Do They Know We’re Here? – Clarification

My post on August 21, 2010, “Do They Know We’re Here?” contains a bit of misinformation, which I hadn’t thought of until reading the latest Planetary Report from the Planetary Society.

While we’ve been experimenting and transmitting radio signal for over a 100 years, those early transmissions were low frequency, typically 1 MHz or less.  The ionized layer of our atmosphere – the ionosphere, reflects those signals back down to the surface, so they won’t escape the planet.

If any of you have listened to AM radio (Remember that?) at night, you find that you can pick up stations from 100s to 1000s of miles away, whereas you can’t hear them during the day.  This is because at night, the sunlight is no longer showering the atmosphere with UV radiation, which knocks electrons off the oxygen and nitrogen atoms charging them – ionizing them.  The lower layer of the ionosphere rapidly dissipates when the Sun sets, leaving the higher layers to reflect the AM transmissions, allowing them to reach much further.

We don’t see this with our FM radios,  because they operate at much higher frequencies (in the 10-100 MHz range) and don’t rely on the ionosphere to  reflect them to your radio.  For your radio to receive your favorite FM station, you have to be able to receive the signal directly from the transmitting antenna, this is why they are called “line-of-sight” transmissions.  These higher frequencies were used commercially for FM radio and early television in the early 1930s, and they blasted right through the ionosphere and out into space.

So, a long winded explanation to say that we’ve really been broadcasting to the cosmos for about 75 years, as opposed to the 100 years mentioned in the earlier post.

Comments always welcome!

Till next time,

RC Davison

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