Antenna Resistance
Resistance (R) is the “real” part of the antenna system’s impedance and is measured in ohms. In a 50-ohm radio system, a resistance near 50 ohms usually contributes to a good match. If resistance is much lower or much higher than 50 ohms, SWR will increase even if the antenna is perfectly resonant. For example, an antenna measuring 25 ohms resistance and 0 ohms reactance is resonant, but it is still not well matched to 50-ohm coax. Likewise, an antenna measuring 100 ohms resistance and 0 ohms reactance is also resonant but mismatched. Resistance is affected by antenna design, feedpoint location, antenna height, loading coils, radials or ground system, nearby conductive objects, losses, and even the feed line itself. It is important to understand that 50 ohms of resistance does not automatically mean the antenna is highly efficient; a 50-ohm dummy load also has an excellent SWR but converts nearly all RF energy into heat rather than radiating it.
Disclaimer: This independent educational material is not affiliated with, sponsored by, or endorsed by the developers, manufacturers, standards organizations, government agencies, or network operators associated with the technologies and services described.
