The small grey box is the USRP software‐defined radio. The USRP digitally downconverts the received (Rx) input signal into I/Q format and sends it via Ethernet to the computer. The USRP also digitally upconverts an I/Q signal from the computer to an RF signal at the transmitter (Tx) output. The USRP’s Rx input is connected to a VHF antenna on the ELW roof. The Tx output can be connected to the oscilloscope and spectrum analyzer.