I'm building a remote tuning device for a magnetic loop antenna. It will use a stepper motor to turn the shaft on the mag loop's variable capacitor. The electronics will consist primarily of a microcontroller and a stepper motor controller. Control and power will be provided through a shielded twisted pair cable. One twisted pair will carry a half-duplex RS-485 signal at a low baud rate. The others will carry DC power and ground, somewhere around 7 – 12 volts depending on which motor I choose. I have a draft schematic here.
The transmitter must be on during tuning, so this device will be next to a source of around 5 – 10 watts of RF at 7 – 30 MHz. There is no RF circuitry inside the box and I would like to keep out stray RF as much as possible. I have never done a project like this before so I have a number of questions about making it robust to RFI.
Should I build this inside a metal box? And what connections should exist between the chassis, the control cable shield and the DC supply ground inside that cable? Should I ground the shield at one end or both?
What kind of protection do I need on the power and control lines? I have TVSes on the power supply, as well as both RS-485 lines (using a SM712, designed specifically for this purpose). I put a ferrite bead on the power line as well (but I'm unsure how to choose a specific part) and I have plenty of bypass capacitors. There are also 10 ohm resistors in series with the RS-485 lines to prevent damage due to overcurrent for whatever reason. (I realize this creates a RC low-pass filter with the line capacitance, but I believe this won't be a problem with the low data rates I'm using.) Is there anything else I'm missing?
I'm using a very basic RS-485 transceiver chip; should I switch to an optically or galvanically isolated part? Am I correct in thinking this would be pointless without an isolated power supply? In my current design the sender and receiver share a power supply and will not have a ground offset.
What PCB layout rules do I need to follow, besides "use a ground plane" and "keep traces short"?
Any other advice on making this work well would be greatly appreciated! Thank you.