MAX2014 is a 50MHz to 1000MHz logarithmic RF detector. The test data of the MAX2014 at 5MHz is included in the attachment.
It can still provide around 50-55dB of dynamic range at 5MHz. Slope is 47mV/dB at input between 0dBm and 5dbm input, and 24.62mV/dB at input between -60dBm and 65dBm.