  We developed a scanning DC SQUID microscope with novel readout electronics
capable of wideband sensing RF magnetic fields from 50 to 200 MHz and
simultaneously providing closed-loop response at kHz frequencies. To overcome
the 20 MHz bandwidth limitation of traditional closed-loop SQUIDs, a
flux-modulated closed loop simultaneously locks the SQUID quasi-static flux and
flux-biases the SQUID for amplification of the RF flux up to $\Phi_0/4$ in
amplitude. Demodulating the SQUID voltage with a double lock-in technique
yields a signal representative of both the amplitude and phase of the RF flux.
This provides 80 dB of a linear dynamic range with the flux noise density of 4
$\mu \Phi_0/\sqrt{Hz}$ at 200 MHz for $YBa_2Cu_3O_7$ bi-crystal SQUID at 77 K.
We describe the electronics performance and present images for RF magnetic
field of the travelling wave in a coplanar waveguide, the standing wave in an
open-circuited microstrip, and a surface mounted device antenna.
