One concept when using ~3-meter, Part 15 AM transmit antennas is that the higher they are installed above the surface of Earth, the greater their signal strengths at a given distance (other things equal). The assumed reason for this is that the antenna has better "line of sight" path clearance, which reduces propagation loss.
The graphic below shows the measured results of an experiment to investigate this concept.
Note that the fields measured at the same distance from the transmit antenna are the same for the "blocked" path as for the clear, unobstructed path.
The reason for this result is that obstructions such as buildings and terrain that are small in physical size compared to the wavelength of the radiated signal have almost no affect on them.
So what is responsible for the increased signal strength many have noticed when elevating a Part 15 AM 3-meter transmit antenna?
It is the effect of using the additional conductor length(s) needed when the 3-meter antenna is installed more than a few feet above Earth. All such conductors connected to the circuit "common" bus of an elevated transmitter and/or elevated antenna tuning unit (ATU) will have r-f current flowing on them, which adds to the radiation produced by the 3-m antenna in use.
Such conductors include the outer surface of the outer conductor of a coaxial cable connected between the transmitter and a remote, elevated ATU.
Some of the current on the inner surface of the outer conductor of the coax crosses to its outer surface where it connects to the ATU+3-meter "antenna" conductor, and travels along that outer coax surface for its entire length.
The presence of such r-f current produces e-m radiation into space from the coax cable for the same reason it does when flowing along a 3-meter antenna conductor.
Other radiating conductors (if present) are:
- Unfiltered d-c power and program audio lines leading away from an elevated Part 15 AM transmitter
- Conducting masts/supports used to install the system at those elevations above Earth while connected to the "ground" terminal of an elevated transmitter or ATU
- Conductors connecting the transmit system ground terminal to the "ground" conductor of the a-c mains outlet providing power to the system
CONCLUSION: Such added radiation is responsible for the performance improvement observed from elevated Part 15 AM transmit systems.
Over the years I have pondered this as well, but have no hard data.
As is well known, I have a Procaster mounted on a third floor window frame of a 100+ year old wooden house. It is mounted to a plastic conduit pipe that is then mounted with standoff brackets to the wooden window frame. There is no ground lead attached at all.
The only wires connected is the cable provided by Procaster that carries DC power and audio - 4 wires.
I get pretty solid coverage at about 1.5 miles, sometimes further. Now granted too, I'm in a rural area with no large construction or industry -- just small town homes with their typical infrastructure. I have a very low noise floor to work with.
But it's been discussed here before (years ago now) that I *must* be getting some radiation from that cable, and that somehow that wouldn't happen if I had a ground mounted install.
But the length of the connecting DC and audio cable would be the same or longer if I had ground mounted the transmitter. One way or the other the cables will need to connect between the studio and the transmitter. As my studio is on the second floor of the house, moving the transmitter to ground level would add nearly 25 feet of cable to the system in order to get the cables to ground level, which adds then 25 feet of alleged radiating cable vertically from the studio to the ground which theoretically would add field strength! The only way to avoid this would be if the audio and power came from underground and the cable led from underground to the terminals on the transmitter at ground level. Any transmitter requires power and audio and it must come from somewhere.
I've never tried, as my intent with the elevated install was simply convenience -- as up here the transmitter would be under 4-6 feet of snow half the year with a ground install.
I had intended to do some "real life" testing ground mounted vs elevated to see what effect I could produce in real life in reference to obstructions, since with a ground mount, depending on direction, I could have long chain link fence, an aluminum sided house, wood house, brick buildings, dense trees, all readily available as obstructions.
I scratched this testing idea as I realized that there would be too many other variables to come to any conclusion. Just moving my transmitter from it's present location to ground level directly below it would add a lot of additional cable from the source to the transmitter. Would an elevated ungrounded installation compare to an ungrounded ground mounted install? Were I to mount the ground mounted transmitter to a ground rod in the Earth this would add another variable. Plus the distance to the obstructions would not be constant. The more I thought about it, the more variables I found that would make any test inconclusive.
To do this realistically, moving the transmitter and the testing point where readings are to be taken is not the answer. Being able to move the obstruction into the path would be the way to do it correctly, while leaving the transmitter and metering equipment stationary. That of course is not possible.
So I default to science.
TIB