Yes well worth confirming the fan details, and also if the fan can be removed, or the wires to the fan accessed. Suppressing emi at the source is the best bet. It may be as simple as adding in a local bypass RC or LC in line with the supply rail at the fan, or as close as possible, if that can localise any switching current pulses generated within the fan. If a PWM control line uses very fast switching pulse edges (for no good reason) then those edges may be worth slowing down (again as close to the source as possible, so back in the power supply or where the lead comes off a pcb).
You may also want to look at the wiring of the total system, and see if there are any earth/ground loops from multiple mains supply connections. Is the output DC of the Eltek floating, and so is one output polarity effectively being grounded at your transmitter? If so then switching hash can circulate via capacitive coupling through the Eltek back to the mains protective earth, and loop around through your transmitter.