[aprssig] 50 ohms vs 75 ohms
jimlux at earthlink.net
Wed Mar 29 12:51:49 CST 2006
I couldn't find my old notes, but an old post from Rick Karlquist (N6RK)
covers it succinctly:
I believe this is discussed in the MIT Rad Lab books.
With air dielectric, 74 ohms gives the minimum loss for a given
outer diameter. 35 ohms give the highest power handling capability
assuming air breakdown is the limiting factor. 50 ohms is a compromise
about half way in between.
While it is true that a dipole has an impedance of 72 ohms, it's just a
coincidence (or maybe it falls out from the physics) that this matches the
lowest loss Z.
As far as monopoles.. they're 36 ohms. If you build a "ground plane"
antenna with drooping radials you can arrange it to get 50 ohms. Likewise
a inverted-v dipole.
More information about the aprssig