NC1 said:
The 5/8th wave depends on radiation from a reflection or radiation from a second source to create gain.
The 2nd source NC1 talks about is a 5/8th length radial system preferably sloping down at an optimized length and slope angle to achieve best gain at a low takeoff angle as compared to a 1/4 wave with 1/4 optimized radials or a 5/8th wave with the typical non 5/8th wave radials that most ham or commercial design incorrectly reference.
For some good 5/8th wave antenna info read the following about 5/8th wave myths>
5/8th wave mobile antenna vs 1/4 wave
If you have a copy of eznec or the free 4nec antenna analysis programs build a 5/8th antenna model with various length radials, try 1/4 and then 5/8th antennas with radials beginning with 1/4 wave then increasing to 5/8th wave long and observe the takeoff angle over real ground as the radialls length increases towards 5/8th length and as the radialls slope downward.
Try optimizing by sloping the radialls from horizontal to a downward angle from 90(flat) to ~ 30 degrees downward and observe the change in takeoff angle.
The 4nec optimizer is extremely good at arriving at the optimum radial down slope and radial length model.
This all started After experiencing poor results with a commercial 29MHz 5/8th vertical after swapping it out after using a optimized 1/4 wave with 1/4 wave radialls which worked fine, we were able to finally optimize the 5/8 wave antenna by first comparing to the same 5/8th vertical with first 1/4 wave and finally 5/8 wave length downward sloping radials .
The 5/8th antenna with 5/8th down slope radialls being the only design with measurable performance over a 1/4 wave with similar 1/4 radialls or a properly decoupled 1/2 wave sleeve or vertical dipole or even the typical literature 5/8th wave with 1/4 wave radialls as is offered by some 5/8th wave verticals.
I have just recently built a 5/8th wave on 29.6 MHZ using downward sloping physically shortened (~9feet long), inductively center loaded electrically 5/8th wave radialls, with good results.
Our test bed is a 220MHZ TO 29.6 REMOTE BASE system and we have several mobile stations that have been following their same routes in various directions testing various antennas on 29MHZ for 2 weeks at a time per 29MHZ antennas.
Our test started using the 1/4 wave with 1/4 radials then the 5/8th with 1/4 wave radials, which was considerably worse BTW than the basic 1/4 wave with 1/4 wave radials baseline antenna.
prior to changing to a new design we went back to our 1/4 wave vert with down sloping 1/4 wave radialls to get back to our baseline measurements.
We are now testing the full length 5/8 wave radiator with physically shortened, center loaded DOWN SLOPED electrical 5/8 wave radialls with measurable range improvments over the previous two 1/4 and 5/8th wave designs.