A combined isolator-diplexer device comprising: a combining circuit having a first input connectable to receive radiofrequency electromagnetic radiation having a first frequency from an RF channel, a second input connectable to receive microwave EM radiation having a second frequency that is higher than the first frequency from a microwave channel, and an output in communication with the first and second inputs for conveying the RF EM radiation and the microwave EM radiation to a common signal pathway, and a waveguide isolator connected to isolate the microwave channel from the RF EM radiation, wherein the waveguide isolator comprises: a conductive input section, a conductive output section, a conductive intermediate section, the intermediate section having a first end which mates with the input section, and a second end which mates with the output section, a first DC isolation barrier arranged between the input section and the intermediate section, and a second DC isolation barrier arranged between the intermediate section and the output section, wherein the input section, intermediate section and output section together enclose a waveguide cavity, wherein the output from the combining circuit includes a signal conductor and a ground conductor, and wherein the first DC isolation barrier and the second DC isolation barrier provide a pair of series-connected capacitive structures between the ground conductor of the output from the combining circuit and the conductive input section of the waveguide isolator, the capacitive structure being arranged to inhibit coupling of the RF EM energy and leakage of the microwave EM energy. a combining circuit having a first input connectable to receive radiofrequency electromagnetic radiation having a first frequency from an RF channel, a second input connectable to receive microwave EM radiation having a second frequency that is higher than the first frequency from a microwave channel, and an output in communication with the first and second inputs for conveying the RF EM radiation and the microwave EM radiation to a common signal pathway, and a waveguide isolator connected to isolate the microwave channel from the RF EM radiation, wherein the waveguide isolator comprises: a conductive input section, a conductive output section, a conductive intermediate section, the intermediate section having a first end which mates with the input section, and a second end which mates with the output section, a first DC isolation barrier arranged between the input section and the intermediate section, and a second DC isolation barrier arranged between the intermediate section and the output section, a conductive input section, a conductive output section, a conductive intermediate section, the intermediate section having a first end which mates with the input section, and a second end which mates with the output section, a first DC isolation barrier arranged between the input section and the intermediate section, and a second DC isolation barrier arranged between the intermediate section and the output section, wherein the input section, intermediate section and output section together enclose a waveguide cavity, wherein the output from the combining circuit includes a signal conductor and a ground conductor, and wherein the first DC isolation barrier and the second DC isolation barrier provide a pair of series-connected capacitive structures between the ground conductor of the output from the combining circuit and the conductive input section of the waveguide isolator, the capacitive structure being arranged to inhibit coupling of the RF EM energy and leakage of the microwave EM energy.