A complete model of the Raibeam design must include a transmission line with a given propagation delay, which is connected and impedance matched to both of the driven elements. In this case, small closed loops are used to accomplish the matching. Since there is an inherent inability in NEC based modeling programs to accurately model small closed loops, inaccuracies arise when such programs are used to model this design. 

One can find a more than enough suggestions on how to deal with this problem, but my experience has been that the inaccuracies remain, regardless.  

I have also found found notable inconsistencies in both gain and impedance predictions between differing NEC2d based modeling programs, including EZNEC+, when modeling identical structures...  interestingly, in comparisons between EZNEC+ and EZNEC Pro (NEC4) the models proved to be quite consistent. This leads me to conclude that NEC based modeling programs can, and do, produce artifacts. Granted, these compared programs were, in the most part, unique to one another in how and in what form the output information was presented. 

Presentations aside, perhaps a degree of these inconsistencies are a product of their theoretical basis; Maxwell's (classical) field theory.

Maxwell's field point of view puts forward the idea of local action-- where interactions of all kinds could be successfully described as interactions happening at local points in space and time, with the effects at other places being the result of propagating fields. Thus, the generally accepted (classical) view is that electric and magnetic fields are generated by RF energy being applied to an antenna, and these fields then propagate to combine in the far-field-- despite being out of phase-- to produce an electromagnetic field which also propagates.

A more modern view of electromagnetism holds that an electromagnetic particle/wave (a photon) is produced by the perturbation of an electron (by an alternating current, for example), and propagates naturally without any need for the combining of electric and magnetic fields in the far-field. 

Further arguments against the existence of electromagnetic fields was put forth by a prominent physicist; the late Nobel laureate Richard Feynman.

In his book QED*, Feynman states: "an effect propagates from one point to another but this transmitted effect lacks the physical reality of a field. This concept of a transmitted effect is only a way of describing the literal action of one particle on another at a distance. The idea of particles acting at a distance on one another is a simpler, more satisfying description of electromagnetism than the classical field theory which assigns substance to electric and magnetic fields existing in space."

Setting aside the far-field issue, it appears there are other problems with Maxwell's equations as well. For example, (the late) physicist John Wheeler, in his book "Quantum Foam" states: "Classical electromagnetic field theory does contain some notable internal difficulties however. The worst of these difficulties is the prediction that the field in the immediate vicinity of a point particle is infinite in magnitude, resulting in "infinite self-energy" of the particle, which seems to imply infinite mass. Interestingly, this infinite self-energy can be swept under the rug - just ignored - without disturbing the other successful predictions of the theory. The difficulty is that when applied to quantum field theory this infinite self-energy problem creates infinities."

It seems logical to assume, then, that Maxwell's  electromagnetic field theory, while quite effective in its application for the most part, is not entirely accurate. Granted, NEC based models of straight-forward designs such as wire arrays, ground planes, dipoles, yagis, etc., and their environments, are notably accurate. Nevertheless, it has been my experience that the greater the complexity of the antenna, the greater the chance of artifacts and resultant inaccuracies. 

In conclusion; regardless of the noted shortfalls, all NEC models strongly suggest that the gain produced by the Raibeam's critical coupling is indeed superior to that of an equal-length yagi. This has been proven time and again by side-by-side and other on-air comparisons.

^* Quantumelectrodynamics

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