40MHz replay garage opener trouble

I’m trying to do the classic task of capturing a garage door opener signal and sending it back to open the garage with the Lime.

I performed some first sanity checks:

(i) record signal with garage opener really close, then look at it in time domain to see that the on-off keying corresponds to that of the opener.

(ii) emit the signal with the Lime and record it simultaneously with the Lime to see that it emits at the correct frequency etc.

Both tests worked, however I think my main problem is signal strength. The operating frequency is 40.6 MHz. First, I used a wifi antenna which captured the signal when the opener was very close, good enough to decode the signal as in (i). Then, I took a 1 meter wire and connected it to the inner part of a SMA adaptor, that antenna is obviously better at 40 MHz. I built two such antennas and was able to verify (ii) that way with TX gain at 60.

But the garage itself still does not open. Hence I seek advice on:

  • efficient antenna design for 40 MHz. Wavelength is about 6 meters, so a dipole antenna is a little impractical. are there any compact antenna designs? I saw inside the garage door opener they use a wire looped around a grey cylinder of unknown (possibly high epsilon) material.

  • any recommendations for amplifiers?

Has anyone successfully done a replay at 40 MHz with a Lime? What antennas (and amplifiers) did you use?

For reference, my record and replay flowgraphs in gnu radio companion:

PS: As a side note, I noticed that if I use more than gain 12 in the bandpass filter, considerable amounts of noise and stray signal appear and deteriorate the measured signal. Not sure why 12 is a magic upper limit here…