Hydrogen line

https://en.wikipedia.org/wiki/Hydrogen_line

21.106114054160(30) cm in free space. ...

1 420 405 751.768(2) Hz

=>

Search after (meter or kilometer)

50 * 0,2110611405416030 = 10,5530570

78 * 0,2110611405416030 = 16,4627690

115 * 0,2110611405416030 = 24,2720312

193 * 0,2110611405416030 = 40,7348001