The Holmdel Horn: The Most Important Radio Antenna Ever

By Andrew Tarantola on at

In 1964, two researchers at Bell Telephone Laboratories were desperately attempting to pin down a source of interference that their antenna kept encountering. Little did they know that their antenna, the Holmdel Horn, was picking up the first observable evidence of the Big Bang—cosmic background radiation.

Horn antennas were first developed in 1941 by Alfred C. Beck and Harald T. Friis. They're characterised by the long parabolic reflector that feeds the antenna and acts like an antique ear trumpet. These antennas are uniquely well-suited to radio astronomy—they can observe a broad band of spectrum, and they have a high tolerance against stray radiation, since the reflector blocks signals not directly in the radio beam's path.

The Holmdel Horn is 50 feet long with a 20-foot by 20-foot aluminum aperture that rotates around a central pivot point atop a pintle ball bearing. It was built in 1959 as part of NASA's passive satellite project, Project Echo. Passive satellites, unlike active ones, are little more than reflecting dishes in low Earth orbit. The ones for Project Echo employed aluminised plastic balloons to bounce radio signals over Earth's horizon.

The Bell Labs research pair, Arno Penzias and Robert Wilson, were attempting to bounce signals off of these balloons when they noticed a bit of stubborn radio interference in their readings. They had already eliminated every other source they could think of—they removed the effects of radar and radio broadcasting, they cooled the receiver to -269 degrees Celsius to suppress its heat signature, and they even cleaned pigeon poop from the reflector—but to no avail.

The steady, low-level noise seemed to permeate the entire sky. No matter where or when the two looked, they encountered the mysterious 7.35-cm wavelength energy signature. They could only confirm that the radiation did not originate in the Milky Way galaxy.

The duo only began to realize the significance of this radio source when a friend, MIT Physics professor Bernard F. Burke, showed Penzias a research paper from an astrophysics team at Princeton. This team posited that the Big Bang, still a new idea at the time, would have spread massive amounts of radiation as the universe expanded and coalesced into galaxies. The hypothetical energy that the Princeton team described fit Penzias' and Wilson's findings perfectly and helped strengthen arguments in favor of the Big Bang.

"We live in an ocean of whispers left over from our eruptive creation," Russian theoretical-physicist George Gamow once said. "Nobody was listening." [CMD Wiki, National Park Service, Holmdel Horn Wiki ]