Nigel is right. Not knowing a lot if information makes it difficult to even advise you. In the U.S. you can build a receiver for most frequencies without a problem. But it requires some test equipment to get the receiver to function properly. A transmitter is another question. You can build unlicensed transmitters for certain frequencies, but they are limited to such things as power, antenna and as a result field strength. They too require some test equipment to get them to function properly. Even the ones that operate in the CB and FRS bands have technical specifications that may be difficult to meet for the amateur builder.