These devices are important especially as a means of preventing common mode (CM) current noise from degrading performance of SDR receivers such as but not limited to the KiwiSDR. Each has a distribution function as well.
A problem underlying getting the best performance from a quiet receive system is avoiding unwanted noise ingress which can raise the system noise floor above the noise present within the radiation resistance of the intended antenna. Often this kind of degradation isn't even recognized or is attributed to some other cause, such as differential noise present across a power supply input.
The CM mechanism is one that produces a modest amount of noise current along but not accross differential lines coming into a receiver, including LAN connections, Power Supply connections, RF Antenna and GPS Antenna connections. Currents common to all conductors of one of these lines can flow through the circuit board ground plane of a receiver and be converted to noise across a preamplifier input and thereby reduce the incoming SNR while at the same time appearing to come from an antenna.
Additionally, current on the shield of a coaxial feedline shield can be converted to a differential voltage across the center/shield conductors by imperfect balance at the antenna. Any of these unwanted currents may be sourced from an impedance that is higher than the typical 50 ohm receiver input so requires extra effort to reduce to a level below that which can cause impairment. This can mean that broad band baluns or ferrite chokes/baluns are not sufficient to remove the problem.
For some receivers the unwanted conversions from current through a (common) groundplane to an input across the input may be only 60-80 dB below that of desired input signal paths. For the KiwiSDR a few microamperes of CM noise current flowing through the PC groundplane, from the network/PS end to the Antenna/GPS end, can raise the noise floor and degrade performance. This can be particularly insidious since it may only occur when the antenna feedline is connected and so make the user believe it is truly produced in the desired antenna rather than from an undesirable CM path.
By isolating all connections in and out of a receiver so that CM currents can't flow, receiver performance may sometimes be greatly improved.
The GPS, Antenna and External Clock isolation kits presented here serve to help reduce the common mode currents at the same time they allow multiple receivers to share these devices. Particularly for multiple-KiwiSDR installations, this can remove common mode current paths among the Kiwis that may not have been recognized.