Remember that spectrum analyser can work without cable, just with antenna ?

You can make an air core ("styrofoam core") transformer: draw a 10-15 turns spiral on a board and terminate it with high-Z load. Glue an insulating pad on top of it and slap a 3-5 turns of insulated solid wire soldered to BNC connector. Cover everything with pile of glue.

Connect spectrum analyser to this pickup coil and you are done with impedance/overload/grounding problems.

![enter image description here][1]


  [1]: https://i.sstatic.net/4fNMq.png