The problem that I previously mentioned is that the total length of wire in your coils must be much shorter than the free space half-wavelength of the frequency at which you are operating. The half-wavelength of 50 MHz is 3 metres. Your total length of wire must be much less than this. By "much less" I would estimate no more than 1/10, or 0.3 meters. If your wire length is more than this, you will have uneven current distribution in the coils, and hence an uneven magnetic field, and there is NOTHING that you can add externally, whether it be capacitance or magic pixie dust, to correct this problem.
Meanwhile, I just went back and re-read the posts to make sure that I haven't missed any important specs, and found several confusing statements.
First of all:
Hi, currently I working on a helmholtz coil, to generate a radio-frequency at the centre of the coil, a field strength of 1A to 2A is put into the coil, and the frequency ranges from 10Mhz to 50Mhz.
"field strength of 1A to 2A" makes no sense. The unit A (Amperes) is current, not magnetic field strength. Field strength is measured in Teslas.
Other things that you've posted that are contradictory or confusing:
"Currently, a side of the coil has 74 turns, diameter of 20.5, total length is 48.82m."
"The length of my coil currently is 97.64m, which is adjustable (buy more or cut)"
"Currently, this is my coil, the distance between the two coils are 10.5cm (half the Diameter of the coil)."
"the length of the coil is about 12 cm"
So let's get some basic information:
What is the diameter of your coils?
What is the axial spacing between your coils?
What field strength, in Teslas, do you want to achieve?
And one more confusing item:
"I am trying to make a magnetic coil, to generate a field strength where the maximum strength is at the centre of the coil."
Again, this is confusing, because a Helmholtz coil is intended to create a uniform field. So, do you want a uniform field, or something where the field increases towards the centre?