Post by Rich on Jun 16, 2019 13:59:28 GMT
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 radiated signals 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:
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 radiated signals 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.