An email was sent to me asking about the relative performance of 3-m verticals with and without loading coils and ground connections. Following is my response based on study of the subject in standard antennna engineering textbooks and related papers, NEC evaluations, and career experience.
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NEC calculates the intrinsic radiation patterns and directivity for a "wire structure" having specified sources and loads at defined locations, and also reports its drivepoint Z (among other things). So the pattern and gain of the 3-m vertical alone is always the same for a given set of conditions, regardless of whatever networks precede it.
With no loading coil, the input reactance of a 3-m vertical on 1700 kHz is >3,000 ohms (capacitive). Of course, no practical tx can deliver much power into such a large impedance mismatch. Most of the tx power generated is converted to heat in the output amplifier and the transmission line to the antenna. The amount radiated from the antenna is very low.
With the loading coil the input reactance of the antenna can be set to ~zero, albeit with some added R in the coil. But even with the I^2R loss of the coil, much more of the available tx power can be delivered to the load connected to the tx.
All comments above are independent of ground considerations. The resistance of the ground connection produces I^2R loss in the r-f current available from the tx, and so reduces the power reaching the radiator -- whether or not the radiator is resonant.
And the resistance in the ground connection is not just to the earth, itself, but to the r-f currents induced into the earth by radiation from the antenna. That is why a vertical using a proper radial ground system produces much greater fields than if all of that copper in the radials was buried in one mass, under the radiator. The radial conductors have to be buried where they can collect most of the r-f ground currents from the radiator, and return them to the ground terminal reference of the tx/antenna system with minimal loss. This is a subtle, but very important point.
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NEC calculates the intrinsic radiation patterns and directivity for a "wire structure" having specified sources and loads at defined locations, and also reports its drivepoint Z (among other things). So the pattern and gain of the 3-m vertical alone is always the same for a given set of conditions, regardless of whatever networks precede it.
With no loading coil, the input reactance of a 3-m vertical on 1700 kHz is >3,000 ohms (capacitive). Of course, no practical tx can deliver much power into such a large impedance mismatch. Most of the tx power generated is converted to heat in the output amplifier and the transmission line to the antenna. The amount radiated from the antenna is very low.
With the loading coil the input reactance of the antenna can be set to ~zero, albeit with some added R in the coil. But even with the I^2R loss of the coil, much more of the available tx power can be delivered to the load connected to the tx.
All comments above are independent of ground considerations. The resistance of the ground connection produces I^2R loss in the r-f current available from the tx, and so reduces the power reaching the radiator -- whether or not the radiator is resonant.
And the resistance in the ground connection is not just to the earth, itself, but to the r-f currents induced into the earth by radiation from the antenna. That is why a vertical using a proper radial ground system produces much greater fields than if all of that copper in the radials was buried in one mass, under the radiator. The radial conductors have to be buried where they can collect most of the r-f ground currents from the radiator, and return them to the ground terminal reference of the tx/antenna system with minimal loss. This is a subtle, but very important point.
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