Unlike other RF spectrum users, radio amateurs may build or modify transmitting equipment for their own use within the amateur spectrum without the need to obtain government certification of the equipment.[23][24] Licensed amateurs can also use any frequency in their bands (rather than being allocated fixed frequencies or channels) and can operate medium to high-powered equipment on a wide range of frequencies[25] so long as they meet certain technical parameters including occupied bandwidth, power, and maintenance of spurious emission. As noted, radio amateurs have access to frequency allocations throughout the RF spectrum, enabling choice of frequency to enable effective communication whether across a city, a region, a country, a continent or the whole world regardless of season or time day or night. The shortwave bands, or HF, can allow worldwide communication, the VHF and UHF bands offer excellent regional communication, and the broad microwave bands have enough space, or bandwidth