The technical gurus here earlier, Rich Fry & Neal (Radio8z) both stated that since the wavelengths of an AM broadcast signal are so large, obstructions would make little difference.
I believe that where obstructions do matter is when they absorb the signal, or cause interference through other radio waves (such as LED lights).
@artisan-radio I dont care what anybody says, I know line of sight is neccessary for the relatively near feild. Beyond that obstruction may have no bearing, but those in the imeadiate area absolutely do.
The near field goes out to about 1 wavelength, after which there is a transition zone to the far field.
The drop off in field strength as you move away from the antenna is pronounced within the near field, then less so in the far field.
It does make sense that obstructions within the near field would make much more of an impact (even if it's only in signal absorption and interference), which is probably why getting my Talking Sign wire antenna above the roofline made such a difference in range.
As you (and others have said) Rich, obstructions in the far field probably make far less of a difference.
So what it boils down to, if this is all accurate, is that you want your antenna away from close (within the near field) obstructions, but it doesn't matter as much farther out. A height of 3 feet, with no close obstructions, would be almost or as effective as a height of 30 feet. That's discounting the effect of the long ground wire in the latter case, which may or may not be seen as compliant by the regulatory bodies.
I've moved around a lot, and had a lot of installations; I've found that ground conductivity tends to overshadow most other factors in the far field. These other factors do play a role, but it's not nearly as much of a role. For example, on Bowen Island, with poor ground conductivity (it's basically a rock sticking out of the ocean), I never could get more than a 1/4 mile range (no matter how high I mounted the antenna, and how long the ground wire was, and how many radials I installed). With the same transmitter I was getting 1-2 miles range on the mainland on a flood plain with great ground conductivity (and at a mounted height of 2 feet or so, no radials).
@artisan-radio Good post, and in it you mentioned
"..A height of 3 feet, with no close obstructions, would be almost or as effective as a height of 30 feet.."
- Which, though I'm not sure its entirely accurate, however the same type of observation is what had became apparent in Yellowstone (early 70s) when the FCC made them lower them and apply the ground lead as part of the 10 foot restriction. The result was not what they expected... the ground mounted transmitters still were far exceeding the approximate 300ft range which the 10ft antenna had been calculated to acheive back in 1956.. So in 1974 the FCC proposed to eliminate the 15.219 option (it was never intended for manufactures in the first place but the FCC had failed to specify that). But fortunately that proposal did not go through.
What I'm getting to is that both we and the FCC know that a compliant 15.219 install is capable of around a 3 mile radius according to physics, the equipment and methods used by the FCC to determine feild strengths.
And as you pointed out if you raise it an extra 20 feet it's not really going to make that much of a difference (in far-field range anyway) - but it will still make some difference (most likely, presumably at least).