into the stator plates the input capacitance of the input capacitor section  18 , 12  decreases and the output capacitance section  32 , 12  increases. A roller inductor  16  is shown connected to the center of the rotator plates of the differential capacitor and ground, with a rolling tap  17  providing inductance adjustment. 
   Stator plates  18  correspond to the input capacitor stator plates, which are connected via a switch  20  to input  22 . 
   It is noted that the upper positions of switch  20  result in a direct feed to antenna connectors  26 ,  28  and  30  corresponding respectively to coax  1 , coax  2  and an external dummy load. 
   The stator plates for the output capacitor, here illustrated at  32 , are coupled to a switch  34 , which as illustrated is ganged to switch  20 . Switch  34  connects the output stator of the differential capacitor to the various output terminals  26 - 30 , and to a balanced line transformer  36  to accommodate balanced lines at  38  and  40 . Note a long wire is coupled to switch  34  at terminal  42 . 
   It will be appreciated that, due to cross coupling between the two sections of the differential capacitor, the minimum capacitance is complex, since the input section maximum capacitance is affected by the output section. To solve the problem, an exceptionally large capacitor  50  was coupled between the input section stator plates and ground because the differential T tuner suffers in the higher frequencies from the inability of the input capacitor portion of the differential capacitor  12  to be able to match resistive loads greater than 200 or 300 ohms. This was found to be due to the minimum capacitance associated with the use of a differential capacitor. 
   The differential capacitor tuner described in  FIG. 1  is useful because with only two knobs, namely that controlling the differential capacitor and that controlling the roller inductor, one can theoretically tune the output impedance to the 50-ohm input impedance. 
   While the differential tuner has only two knobs, the inability to control separately the input capacitor capacitance and the output capacitor capacitance does not afford the flexibility to match high impedance loads to the input impedance in all circumstances. 
   Referring now to  FIG. 2 , the subject traditional T tuner has an independent variable input capacitor  60  and an independent variable output capacitor  62 , with the junction thereof coupled through an inductor  64  to ground through a roller tap  66 . It is noted that the output of output capacitor  62  is coupled to an antenna  68 , whose impedance may not be matched to the input impedance at input  70 . More particularly, for a large 500 pf or larger variable input capacitors  60 , the minimum capacitance associated with the input capacitor may be too large to be able to effectively match antenna  68  to the input  70  impedance if the antenna presents a resistance load of over 300 ohms at 10 Meters. 
   In order to effectively lower the minimum capacitance of capacitor  60 , a shunt capacitor  72  is connected between the input and ground. In a preferred embodiment, shunt capacitor  72  has one of its leads directly attached to the input terminal of the input matching capacitor, with the other of its leads connected to ground immediately adjacent the input matching capacitor. In this manner, the shunt capacitance is available directly at the input end of the input matching capacitor. 
   As described in  FIG. 3 , the effect of putting the shunt capacitor to ground is effectively connecting a capacitor in series with input capacitor  60 , thereby lowering its minimum capacitance, which allows the traditional T tuner to match ever-increasing resistive antenna loads. 
   What is shown in this figure is the effect at the higher frequencies of providing a shunt capacitor across input  70  to ground. The shunt capacitor here shown in dotted outline  72  and the signal source resistance can be characterized as a series-connected RC circuit illustrated in dotted box  74 , composed of a resistor  78  and a capacitor  80 . 
   Assuming that input-matching capacitor  60  is at minimum capacitance, the effect of shunt capacitor  72  is to place capacitor  80  in series with capacitor  60 . This reduces the equivalent minimum capacitance associated with capacitor  60 . The result is that one can obtain excellent matching at the higher frequencies for high antenna impedance by solving the minimum capacitance problem. 
   Referring now to  FIG. 4 , what is shown is the effect of providing a shunt capacitor across an input matching capacitor for a T network. The curve labeled  82  illustrates that for the case in which no shunt capacitance is used, the maximum load resistance matchable is less than 300 ohms when C 1 , the capacitance of the variable input matching capacitor, is at its minimum point and the variable output matching capacitor C 2  is at maximum. 
   As shown by curve  84 , when using a fixed shunt capacitor at 100 pf, the maximum load resistance matchable at C 1  min and C 2  max is close to 700 ohms. This means that using a shunt capacitor increases the maximum matchable load resistance. 
   What can be seen is that the introduction of a fixed shunt greatly improves the maximum resistance load that a T network tuner can match for C 1  min. 
   This provides excellent matching for high resistance loads across the entire 10-meter band. Moreover, the use of a shunt capacitance extends the high impedance matching range over the entire 160-10 meter bands, but its effect is less as the frequency is lower. 
   Noting that in the above example a maximum shunt capacitance of 100 pf permits matching to a 700-ohm load, increasing the shunt capacitance to 200 pf expands the top end to 1,500 ohms. 
   While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications or additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.