Gamma match is not a broadband impedance transformer, so it must be classed as a narrowband match.
But, does that mean that ham folk-lore that it necessarily degrades antenna VSWR bandwidth is soundly based?
The following analysis uses an RLC load that has about the same bandwidth as a well designed Yagi, half power bandwidth is 14MHz, ie antenna Q is 10. This is a medium Q Yagi, and quite representative of a practical narrowband antenna.
Above is a model of the antenna which has a native feed point impedance of 12.5+j0Ω at mid band, transformed with a lossless 4:1 broadband transformer (2:1 turns ratio in WinSmith). A properly adjusted folded dipole driven element would have this response. Note that the sweep from 137-151MHz is contained within the VSWR=2.62 circle shown.
Above, the same antenna transformed with a lossless gamma match consisting of equal tube diameters. The equal tubes result in a 4:1 transformation (again 2:1 turns ratio shown in Winsmith), a shunt s/c stub to represent the gamma ‘loop’ and series capacitor. In this case, all of the sweep from 137-151MHz is contained within the VSWR=2.35 circle shown.
Above is a sweep to the half power points (VSWR=2.62). The bandwidth has increased to 18MHZ over 14MHz, 29% wider, or Q=8.
A practical gamma match is not lossless, and practical losses would further increase the bandwidth, but with a good implementation that increase would be insignificant.
The example analysis demonstrates that a gamma match, although regarded as a narrowband transformer, need not reduce the VSWR bandwidth of a narrowband antenna, indeed it may increase it.
- Balanis, C. 1997. Antenna theory – analysis and design 2nd ed. John Wiley & sons New York.
- Cebik, L. Oct 2006. Some Preliminary Notes on the Gamma Match. Restricted.
- Duffy, O. Apr 1998. Novel Gamma Match Construction. VK1OD.net (offline).
- ———. Oct 2013. Designing a Gamma match using a Smith chart. VK1OD.net (offline).
- Healey, D. Apr 1969. An examination of the gamma match In QST Apr 1969.
- Smith, P. 1995. Electronic applications of the Smith chart 2nd ed. Noble Publishing Tucker.
- Tolles, H. 1973. How to design gamma matching networks In 73 Magazine May 1973.
- Folded dipole step up ratio calculator