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Critical Coupling to achieve Supergain Probably the most familiar use of the term critical coupling is normally applied to tuned LC circuits, and describes a condition in which 2 resonant inductors in close proximity achieve maximum energy transfer. Indicative of such coupling is equal amplitudes of current in both inductors. This same condition can occur between two parallel ½ wavelength (resonant) dipole elements as well, and can be achieved through several methods of coupling. i.e.; mutual inductance, capacitive coupling, physical coupling, or by a combination of these methods. One popular design in use today applies capacitive coupling, where two parallel ½ wave dipoles (usually a driven element and a parasitic element) are folded or bent in such a fashion that their tips come into close proximity to each other. Another popular design feeds two parallel resonant dipoles spaced at 1/8 wavelength and fed with a phase difference of 135º. Attaining an effective degree of critical coupling using either method as described above is certainly difficult, and if not carefully designed and adjusted, their directional gain is no greater than a two element log cell. The classical yagi antenna design utilizes mutual coupling to achieve directional gain but, unfortunately, one cannot apply critical coupling methods to this design. When the driven element of a yagi is placed in close enough proximity to a parasitic element for the currents in both elements to become equal (or nearly equal), the spacing between them has become too close to develop good directional gain and will also reduce the bandwidth to a point of un-usability. A practical two element yagi design will have unequal currents, a rather poor front-to back ratio, and a nominal directional gain of approximately 6.2 dBi ±0.5dB in free space. The patented Critically Coupled bi-periodic method This method is similar to the dual fed system mentioned above except the element spacing is reduced to approx. 1/10 wavelength, which is the optimum spacing for maximum mutual coupling. Critical coupling (equal currents) is achieved through the use of a coaxial delay line that is impedance matched to both elements. Unlike the aforementioned dual fed system, the rear element is not a typical "fed" element, rather, it is adjusted to achieve a negative impedance characteristic at one point in the cycle, which allows the induced energy that has not been dissipated by radiation to flow into the delay line and back to the feed-point in phase with the input current, thus reinforcing the input current similar to what occurs in a feedback loop. See graphic below Computer Modeling The popular notion that my critically-coupled driver system can be modeled as two parallel resonant (or near resonant) dipoles with two independent voltage/current sources applied (ignoring the coaxial delay line and matching systems with a single feed-point) is woefully insufficient. All such models are incomplete thus incorrect. Problematic as well, is that models of the matching systems using the currently available NEC2-4 applications, cannot be relied upon either, as the matching systems are, in essence, closed loops constructed of various diameter tubes and a feed-point. It has been well established that the NEC engine cannot model architectures of this type accurately. Even in cases where equal diameters and equal segment lengths are applied, the results will not be consistent with the empirical. A good example of this comes to light when modeling a series capacitor gamma match mounted on a 1/2 w. dipole. Modeled with equal diameter wires and equal segment lengths (as recommended), no matter what value of capacitance or length of inductor is applied, or where the capacitor or feed-point is placed, the model fails drastically. One will find that in order to achieve a model that is consistent with both the known gain of the dipole, a 50 ohm gamma input impedance, and a good degree of physical consistency with its real-world counterpart, the model description must take a significant departure from the accepted paradigm. When I applied the knowledge gained from solving the preceding gamma match problem to my matching systems, a somewhat successful model of my critically-coupled driver system was produced in EZNEC. to view, click here. To read about further modeling problems click here. All known of versions MININEC do not offer a virtual model of a coaxial transmission line. |
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