Antenna restrictions here in HB9-land are severe, but I want to get on
the air from my 3rd-floor apartment. I have draped a horizontal
longwire outside along the wall of the building. This wire is now just
short of 20m.

Before I transmit, I want to make sure to minimize any RFI that
bothers the neighbors' stereo systems and other electronic equipment.
First, I will put on a lowpass filter at the tx.

Past experience at other QTHs tell me that I will also need a
counterpoise. But here's the question: does the counterpoise radiate,
even if it's cut to 1/4 wavelength? Then, if I put the counterpoise on
the floor in my shack I haven't really solved the problem because the
CP radiates into my shack and my neighbors' apartments?

I realize that a counterpoise is the "second half" of an antenna. That
being the case, what's the difference between a traditional dipole and
a longwire/counterpoise configuration? Both legs radiate, correct?

Thanks for any clarification anyone can shed on this mystery!

73,
Paul HB9DST
An expat-American cliffdweller in Switzerland