Total posts : 45366
I think that the discussion so far has been mostly on-topic. Rattan reported excellent results with his completed SSTRAN kit while transmitting to AM receivers inside his house. This is a common experience. Good signals can be obtained inside the house even if the antenna is much shorter than three meters, and there is no ground connection. This is why Part 15 AM devices were originally used as “phono oscillators,” which allow a phonograph to make use of the audio power amplifier in an AM receiver. The fact that the near field of a short antenna has a minimum radius not limited by the short antenna length helps explain why signals inside a residence of typical size are so strong.
There was some difference in opinion about how far the near field extended, and so much of the discussion was to determine the extent of the near field.
The other discussions about noise in signals beyond the near field (including comparison between noise in AM and FM systems) were, I think, also on-topic because they relate to performance the SSTRAN might have when used outside with an antenna using a loading/tuning coil and a ground.
Part 15 AM systems are small, but they are not therefore simple. The environments they are used in can make them even more complex, in principle, than commercial AM systems. Perhaps the antenna of a commercial transmitter can (for example) be located more than a half wavelength from the nearest obstruction, as is recommended, but that is not going to happen if a Part 15 AM transmitter is operating in a typical back yard. So, if a discussion in the Forum appears to get “technical,” this is justified because of the difficulties and complexities under which the Part 15 operator must labor.
Here’s another digression: Part 15 AM can be used across all of the broadcast band, and even slightly below. Part 15 AM operators stick to the upper end of the band because of the much higher antenna efficiency there. However, the near field extends a lot further at the lower end of the range because of the longer wavelengths there. Maybe Part 15 AM is not hopelessly bad at the low frequencies.
Poynting’s theorem applies to the transmission of energy by electromagnetic fields from one place to another. Since the waves in the near field are mostly stationary, there is little transmission of energy there. It is in the far field that energy is transmitted, and that is where the Poynting vector is applicable.
Poynting’s theorem applies not only to antennas, but to transmission lines. It even applies to DC circuits. The transmission of energy from a battery to a resistor along wires can be calculated by determining the electric and magnetic fields along the transmissin path, and using the Poynting vector. I would not recommend such a complicated method for solving such a simple problem, however.