Tu Zuochao invented his shapeless signal receiver in Shanghai in 1940  when he was training radio engineering for Li Bai from 1939 to 1941. The truth  was that 1 thin wire (or 2 if necessary) temporarily between the output and input  of the IF (intermedium frequency) amplifier in a family radio set causes a positive feedback thus a temporary beater. So that the radio set receives signals. 

Before Japs caught Li on Sept 15, 1942, he took off the wire(s) and threw the wire(s)  away in a couple of seconds. The Japs found Li's transmitter was in a good  condition but found Li's family radio set could not receive signals but broadcastings only. Li could not be convinced for any active radio operation. Li was released 8  months later. He continued his underground work till Dec 30, 1948. He was executed on May 7, 1949. 

Li‘s story was filmed for The Eternal Wave in China in 1958. Tu was asked to provide background and to to make props. He was determined not to disclose the truth of his shapeless signal receiver. However he had to convince audience with radio telecom  knowledge. So there came this fake beater prop. 

Radio operator Li Xia's family radio set without the back cover facing the audience  shows that this is a normal family radio set. Li installed his theorectical beater and  placed it on his radio set. He sent and received both successfully. His back to normal radio set was back in his bedroom. 

 涂作潮1939带1941年间在上海培训报务员李白时，于1940年研发了无形收报机，并 倾囊相授。无形收报机的秘密是一根（必要时2根）细铜丝，临时安装在收音机的中频

放大的输出和输入级，形成的正反馈就是收音机收报必须的差频震荡器。 

   1942年9月15日日寇捕获李白前，李白在几秒钟内卸下并扔掉了这（两）根细铜丝。 日寇发现李白的发报机完好；但是他的收音机只能收音，无法收报。由于没有现行电台的证据，李白8个月后获释。他继续着他的地下电台到1948年12月30日。他在1949年5月7日牺牲。 

   1958年的《永不消逝的电波》讲述了李白的故事。涂作潮为影片提供了背景素材并制作了道具。他决心不泄露无形收报机的秘密。但是他又要取信于懂得无线电收发报的观众。于是这个无形收报机的假道具登场了。