On 15 Aug, 1977, Jerry R Ehman received what may be our first, and possibly only, communication from extraterrestrial intelligence. Ehman was working at Ohio State University's Big Ear radio telescope and he scribbled "Wow!" on the paper printout which was recording the telescope's signal. The website below has the technical details, if you are interested.https://en.wikipedia.org/wiki/Wow!_signal
PS:For some reason, I can't just click on this text and go to that page, you may have to cut and paste it into your browser, type it in by hand, or Google "wow signal" to take you there.
The telescope could be pointed in the North-South direction, but depended on the Earth's rotation to sweep across the sky, from West to East. The signal was picked up as a loud burst of 1420 MHz radio emission from a point in the Southern Milky Way, in the constellation of Sagittarius. Although the point is not known with great precision, it can be calculated because the geometry and electronics of the telescope are well understood. The signal must have originated from a box roughly one degree on a side (a degree is about twice the width of the full moon. Furthermore, the signal can be constrained to two narrow strips at the E and W edges of that box. As a check, the radio telescope alternated between two feed horns to make sure the signal was actually coming from the sky and not accidentally generated by a glitch in the receiver's amplifiers, but this introduces an ambiguity as to which one of those strips is the actual origin. The website has a star chart which should make this clearer than my explanation.
The telescope was tuned to 1420 MHz because that is the 21 centimeter (wavelength) line of neutral hydrogen gas. The entire galaxy resonates at this frequency which is the natural emission of the hydrogen clouds floating between the stars. This frequency would no doubt be familiar to any alien civilizations who knew anything about astronomy. It is also in a part of the radio spectrum which is relatively free of static and noise. In other words, its a natural place for ETs to be transmitting, and they would be assured that there might very well be someone listening at that frequency. Its the logical place to transmit, so its the logical place to listen.
There are a variety of reasons to suspect this might be the real thing. The twin horns of the telescope assures that the signal is external, not a land-generated signal or noise from an Earth satellite or from within the receiver. It was also a narrow-band signal, which suggests it was artificially generated. This equipment was designed to search for just this type of signal, and this is exactly what they were expecting to hear, That frequency is also reserved for astronomical use by international treaty so it is not likely there is anyone else, military or industrial, transmitting on it. We also know the noise must have come from very far away because it did not move while it was scanned. The burst was much louder than the normal 21 cm background hum from galactic hydrogen. As the Earth turned, the Big Ear heard a single clear note from deep space for 72 seconds.
The discovery of an artificially generated radio signal from outside the solar system would be the greatest scientific discovery of all time, so the experiment must be repeated to be verified. 24 hours later, when the scope scanned that spot again, it listened. There was no signal. Urgent telegrams were sent to other observatories to try and repeat the observation, there was nothing heard. In fact, nothing since has been heard from that location, although many have listened. Star charts show nothing at that spot except a few faint, unremarkable stars. Photographs through great telescopes reveal nothing, just a mist of stars--this is the southern Milky Way, lots of faint stars, but so is the view in any other direction. Like sand on a beach.
All we know about the Wow! signal is its frequency, strength, location and that it was on for at least 72 seconds but never repeated. It is highly unlikely it was a hoax, a natural phenomenon, or some Earth or satellite transmitter operating at an unauthorized frequency, although none of those things can be ruled out completely, either. Although you would expect an attempt to communicate would last for more than 24 hours, it is always possible that we just accidentally intercepted a real alien signal meant for someone else, or we were listening to some kind of industrial activity or automatic beacon on another world that occasionally gives off a burst of radio noise. We just don't know.
The Big Ear Radio Telescope has since been torn down, and a golf course erected on that site..