A mobile surveillance apparatus including a trailer and mast pivotally connected to the trailer and movable between a vertical position and horizontal position. At least one compensation capacitor is coupled to provide negative feedback through the capacitor from the second amplifier stage to the first amplifier stage. Filters incoming messages from an unrecognized sender, asking user whether to add sender to correspondent list, and if so prompts user for needed information. The power supply circuit in the accessory device generates at least a first regulated voltage output.