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[aprssig] 50 ohms vs 75 ohms

Jim Lux jimlux at earthlink.net
Wed Mar 29 18:51:49 UTC 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.

Jim, W6RMK






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