Common Mode Isolation Transformer

This design is intended create a high impedance common mode path to reduce unwanted KiwiSDR noise floor degradation from CM ==> differential conversion through the KiwiSDR's groundplane.  Current through the PCB, from the LAN/PS end to the GPS/RF end can be converted into unwanted response that dwarfs the desired signals.  Raising the impedance for this path can sometimes greatly improve receiver performance.

A simple test to see if this PCB might be a worthwhile addition is to simply touch the shield of the antenna coax to the shield of the KiwiSDR's antenna SMA connector on a functioning KiwiSDR  with no involvement by either center conductor.  Any increase in noise floor is probably due to unwanted conversion and may be largely eliminated by sufficient CM isolation. 

The amount of isolation that is achieved depends upon the impedance sourcing and sinking the noise current and by the inter-winding capacitance of the transformer used on this PCB.  Use only a flux-coupled rather than a transmission line type transformer here. Parts with broader frequency range are advertized but may not be flux-coupled and so have much higher inter-winding capacitance.

So far the best choice has been the non-catalog MiniCircuits T1-1 or T1-1T which may have as little as 1 pF coupling. The MiniCircuits ADT1-1 will fit on this board and may also suffice but has been measured to have significantly higher inter-winding capacitance so might not be as good, particularly at upper HF.