So I found a lighted passenger car in my stash that hasn't been converted to LEDs so it draws good current and conveniently had noisy rollers/wheels so it flickered when moving. I attached my white FM transmitter as shown with both L and R inputs going directly to the PS2-3V speaker.
With track powered to full DCS voltage with the engine at idle and the passenger car nice and bright, I pushed/dragged both cars back and forth many times quickly, slowly, etc.. NO NOISE even as the passenger car flickered madly.
I ran the assembly under DCS at various speeds both F and R. If you can play the attached recording, you can hear a burst of static-like noise at about 4, 6, and 10 sec into the recording. The engine is running at 50 MPH and obviously the sound has fallen into the clickety-clacking. But this noise would occur at the same point in the layout at any speed. So I'd move the receiver around the room going to a different AC outlets for the receiver and the locations of these static-noise would change and again always occur when the engine passed the same points around the layout. My conclusion is this is FM transmission noise given a relatively weak transmitted signal and reflections/ghosting/whatever given an FM wavelength of about 10 feet (100 MHz). Is this what your noise sounds like?
So I could not get any "noise" pushing/dragging the powered main unit and/or slave (lighted passenger car).
I also now recall that the reason these gadgets do not have an antenna wire is they assume you are hooking this up via a "long" cable to a CD player or whatever. So they couple the RF signal onto the "ground" wire of the audio input and let that be the antenna. After all, you only need to transmit from your front seat to your car radio...which is different that running a train around a basement or larger. And of course these widgets presumably were tested to pass FCC power limitations. This can devolve into a very nerdy discussion so I'll just leave it at that but jot this down almost as a note to myself as I need to keep the antenna issue in mind if I am able to cram the components into just the main unit.
