A reader of my article Are gamma matches as bad as all that asked whether the W5VJB dipole was a gamma match given the hint in the article that the traditional gamma tuning capacitor
is not essential to a gamma matched antenna.
Above (Britain 2006) is the subject dipole which Britain describes as a
partial folded element having a J shape which is grounded at the midpoint of the longest portion of the element. ((Duffy2010b) refers to it as a half folded half wave dipole.)
(Britain 2001) suggests that the dipole in free space has a feed point impedance of 150Ω:
One solution is to use a partial folded J element, having an impedance of 150Ω as the driven element…
However, that portrays a failure to understand how it actually works.
The sketch above shows the current paths at the centre of the W5VJB dipole (conductors all of the same diameter). The current 2I would flow in an ordinary half wave dipole which has a centre feed free space point impedance of around 72Ω, and since only half that current flows from the source shown above for the same total dipole current moment, its load impedance is four times that of a centre fed dipole, or around 288Ω (often rounded to 300Ω).
This is quite the same operation as a gamma match with equal size conductors.
So, what we have is a gamma match where the gamma section is an electrical quarter wave, the centre impedance of the half wave plain dipole is multiplied by four and shunted by a quarter wave o/c stub which has a very high impedance and negligible effect on the feed point impedance, The feed point impedance will be around 288Ω for such a dipole in free space.
When such a dipole is used to replace a plain dipole driven element in a Yagi where the feed point impedance was around 12.5Ω (as in the W5VJB cheap Yagis), the W5VJB dipole provides a 4:1 impedance step up to 50Ω (Duffy 2010b).
But yes, the W5VJB J dipole is simply a gamma match for transformation from 12.5+j0Ω to 50+j0Ω, a specific application of the gamma match. W5VJB might not have designed it in that way, but it is exactly what would be obtained using the method at (Duffy 2013) designing a gamma match for transformation from 12.5+j0Ω to 50+j0Ω with equal conductors, see the Smith chart below.
Does it work? Yes, I think so, but I am sure it works differently to W5VJB’s patent explanation.
Is it novel? I don’t think so, it is a specific case of a gamma match, and like all gamma matches, it shares its current division behaviour with a folded dipole.
- Balanis, C. 1997. Antenna theory – analysis and design 2nd ed. John Wiley & sons New York.
- Britain, K. Feb 2006. Controlled impedance “cheap” antennas. http://wa5vjb.com/yagi-pdf/cheapyagi.pdf (accessed 10/11/14).
- ———. 2001. US Patent 6307524 Yagi antenna having matching coaxial cable and driven element impedances.
- Cebik, L. Oct 2006. Some Preliminary Notes on the Gamma Match. Restricted.
- Duffy, O. Apr 1998. Novel Gamma Match Construction. VK1OD.net (offline).
- ———. 2010a. Folded half wave dipole. VK1OD.net (offline).
- ———. 2010b. Half folded half wave dipole. VK1OD.net (offline).
- ———. 2013. Designing a Gamma match using a Smith chart. VK1OD.net (offline).
- ———. Nov 2014. Are gamma matches as bad as all that. http://owenduffy.net/blog/?p=2822.
- 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