frequency: medium wave transmission sites usually broadcast only on one to four frequencies, while FM-broadcasting sites often uses four and more frequencies. Diplexers may be used as a back-up device. An example is maintenance work at one antenna of a medium wave transmission site that has two antennas transmitting on two frequencies. Then the other antenna can be used for broadcasting both channels. If it is not possible to build a second antenna for the second transmitter due to space constraints, then the diplexer is used permanently. At long wave broadcasting sites diplexers are normally not used since these stations usually broadcast on only one frequency. A realization of diplexers for long wave broadcasting stations may be difficult, as the ratio of bandwidth (9 kHz) to transmission frequency is high. Diplexers are not used at VLF transmitters. In this frequency range their realization is very difficult because of the very high voltages that occur in the huge tuned loading coils that are used in the antenna feed. Diplexers are also used for non-broadcast applications such as amateur radio. Residential Diplexers are also used in the home to allow a direct broadcast satellite TV dish antenna and a terrestrial TV antenna (local broadcast channels) to share one coaxial cable. The dish antenna occupies the high frequencies (typically 950 to 1450 MHz), and the TV antenna uses lower television channel frequencies (typically 50 to 870 MHz). In addition, the satellite also gets a DC to low frequency band to power the dish's block converter and select the dish antenna polarization (e.g., voltage signaling or DiSEqC). The diplexer is useful in homes that are already wired with one cable, because it eliminates the need to install a second cable. For the diplexer to work, the existing cable must be able to pass the satellite frequencies with little loss. Older TV installations may use a solid dielectric RG-59 cable, and that cable may be inadequate. RG-6 cable is typically used for satellite feed lines. In this application, there would be a diplexer on the roof that joins the satellite dish feed and the TV antenna together into a single coaxial cable. That cable would then run from the roof into the house. At a convenient point, a second diplexer would split the two signals apart; one signal would go to the TV set and the other to the IRD of the DBS set-top box.