[MikeS (OP)]

As space expands so the electromagnetic spectrum becomes progressively more red-shifted.  The longest wavelength being radio. 

My question, is there a limit to how far radio waves can be red-shifted?  Do we know what that limit is?  Do we have the technology to receive the longest possible radio waves?

[damocles]

no, not applicable, and no

[yor_on]

Nice question Mike, and one I wonder about too.

[imatfaal]

You could say that the max wavelength is twice the size of the universe - i.e. a standing wave with nodes separated by entire universe

[Soul Surfer]

There is no lower limit in theory but once it drops into the audio range it is best considered as being a variable electric or magnetic field rather than a wave.   Detection is quite easy with a simple capacitative aerial (rather than an inductive one)

A few kilohertz has been used for long range communications to submarines and even lower frequencies around 50 Hz have also been suggested.  The lowest frequencies I have worked with were around 1 Hz or less  these are the  frequencies produced by your muscles as you move they can work as a person detector and it will detect you tensing and releasing your arm muscle at a couple of metres.  All you need is a mosfet with a capacitive plate to sense the field.

The big problem is there are all sorts of things that create noise at these very low frequencies particularly in our electrical world and sorting any signal out from the noise is the real problem.