Measured and Field- Based Theoretical Q of Gravitationally-Small Gravitational Antennas

Abstract

Recent breakthroughs in detection of gravitational waves not only demonstrate the transmission and reception of such waves, but also prompt comparisons to electromagnetic antennas. Although the transmitting gravitational antenna and signal generator may be the inspiral of a binary neutron star orbit, such new gravitational antenna observations raise interesting questions from an antenna engineering perspective. In particular, measured data are used to determine the Q (quality factor) of the gravitationally-small gravitational antenna comprised of the neutron star quadrupole, along the lines of Wheeler-Chu limits for electromagnetic antennas. The recent gravitational wave event GW170817 had signal frequencies below 75 Hz and implied orbital separation near 250 km, which suggest a gravitationally-small radiator below 0.07 wavelengths. In addition, qualitative arguments based on quadrupole gravitational field components suggest a theoretical Q inversely proportional to the seventh power of gravitational quadrupole antenna size. Measured data are presented that somewhat better support quadrupole gravitational antenna Q inversely proportional to the fifth power of gravitational antenna size, more so than seventh power. Lastly, an antenna engineering perspective should provide insight for metamaterial and non-Foster enhancement of gravitational antenna designs.