Last night I monitored 1296 MHz with the RTLSDR receiver and the homebuilt biquad antenna, fixed in one direction.
Apart from a strong station, I could hear nobody else. Then the usual HB9 placed a CQ and he was weaker than expected, so I switched back to the indoor 23cm dipole and his signal was at least 5 dB higher: something is wrong with the biquad! Last night's test confirmed my suspicions.
The outdoor antenna should gain about 10 dB over the dipole. The cumulative coax and connectors loss could be 3 dB, leaving a 7 dB boost in favor of the biquad. But according to my late night observations the difference is 7+5 = 12 dB in favor of the (indoor) dipole!
Either the biquad I have built has a weird radiation pattern or something is really wrong. Troubleshooting begins.