Quantim efficiency of a radio receiver In optical communication we often know quantum efficiency of the receiver (probability of single photon detection and bitrate per photon).
Are there any estimations on typical achievable quantum efficiency and bitrate per photon of typical radio receivers? (HF and sattelite bands - L S C X Ku)
 A: Photon energies in that frequency range are far below the input noise of the receiver, so one can't do photon counting. The smallest detectable signal will consist of a large number of photons and can be treated like a classical electromagnetic wave. 
The technically most often used figure of merit that is similar to the quantum efficiency would be the reflection coefficient $\Gamma=V_r/V_f$, i.e. the ratio between reflected voltage and full voltage on the input of a mismatched receiver. From this the VSWR (Voltage Standing Wave Ratio) is derived as 
$VSWR={{1+|\Gamma|}\over{1-|\Gamma|}}$
Similarly, for reflected power to full power (rather than voltage) the Standing Wave Ratio SWR can be expressed as
$SWR={{1+\sqrt{P_r/P_f}}\over{1-\sqrt{P_r/P_f}}}$.
For monochromatic waves the power is proportional to the photon flux, so I would say that one would practically use the power ratios or the SWR as an engineering (not physics) equivalent of the quantum efficiency. For modulated signals and wide frequency bands one would have to integrate over the time and/or frequency domain to arrive at an average figure.  
