I see another one of my favorite antennas is being discussed here. About 15 years ago I built a PDL-4 out of a PDL and a shooting star. Sorry to say at the last minuet I allowed myself to be deceived into using a field strength meter in the near field for tuning its gain and ruined that project quickly. Impedance changes fairly drastically when adding other parasitic elements so you can expect to have to retune those gammas. I'm in the process of building a PDL-3 with one set of Yagi directors as we speak and had been considering the Quagi versus Quad ideas.
With 3 elements is should be close to the same gain Quagi or Quad. There was some talk about how the Yagi directors may interact with the folded dipole portion of the driven element in a negative way and that just may be the case. First, lets understand the term "folded dipole" is only being used here because the 1/2 wave dipole is not inline. The real folded dipole would consist of an electrical full wavelength of conductor, folded in half, over itself. What we really have on the PDL is more of a loaded dipole that is an electrical 1/2 wavelength but the loop inductors shrink its physical length to fit the quad loop around it.
The next thing to notice is it's the vertical dipole that feeds the horizontal loop and the horizontal dipole that feeds the vertical loop. If the dipoles radiated significant energy, the PDL might have less then 3 db isolation between vertical and horizontal since the dipole and quad would always be radiating in the opposite polarity. That is not the case since the dipole is delivering most of its energy to be radiated on the quad loop. However, we also have to consider there is most likely some small degree of radiation from the dipoles. That could make it a poor choice to add a Yagi element directly in front of a PDL driven element.
The Yagi is more likely to parasitically couple to the energy in the dipole of the driven element rather then the energy in the quad of the driven element. That energy is in the reverse polarity and also takes away some energy that would be driving the quad driven element to parasitically excite the wrong Yagi director. For a PDL-3 or even a 4, I would probably go all quad. Anything larger and I would make the reflector, driven and first director as quads and all other directors as Yagi's. That's because each director you add is excited by the element in front of it. As long as you have one quad director in front of the PDL driven element, any Yagi directors in front of it should only be parasitically coupling to the desired polarity.
With 3 elements is should be close to the same gain Quagi or Quad. There was some talk about how the Yagi directors may interact with the folded dipole portion of the driven element in a negative way and that just may be the case. First, lets understand the term "folded dipole" is only being used here because the 1/2 wave dipole is not inline. The real folded dipole would consist of an electrical full wavelength of conductor, folded in half, over itself. What we really have on the PDL is more of a loaded dipole that is an electrical 1/2 wavelength but the loop inductors shrink its physical length to fit the quad loop around it.
The next thing to notice is it's the vertical dipole that feeds the horizontal loop and the horizontal dipole that feeds the vertical loop. If the dipoles radiated significant energy, the PDL might have less then 3 db isolation between vertical and horizontal since the dipole and quad would always be radiating in the opposite polarity. That is not the case since the dipole is delivering most of its energy to be radiated on the quad loop. However, we also have to consider there is most likely some small degree of radiation from the dipoles. That could make it a poor choice to add a Yagi element directly in front of a PDL driven element.
The Yagi is more likely to parasitically couple to the energy in the dipole of the driven element rather then the energy in the quad of the driven element. That energy is in the reverse polarity and also takes away some energy that would be driving the quad driven element to parasitically excite the wrong Yagi director. For a PDL-3 or even a 4, I would probably go all quad. Anything larger and I would make the reflector, driven and first director as quads and all other directors as Yagi's. That's because each director you add is excited by the element in front of it. As long as you have one quad director in front of the PDL driven element, any Yagi directors in front of it should only be parasitically coupling to the desired polarity.
