There has been some discussion about ways to install a Part 15 AM transmitter on a rooftop or other elevated mount that would allow functionally legal operation according to FCC Part 15.219, but still provide some means of lightning protection for the elevated system.
One manufacturer has supplied a choke for this purpose, for installation between the transmitter chassis/r-f ground terminal and the "ground lead." However that was not shown to be acceptable when the FCC tested an elevated system before and after installation of the choke.
The problem here is that a choke that is effective in eliminating radiation from the conducting path from the transmitter chassis to a true r-f ground (something buried in the earth, typically) also prevents radiated r-f currents from returning to the transmitter chassis from the ground plane below the transmitter. This means that such an elevated system with an effective choke will radiate less than it would if it was installed with its base at earth level, and used no choke in its ground lead.
Several computer studies were done using NEC-2D to investigate a system configuration that would produce a better result.
One configuration that has promise is an elevated Part 15 AM transmitter with a 3-meter conductor attached to its antenna output connector, and with two horizontal conductors each 3 meters in length attached to the tx chassis/r-f ground connector, and aimed in opposite directions.
For such a system elevated 20 feet above a perfect earth, and with no wires or other conductors such as towers/masts/flagpoles connecting the transmitter to an earth ground, resonance on 1700 kHz is achieved by a coil of about 4400 ohms of inductive reactance. Assuming that the coil has 10 ohms of r-f loss, the radiation pattern is circular in the horizontal plane, and has a peak gain in the horizontal plane of about -14.6 dBi. The vertical radiator is being driven against the two horizontal wires, which serve as a counterpoise, and a source for returning r-f currrents back to the transmitter to complete the circuit needed for best possible radiation from the system.
Comparing the above to a ground-mounted system using a total radiating length of 3 meters, including the exposed length of a short wire connecting to a buried ground rod with 30 ohms of r-f resistance and no horizontal wires, it could be resonated on 1700 kHz with a coil having about 3026 ohms of inductive reactance (perfect earth). If the loading coil has 10 ohms of r-f loss then the peak gain of the system is -20.5 dBi.
These gain numbers are negative, which means that the elevated system has almost 6 dB more gain than the system at ground level.
At least one popular design of a Part 15 AM transmitter includes a high-resistance path and a "surge suppressor" across the r-f output to chassis/r-f ground. These serve the need to drain off static charges that build up on the antenna, and to bypass the transmitter circuits for high voltage transients from nearby lightning strikes. But they do need a fairly low impedance path to an earth ground in order to function well.
That path can be provided in a way that will prevent most radiation from the ground lead by using a high-Q, parallel tuned circuit installed where the ground conductor connects to the r-f ground terminal of the transmitter. It could add several thousands of ohms of loss to the ground conductor on the operating frequency, while having little affect to frequencies below the broadcast band where lightning energy is highest. It would also provide a d-c path to a buried ground rod, which will drain static from the elevated whip.
But the added loss in the ground conductor on the operating frequency will have little affect on the elevated system with the two horizontal wires, as it doesn't depend on the ground conductor for returning radiated r-f current to the system.
It would be useful if one or more of the manufacturers of commercially sold Part 15 AM transmitters would invest a bit of R&D money into further investigations of this, possibly leading to the introduction of a product line accessory that would benefit their customers.
Part of that investigation probably should include getting an official response from the FCC with regard to functional, legal compliance of such systems with Part 15.219.
RF
One manufacturer has supplied a choke for this purpose, for installation between the transmitter chassis/r-f ground terminal and the "ground lead." However that was not shown to be acceptable when the FCC tested an elevated system before and after installation of the choke.
The problem here is that a choke that is effective in eliminating radiation from the conducting path from the transmitter chassis to a true r-f ground (something buried in the earth, typically) also prevents radiated r-f currents from returning to the transmitter chassis from the ground plane below the transmitter. This means that such an elevated system with an effective choke will radiate less than it would if it was installed with its base at earth level, and used no choke in its ground lead.
Several computer studies were done using NEC-2D to investigate a system configuration that would produce a better result.
One configuration that has promise is an elevated Part 15 AM transmitter with a 3-meter conductor attached to its antenna output connector, and with two horizontal conductors each 3 meters in length attached to the tx chassis/r-f ground connector, and aimed in opposite directions.
For such a system elevated 20 feet above a perfect earth, and with no wires or other conductors such as towers/masts/flagpoles connecting the transmitter to an earth ground, resonance on 1700 kHz is achieved by a coil of about 4400 ohms of inductive reactance. Assuming that the coil has 10 ohms of r-f loss, the radiation pattern is circular in the horizontal plane, and has a peak gain in the horizontal plane of about -14.6 dBi. The vertical radiator is being driven against the two horizontal wires, which serve as a counterpoise, and a source for returning r-f currrents back to the transmitter to complete the circuit needed for best possible radiation from the system.
Comparing the above to a ground-mounted system using a total radiating length of 3 meters, including the exposed length of a short wire connecting to a buried ground rod with 30 ohms of r-f resistance and no horizontal wires, it could be resonated on 1700 kHz with a coil having about 3026 ohms of inductive reactance (perfect earth). If the loading coil has 10 ohms of r-f loss then the peak gain of the system is -20.5 dBi.
These gain numbers are negative, which means that the elevated system has almost 6 dB more gain than the system at ground level.
At least one popular design of a Part 15 AM transmitter includes a high-resistance path and a "surge suppressor" across the r-f output to chassis/r-f ground. These serve the need to drain off static charges that build up on the antenna, and to bypass the transmitter circuits for high voltage transients from nearby lightning strikes. But they do need a fairly low impedance path to an earth ground in order to function well.
That path can be provided in a way that will prevent most radiation from the ground lead by using a high-Q, parallel tuned circuit installed where the ground conductor connects to the r-f ground terminal of the transmitter. It could add several thousands of ohms of loss to the ground conductor on the operating frequency, while having little affect to frequencies below the broadcast band where lightning energy is highest. It would also provide a d-c path to a buried ground rod, which will drain static from the elevated whip.
But the added loss in the ground conductor on the operating frequency will have little affect on the elevated system with the two horizontal wires, as it doesn't depend on the ground conductor for returning radiated r-f current to the system.
It would be useful if one or more of the manufacturers of commercially sold Part 15 AM transmitters would invest a bit of R&D money into further investigations of this, possibly leading to the introduction of a product line accessory that would benefit their customers.
Part of that investigation probably should include getting an official response from the FCC with regard to functional, legal compliance of such systems with Part 15.219.
RF