Im not sure i fully understand your question, this obviously is due to my language barrier, my apologize...ill give it a shot:
If you mount a 5/8 wave above your beam, You should remove the radials and place it direct on top of the yagi, in that way the beam can work as radials.
Or...
You should place it "sufficient" higer and rotate the vertical in such a way the radials of the vertical are not inline with the elements of the beam. (but in a 45 degree angle ...if you have 4 radials)
I could model that for you...if you know the distance between the beam and vertical on the same mast.
In either case, performance wont be enhanced nor degraded when compared to both antennas on a single mast of theire own
For local work...antenna tip height rules, the type of antenna comes in secondary.
(that is ...if the individual vertical antenna is working without issues...)
Often it is better to just "add" mast instead of buying a new antenna.
The main reason why you hear so many different stories about antennas like antron 99 / Imax etc is that those type of antennas ( end fed verticals without radials) can have different outcomes in various situations.
That is cause the "second" half of the antenna is missing.
all antennas need "two parts".. including 1/2 wave verticals.
Just imagine you have a mobile antenna... but you dont attach it to a car.. You know it needs ground.. .SWR will be terrible..the pattern serously skewed etc.
That is also true in a lesser degree for an end-fed vertical.
The impedance is much higher, the voltages is much higher therefor the current is low... at the end of the end-fed,
Due to that relative low current at the feedpoint, the demand of "good ground" can be much less... but it still needs some...for that current to flow.
Normally this is ground is provided by "the mast" or "the outshield" of the coax.
And that is where problems could rise... if you have "the wrong" length of coax or mast heigth.. and it starts to radiate it could drop perfomance.
But that isnt always the case, hence the different outcomes for various situations.
Hope i did answer your question ?
If not please do ask...
Kind regads,
Henry
