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.
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