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If the quality of the ground varies due to say rain, then the transmitter has to be retuned?
The table below shows how the load on a Part 15 AM transmitter is dependent on the values of the loading coil a-c loss resistance, the r-f ground resistance, and the reactance of the antenna system.
The coil loss was set to 15 ohms, which might be typical for the ferrite loading coil used in some modern Part 15 AM transmitters.
The reactance (j value) was set to 10 ohms, in recognition that it is difficult to achieve and maintain zero ohms (perfect resonance) when adjusting the loading coil.
The table shows how the reflection coefficient and SWR of that complex load varies with different values of loss in the r-f ground connection over a practical range for a typical installation, assuming that the transmitter was designed to drive a load of 30 +j 0 ohms.
The reflection coefficient shows what percentage of the transmitter output power is reflected by antenna system for those conditions, returning back to the transmitter and typically dissipated there as heat.
The loss in the r-f ground system (rods, buried radials) can vary depending on the composition, moisture and mineral content of the soil as far away as 1/2 wavelength from the antenna.
|Ground R||Coeff. %||SWR|