One of the feed arrangements used for small transmitting loops (STL) is the so-called gamma match, though it is questionable if these are quite the same as matches of the same name on a linear dipole.
Rectangular gamma match
There are many different implementation, and some would really appear to be little more than an auto transformer with the windings being the so-called gamma loop and the main loop. This type of arrangement is quite equivalent to the model used for the isolated auxiliary loop feed at Small transmitting loop – linear circuit analysis, and Calculate small transmitting loop gain from bandwidth measurement should be directly applicable.
Twisted gamma match
(Underhill & Harper 2002) shows a feed arrangement above used in some of Underhill’s STL.
This so-called “twisted gamma” was measured by them and is reported above. Though this one is not perfectly matched at resonance, it is close and the resembles the measurements reported at Simple resonators might not appear as such initially.
The impedance arc is of similar radius to the R=50 circle, passes almost through the chart centre (the lack of perfect match mentioned earlier), but is rotated from the R=50 circle by about 142° towards the generator.
A length of approximately 38° of 50Ω line would rotate the arc to be roughly coincident with the R=50 circle, meaning the R is approximately constant and X varies about resonance.
This type of arrangement behaves similarly to the model used for the isolated auxiliary loop feed at Small transmitting loop – linear circuit analysis, and Calculate small transmitting loop gain from bandwidth measurement should be directly applicable.
Underhill, M & Harper M. 2002. Simple circuit model of small tuned loop antenna including observable environmental effects. http://personal.ee.surrey.ac.uk/Personal/Marc.Harper/simple.pdf (accessed 01/06/14).