I'm looking to set up a directional antenna for a Motorola SBG6580 while keeping the existing wifi antenna that is part of this cable modem gateway. I need to transmit line-of-sight for about 1/2 mile. Does anyone have any product recommendations for this? Since I don't want to trash the current wifi antenna, I would need a device for one of the four ethernet ports, correct? The SBG6580 is near an external wall and I can run a short cable to an exterior antenna location.