More than 500 sets of equipment for making unauthorized radio broadcasts in China were seized in a national campaign aided by more than 30,000 airwave monitors.

The campaign, launched on Feb 15 by the State Council, resulted in 1,796 cases related to illegal radio stations, after 301,840 hours of monitoring from February to July, according to an online statement by the Ministry of Industry and Information Technology.

The number of incidents was down by 50 percent from April to August, the statement said.

So-called pirate radio has appeared in most parts of China since 2015 and “has been a channel for criminals to defraud and promote aphrodisiacs, along with counterfeit and poor-quality medicine”, according to the Ministry of Public Security’s Criminal Investigation Department.

The operating cost of pirate radio is low, but profit can be high. A pirate radio station that broadcasts advertisements for aphrodisiacs can pocket more than 70,000 yuan ($10,500) a month, with an overhead cost of no more than 10,000 yuan, investigators said in a post on Sina Weibo. It said most spare parts for broadcasting equipment can be bought on the internet.

In a recent case in Shanghai, an operator of a pirate radio station bought eight sets of equipment for less than 10,000 yuan via the QQ instant messaging platform.

Another person, who was in the same QQ group with the seller, helped the operator record the audio advertisements and charged 380 yuan for each piece, according to a media report. The suspect bought a certain medicine for 9.9 yuan per box, which he then sold for 330 yuan through the radio broadcast.

“The broadcast power of pirate radio stations can be 2,500 to 5,000 watts, which is several hundred of times that of commercial radio, and the signal can be received 300 kilometers away,” the department said.

In the Shanghai case, the suspect rented an apartment to accommodate the equipment. The electromagnetic radiation of the equipment was so high that the refrigerator of a nearby family malfunctioned, with TV screens flickering frequently. The suspect didn’t live in the apartment but used his cell phone as a remote control to turn equipment on and off.

Pirate radio may also pose a threat to communication between aircraft pilots and ground controllers, as its frequency band neighbors that of flight navigation signals and can create interference, the department said.

“The development of e-commerce and social networks has facilitated the criminals in selling illegal equipment for pirate radio”, Zhang Jianming, director of Shanghai Radio Administration Bureau, told CCTV.

Zhang said the public should be cautious when listening to radio advertisements, and he also called on the public to report suspicious activity to the police.

Under Chinese law, the unauthorized use radio frequency bands can bring up to seven years in prison.