We are using RF radiation in between 0.3 and 100MHz for evaporative cooling of Rb87 atoms in vacuum. For example, in one application we ramp our frequncy in a three-part linear ramp from 80 to 1MHz during 8 seconds.
What would be the ideal antenna design for this frequency range?
Currently we are using a coil antenna with 3 windings of copper wire (1mm diam.) with a diameter of 4cm. We have no R or C present, our antenna is directly connected to an RF amplifier (Mini-Circuits ZHL-3A +).
The efficiency of our cooling depends on the RF power we can deliver to our atoms (Rb87). So far, we have measured a strong dependence of efficiency vs. distance (the closer the better, roughly linear dependence), but we are limited to ~3cm distance by our vacuum chamber. We have not observed any strong efficiency dependence on the angle of the antenna axis w.r.t. to the atoms. It seem the antenna is radiating, roughly speaking, homogenously in all directions (c.f. near field?).