Jim, K8ELR sent me a link to the United States Marine Corps Field Antenna Handbook, MCRP 6-22D. I thought that I’d covered this before, but couldn’t find it by doing a search. This manual covers antennas and propagation in a very straightforward way, and you can’t beat the price (FREE!)
Here’s what the manual has to say about antenna impedance:
Impedance is the relationship between voltage and current at any point in an alternating current circuit. The impedance of an antenna is equal to the ratio of the voltage to the current at the point on the antenna where the feed is connected (feed point). If the feed point is located at a point of maximum current, the antenna impedance is 20 to 100 ohms. If the feed point is moved to a maximum voltage point, the impedance is as much as 500 to 10,000 ohms.
The input impedance of an antenna depends on the conductivity or impedance of the ground. For example, if the ground is a simple
stake driven about a meter into earth of average conductivity, the impedance of the monopole may be double or even triple the quoted values. Because this additional resistance occurs at a point on the antenna circuit where the current is high, a large amount of transmitter power will dissipate as heat into the ground rather than radiated as intended. Therefore, it is essential to provide as good a ground or artificial ground (counterpoise) connection as possible when using a vertical whip or monopole.
The amount of power an antenna radiates depends on the amount of current which flows in it. Maximum power is radiated when there is maximum current flowing. Maximum current flows when the impedance is minimized—when the antenna is resonated so that its impedance is pure resistance. (When capacitive reactance is made equal to inductive reactance, they cancel each other, and impedance equals pure resistance.)