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Some Part 15 AM txs have a “loading coil” inside the tx enclosure (ex: Rangemaster), and some require an external loading coil (ex: SSTRAN).
Once the capacitive reactance of the short antenna is “zeroed” (brought to resonance) by the necessary inductive reactance of the loading coil, then the transmitter will see a non-reactive load — which can allow for maximum power transfer to the load from the tx output circuits, which produces the maximum amount of r-f current flow on the antenna, which results in maximum radiation for the available power.
But it isn’t enough for the Part 15 AM antenna to have zero reactance at the feedpoint, if the tx output circuits are unable to accommodate the variation possible in the resistive term of the load impedance. And the largest contributor by far to that load resistance term is the resistance of the r-f ground path.
This r-f resistance in the ground path is not the DC resistance of the conductor(s) leading to whatever is considered “ground,” but the resistance of the r-f ground itself plus the conductors connecting it to the tx, at the operating frequency.
Consider a tx designed for a 50 ohm, non-reactive load. When its output is connected to a load that is non-reactive (resonant), but because of a poor r-f ground has a resistive term of 150 ohms instead of 50, then the load SWR seen by the tx will be 3:1, and the return loss from the load (antenna) will be 6.02 dB. The power delivered to the antenna feedpoint will be around 25% of the power that the tx could deliver there, if it was capable of matching into that 150 j0 ohm load. Most of that “missing” power would be lost as heat in the tx output networks. And this doesn’t include the power lost in the r-f ground connection, which is even worse for these conditions.
So a Part 15 AM tx needs to be able to match into a fairly wide range of load resistances, even if those loads have zero reactance (are resonant).
The r-f resistance of the “ground” path in a typical Part 15 AM setup might vary from a low value of 10 ohms to a high value of several hundred ohms.