This article is an addendum to G4CQM 50Ω coaxial dipole, it is another study of models of insertion of the G4CQM 50Ω coaxial dipole into a Yagi with a 50Ω dipole driven element. The models are derived from a manufacturers published model.
Fig 1 shows the VSWR and |Reflection Coefficient| of the base configuration. The design is the result of optimisation, and delivers extremely good VSWR over the frequency range plotted. (The model is in free space, all model elements are lossless, and has zero length feed line, so there are no feed line common mode current issues.)
Fig 2 shows the antenna with the addition of the G4CQM 50Ω coaxial dipole stubs without retuning. As expected, the shunt reactance of the stubs degrades VSWR.
Fig 3 shows the antenna with the addition of the G4CQM 50Ω coaxial dipole stubs with retuning of the dipole length for lowest VSWR at 146MHz. Although the dipole has been retuned, best VSWR is not as good as the base configuration.
This should not be surprising. If we take a perfect 50Ω load (and the configuration in Fig 1 is near to perfect), and shunt it with any impedance, the VSWR must be degraded. It would be wishful thinking that adjusting the length would at the same time deliver a basic dipole impedance with R and X both altered exactly to compensate for the shunt stub reactance.
Fig 4 shows the antenna with the addition of the G4CQM 50Ω coaxial dipole stubs with retuning of the dipole length and position for lowest VSWR at 146MHz.
The G4CQM 50Ω coaxial dipole has not improved the match or the VSWR bandwidth of the base configuration, in fact it has transformed a superb antenna into a rather narrow one. The stubs introduce a reactance than hasn't compensated the bare feed point impedance, but exacerbated impedance change with frequency.
It is arguable that the configuration shown in Fig 3 is better than the optimised configuration in Fig 4, although the VSWR is a little poorer, VSWR bandwidth is better and overall, a more forgiving, more practical antenna.
The best configuration is clearly the base configuration shown in Fig 1 without the G4CQM 50Ω coaxial dipole stubs, and on the surface of it, it looks a very good antenna for its type.
None of this is to suggest that the antenna is a poor one, or to comment on other parameters such as gain, spillover noise, front to back, etc. It is entirely an exploration of the claimed benefit of the G4CQM 50Ω coaxial dipole stubs using NEC models.
VK1OD on the 'net. I appreciate feedback, click on the ... in webm...@vk1od.net for my email address.
© Copyright: Owen Duffy 1995, 2012. All rights reserved. Disclaimer.