From the article: > any modulation with a period of less than 10 seconds or longer than 72 seconds would not have been detectable. With a carrier of 1420 MHz,

From the article: > any modulation with a period of less than 10 seconds or longer than 72 seconds would not have been detectable. With a carrier of 1420 MHz, I'd expect modulation at a much higher frequency than 0.1 Hz. The narrow band and power of the signal is what made it stand out.