This 1 square meter antenna has radiation resistance of about 10 nanohoms, making the 0.4 Ohm resistance (including skin effect, which about doubled the resistance from the DC value) the dominant loss in the antenna. The lower the resistance, the more efficient it would be - and from these numbers, efficiency is nearly directly proportional to 1/R; cut the resistance in half and double the efficiency. The antenna, which is a square loop of 0.5 square mm copper wire 1 meter on each side, measured 6.2 uH and under 0.2 ohms DC.
In the U.S., the Federal Communications Commission rules limit unlicensed transmitters in the 160 to 190 kHz band to a maximum input power to 1 watt and maximum antenna length of 15 meters. With such a small antenna compared to wavelength, the radiation resistance would be very low, and real (Ohmic) resistance in the antenna would tend to be a lot larger than the radiation resistance, making the antenna even less efficient. This puts a premium on getting the real resistance of the antenna as low as one practically can. Some have made antennae from copper water pipe and RG 8/U coax shield, and they would be more efficient for their size than this one, which used 0.5 square mm copper wire.
This output stage has a rich lineage. Murray Greeman, ZLBPU is credited as the originator of the circuit as an LF output stage. Bill Ashlock found that the output signal was cleaner when the transistors has separate base driver resistors. I chose to move the clamping diodes to the end of the base resistors connected to the transistor bases and eliminate the third resistor in the base circuit that was in common with both inputs.
I encourage you to do an internet search on Murray Greeman ZLBPU, Bill Ashlock, and Lyle Kehler) for more information on Lowfer transmitter and antennas. Here are some URL's to get you started. Though these links worked on the day I created this page, I don't make any promises to keep them up-to-date.