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I think that ArtisanRadio has the right idea that the effect reported in this thread has to do with capacitive coupling from the antenna to the metal roof.
All Part 15 AM operators know that their antennas are very inefficient. The FCC rules are designed to make them inefficient. But sometimes Part 15 AM operators don’t appreciate just how inefficient their antennas really are. When someone takes a 10-foot length of copper pipe home from Home Depot, the pipe seems to be enormous; but it is quite tiny compared to a quarter wavelength anywhere in the AM BCB. Good efficiency can be obtained with a 10 foot monopole on the 15 meter ham band, but not at 1.7 MHz. It seems to me that the electrical properties of the barn with its metal roof, and whatever other conductors are present, are such that they provide a radiator with higher radiation resistance at the upper end of the AM BCB than the CB whip used. When the ground wire is connected to the metal roof, the roof serves as a counterpoise, which radiates to some extent, since it is some distance above the earth. When the ground wire is disconnected, the coupling to the metal roof is capacitive. In this particular case, the metal roof and the other conductors related to it, radiate better when capacitive coupling is used rather than direct coupling via a ground wire. The electric field near the CB whip is quite high, and there seems to be a happy accident that it couples well to the barn and its metal roof so that the barn radiates the RF power from the SSTRAN with higher efficiency than the CB whip.
The better radiation efficiency of the barn compared to the CB whip is due to the much larger size of the barn.
Soundcrafter may have rediscovered an effect that has been put to good use by promoters of electrically short antennas that supposedly have higher efficiency than their small size would normally allow. For example, the CFA promoters have tested their short antenna by disconnecting the quarter wave tower at an AM radio station, and replacing it with the CFA. The disconnected quarter wave monopole was close to the CFA, and capacitive coupling between the CFA and the original antenna caused the original antenna to radiate with higher efficiency than was possible with the CFA.
In another CFA experiment, a slightly more subtle deception was performed by installing the CFA near a microwave antenna tower that had a height that was roughly a quarer wavelength of the operating frequency. The microwave antenna tower radiated the RF power with higher efficency than the CFA could have. Nearby large resonant structures were, however, not avilable for all of the CFA experiments. In those cases, the CFA was mounted on a roof, and a long conductive path to earth ground provided most of the radiation.
So, my guess is that the barn with its metal roof is a much better radiator at the operating frequency of the SSTRAN than the CB whip is.