  We present enhanced sensing of radio frequency (RF) electric fields
(E-fields) by the combined polarizability of Rydberg atoms and the optimized
local oscillator (LO) fields of supergheterodyne receiving. Our modified
theoretical model reveals the dependencies of sensitivity of E-field amplitude
measurement on the polarizability of Rydberg states and the strength of the LO
RF field. The enhanced sensitivity of megahertz(MHz) E-field are demonstrated
at an optimal LO field for three different Rydberg states $\rm 43D_{5/2}$, $\rm
60S_{1/2}$, and $\rm 90S_{1/2}$. The sensitivity of 63 MHz for the $\rm
90S_{1/2}$ state reaches 0.96 $\mu \rm V/cm/\sqrt{Hz}$ that is about an order
of magnitude higher than those already published. This result closely
approaches the theoretical sensitivity limit of RF dipole antennas, and
indicates the potential for breaking the limit in measuring sub-MHz E-fields.
This atomic sensor based on Rydberg Stark effect with heterodyne technique is
expected to boost an alternative solution to electric dipole antennas.
