It depends strongly on frequency (coax loss is frequency-dependent), but assuming you mean something in the VHF–UHF / radio astronomy range (e.g. ~1.4 GHz):
To get ~6 dB over 25 m (≈24 dB/100 m), you are looking for a fairly lossy coax, not a low-loss one.
Typical real-world options
At ~1–1.5 GHz (i.e. covering hydrogen line at 1420 MHz):
RG-58
~60–80 dB/100 m
→ ~15–20 dB over 25 m (too lossy, well above 6 dB)
RG-213/U
~25–35 dB/100 m
→ ~6–9 dB over 25 m (very close to your target)
LMR-240
~15–18 dB/100 m
→ ~4–5 dB over 25 m (a bit too low loss)
LMR-400
~9–11 dB/100 m
→ ~2–3 dB over 25 m (far too low loss)