Two 5 foot elements will not get you anything like 5/8 wavelengths. 5/8 wavelength antennas are in the range of 22 feet long, and no matter what you do, short of mounting it higher, 10 feet or so of antenna is not anything like a 5/8 wavelength antenna.
Also, you got your quarter wavelength off as well. A quarter wavelength antenna for CB is between 8.5 and 9 feet long. A five foot antenna is a shortened quarter wavelength antenna, and will not perform as well as a full length quarter wavelength antenna. As a matter of fact, the 5 foot length is right about where performance begins to drop off very quickly. You are working with a compromise length from the start, which is why longer is highly recommended here...
Some mobile manufacturers, including Firestik, claim their mobile 5 foot antennas are 5/8 wavelength antennas. This is nothing but marketing. Those antennas do not perform anything like true 5/8 wavelength antennas, and instead perform like what they are, shortened quarter wavelength antennas.
If you are looking at actual quarter wavelength antennas alongside 5/8 wavelength antennas, and you have the same tip height, their performance will be similar. There are cases when some shorter antennas at the same tip height will actually outperform the longer antennas (sacrilege to many I know, but true none the less), those cases, however, are limited. In the case of using 5 foot antennas as elements, I highly doubt that will happen.
The DB
