Air-gapped networks are wired with Ethernet cables since wireless connections are strictly prohibited. In this paper we present LANTENNA - a new type of electromagnetic attack allowing adversaries to leak sensitive data from isolated, air-gapped networks. Malicious code in air-gapped computers gathers sensitive data and then encodes it over radio waves emanating from the Ethernet cables, using them as antennas. A nearby receiving device can intercept the signals wirelessly, decode the data, and send it to the attacker. We discuss the exfiltration techniques, examine the covert channel characteristics, and provide implementation details. Notably, the malicious code can run in an ordinary user-mode process and successfully operate from within a virtual machine. We evaluate the covert channel in different scenarios and present a set of countermeasures. Our experiments show that with the LANTENNA attack, data can be exfiltrated from air-gapped computers to a distance of several meters away.