When building or tuning an antenna, it is important to remember that radio waves move slower through solid material than they do through space. Also, the amount of difference in speed between the free space and the material changes depending on the specific material the waves are traveling through.

Because of this, the physical length that an antenna is going to have to be in order to radiate properly will be shorter than the theoretical and electrical length that is calculated by my antenna length calculator. There are those who may use different formulas to arrive at the correct length for an antenna and a rule of thumb that seems to be followed quite often is to multiply the results by .95 when converting from theoretical to actual length.

The most accurate way to calculate the correct length that an antenna would need to be, however, would be to not actually calculate the precise length at all. Because the radio waves are going to slow down when they get to the antenna, it will almost always need to be shorter than the theoretical length. The surest way, then, to make sure that an antenna is tuned properly would be to build it to the theoretical length, connect it to the radio and a watt meter, and begin trimming off a little bit at a time until the SWR is optimal. By doing this, you will be assured that your antenna is tuned to an optimal length on a given frequency and not be as concerned with the exact length of the antenna as it can very between different types of coax and different kinds of wire.