I’d like to point out that, although the image of radio signals fleeing the surly bonds of Earth into the cosmos included in Contact makes for a great Hollywood visual, dating this expanding wavefront all the way back to the introduction of radio communication is scientifically inaccurate. In fact, until the introduction of VHF (Very High Frequency) transmissions, radio signals were pretty much trapped on Earth by the ionosphere.
Over the years the general pattern of radio development has been to use higher frequencies as more bandwidth becomes needed. The original longwave and mediumwave signals, used for Morse code communication, and, beginning in the 1920s, for broadcasting, did not escape Earth, but instead were either absorbed by the ionosphere (originally known as the “Heaviside-Kennelly layer”) during the day, or reflected (okay, “refracted” to be technically correct) back to Earth at night.
Shortwave transmissions, developed in the early 1920s, also are predominantly reflected back to Earth. Thus, through the 1920s, there was speculation that the ionosphere had cut the Earth off from ever communicating via radio signals with the rest of the universe. Fortunately, it turned out that VHF and higher frequencies signals, used by FM and TV stations beginning in the late 1930s, can generally pass the ionosphere unimpeded.
Also, in order to reduce costs and increase efficiency, most broadcast FM and TV signals are to a degree focused. FM and TV antennas use stacked elements known as “bays”, and the more bays, the more concentrated the signal into a narrow horizontal plane, aimed at the horizon. This is from memory of the 1980s, but if I recall correctly, a 16-bay antenna gave about a 10-times improvement – i.e. a 500 kilowatt “Transmitter Power Output” (TPO), focused by a 16-bay antenna, resulted in a 5 megawatt “Effective Radiated Power” – (ERP) i.e. the resulting effective signal strength for a receiver located in the plane of the transmitted signal. Thus, as the Earth rotates, for an individual station any extraterrestrial observer would see two pronounced signal peaks, as the turning Earth aimed first the front, and then the back, sections of the transmission plane. And of course there likely would be a jumble of signals, since multiple FM and TV stations share the same operating channels.
The higher the frequency, the more the signal wants to travel line-of-sight rather than follow the curve of the Earth, so, the U.S. Federal Communications Commission lets TV stations operating on higher frequencies use higher power in order to compensate. Thus, during the analog TV days, for low-VHF channels (2-6), the power limit was 100 kilowatts ERP, 316 kilowatts for high-VHF channels (7-13), and 5000 kilowatts for UHF (Ultra High Frequency) stations (originally channels 14-81). (The power limits were higher than needed for the UHF band to have coverage comparable to that of the VHF stations, as an incentive for people to build UHF stations). As Cecil noted, with the transition to Digital TV, most TV stations now operate on the UHF band (now contracted to channels 14-51), although with more stations crowded on fewer channels, their transmission powers have been reduced, but UHF signals are still generally stronger than VHF.
I bring this up because I believe the statement “The most powerful passive leakers are VHF television stations and military radar…” should actually read “UHF”.