Antennas and Radiation
https://www.sciencedirect.com/topics/computer-science/antenna-impedance
Scroll Down to:
Resonant Frequency and Bandwidth
"The input impedance Zin of the antenna is a function of the frequency of operation. Figure 6.5 shows the magnitude of the input impedance of an example antenna as a function of frequency. In this case, the antenna impedance looks like a parallel RLC resonant circuit. The frequency, fr, for which the impedance magnitude is maximum, or equivalently the reactance is zero, is often defined as the resonant frequency. It is desirable to operate the antenna at this resonant frequency so that it can be easily matched to an input transmission line with a real characteristic impedance. The frequency span Δfr, beyond which the impedance magnitude falls below half of the resonant value, may be defined as the impedance bandwidth of the antenna. The antenna may be usable in this frequency band. Outside the antenna's bandwidth, the input power to the antenna would be significantly reflected due to impedance mismatch, resulting in poor radiation."...
"In such cases, the resonant frequency is defined as the frequency of best performance and the bandwidth as the frequency span over which the performance can be tolerated, as dictated by the application. It is meaningful to call the resonant frequency and the bandwidth of an antenna with an appropriate parameter prefix, such as the impedance–resonant frequency, the impedance–bandwidth, gain–resonant frequency, the gain–bandwidth, ..."
More: X (=0) the reactance tells you when the antenna "resonate" in that specific frequency. That is what you usually need first, to make the antenna resonant at a specific frequency.
Then, when you got an antenna close to X=0, then you check R, the impedance, usually you need to go close to 50 Ohms. R=Z
Z is the total impedance, from R + X. If you get R=50 and X=0 then SWR will be 1.
Not all antennas are designed for 50 Ohms.
https://ham.stackexchange.com/questions/13157/rigexpert-aa-35-interpreting-the-information
[MEDIA=scribd]21928505[/MEDIA]
