this post was submitted on 15 Mar 2026
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Something that I found super interesting learning about amateur (ham) radio was that antennas don't always work "backwards" as you'd expect. From Maxwell's equations they obey reciprocity, so it stands to reason (or so I thought) that an antenna that's good at receiving is also good at transmitting.
But it's not true! It turns out that the noise floor of the environment
in part due to atmospheric stuff like lightning
is so much higher than the sensitivity of radios (well above thermal/Johnson noise) that an inefficient antenna can be a really good antenna for receiving, in certain circumstances. Namely, if a receive antenna is inefficient but has good directionality, it can be useful...but probably no good for transmission!
It's not super profound or anything, but I found it pretty interesting.