Total posts : 45366
Unfortunately there is not nearly as much solid technical information published about short antennas at low frequency as there is about more conventional antennas. I would love to see a really good technical analysis published about elevated ground planes for short antennas at low frequencies. Their effectiveness at AM frequencies seems to be more a matter of trial and error which is difficult due to the heights and lengths of wire involved.
A more conventional tuned part 15 antenna with a standard ground radial system is pretty well understood and has a simple mathematical model. As long as the antenna is elevated up to about 20 ft above ground, the model seems to apply. When a tuned antenna is elevated too far above a standard ground system, the model changes. 20 ft is a small fraction of a wavelength at AM frequencies, so the ground situation doesn’t change much. Beyond that, when the height becomes a significant fraction of a wavelength, then the model changes.
I know that some have reported success with elevated ground planes, but I have never seen any real experimental proof that they are of real benefit. For example, someone may report that their signal improved when they strung some ground wires out from the base-point of their elevated antenna, but this is not meaningful unless it can be compared against a description of the ground system in use before the elevated radials were added.
The Part 15 broadcast community is populated by a significant number of people who have little or no knowledge of antenna theory, but they are typically very enthusiastic about part 15 broadcasting. It is often difficult to sort out the lore from the facts.
It would be wonderful if there were a book written by a real professional antenna engineer focused on 3 meter AM broadcast antennas. If anyone knows of one, I would like to know about it! Otherwise we are stuck with the simple model which has its roots in mobile and portable whip antenna analysis by ameteur radio people for the 160 meter band (just above the AM band).