A directional antenna has to be much larger than wavelength. Won't fit in a van if we're talking about HF (with wavelengths measured in tens of meters).
Not[0] really[1]. Loop antennas (active or passive) have good gain perpendicular to the loop. It would be easy to mount on the inside of a wood paneled moving van with some nice receiving equipment. You park the van so the side faces the target's apartment and voilà you can listen to the mixer of their superhet radio. If your searching for receivers a couple vans driving around could triangulate and detected signals. Just getting the right apartment building would be enough for the KGB to turn the place over.
You can make a small (relative to wavelength) loop antenna that has a sharp minimum, but it will be very inefficient. You want your directional antenna to have a sharp maximum (high gain) so that it can increase signal to noise ratio.
[0] http://www.kr1st.com/swlloop.htm
[1] https://www.fmuser.org/fm-receiver/receiver-antenna/DE31MS-l...