I assume I understand what you are asking now.
You are asking about the effect of conductor diameter onto bandwidth and antenna efficiency. Those are all related to antenna efficiency and bandwith but different effects are involved. It is always a tradeoff what is sensible to do for a given frequency, and in consequence if you get an improvement and how much it improves operation.
These effects are
1. The resistance of conductor reduces the antenna efficiency. Silver being the best but also most expensive conductor, than cooper, aluminum and steel.
2. The so called skin effect, describes that the higher the frequency is, an increasingly thinner diameter of the outer surface a wire conductor is used for RF. Therefore at higher frequencies you don’t need a solid conductor any more and tubing is sufficient.
To improve conductivity often a thin silver coating on a cooper basis e.g. inductor coils or pcb, only have the outer surface is coated by silver to improve conductivity/reduce resistance. When a gold plating is applied e.g. coax connectors instead of silver, the reason is to eliminate corrosssion.
The exception of this is the use of threaded wires, e.g.those coils wound around ferrite antennas for AM (<1700 kHz) and lower frequencies.
For Skin effect see https://en.wikipedia.org/wiki/Skin_effect
3. The thicker the wire/surfaces of tubing is, the more the antennas capacity and in consequence bandwidth can be increased within limits.
As said before, you can only achieve a compromise and it depends on the frequency which material and diameter of the conductor gives you the best performance and bandwidth.
keep getting some "ghost signals" I call them. Just to one side of a signal, is like a slightly smaller power image of the same signal sometimes.
Can you provide sreen shot how the signal looks like? I assume you have here some not antenna related effects.