In the prototype antenna system, an additional common-mode choke was inserted about 2.5 m away from the matching network. This choke reduces the likelihood of the excitation of the common-mode current in the coax due to antenna radiation. It is a good practice to apply such a precaution not just for the LPi antenna but for any other end-fed antenna as well as GP category antennas. This additional choke should ideally be connected about 1/4 wavelength away from the matching network. In this case, the wavelength is related to the maximum operational frequency (Fmax). It was about 2.5 m for Fmax=29.7 MHz.
If you would like to experiment a bit with the LPi for the same frequency range, a good idea would be to make D a little larger, say, 16 cm. You should then decrease SWR at the band edges and increase it a little in the center of the band. It may be important for the transceivers not equipped with internal ATU. I would not bother though if my transceiver had the ATU.
Speaking about experimenting, I also built an LPi on an aluminum mast. My simulation predicted feed point impedance closer to 100 ohms instead of 200 ohms. I built a 50 ohm to 112.5 ohm step-up unun. 9:4 unun, if you will. Unfortunately, I got smaller bandwidth than expected. Perhaps the unun I built was not perfect enough and its impedance ratio was different? Or the simulation was inaccurate? It is a well-known fact that NEC-2 and NEC-4 based simulators introduce errors in the simulation results when closely spaced wires of very different diameters are used (as it was in this case). It is possible that the newest NEC-5 engine still has this flaw? Chances are, I would finally get the desired bandwidth by changing antenna geometry or/and the unun construction by trial and error. But I was not patient enough and disassembled this antenna version. I just want to let you know that if you decide to use an aluminum mast rather than a fiber glass one, be prepared to spend more time (and work) for antenna tuning.
I compared the newly built LPi with my proven GP-7DX vertical antenna installed on the other side of the house. I could not tell which one works better. They performed equally well. The LPi efficiency leaves nothing to be desired. This is an inexpensive, easy to build and easy to tune antenna. Even a beginner can build it. When installed vertically, it radiates at a low angle, just like a vertical dipole. That's why it is good for DX hunting. However, if your QTH is surrounded by other buildings, metallic fences, lamp posts, it is recommended to install it not just above the ground but a couple of meters higher or even on your house roof. Nothing will block your signal then. You can install the LPi practically everywhere because it has minimal footprint and does not require grounding or counterpoises. I have been using this antenna for almost two years now, and I really recommend it to every ham.