There are a couple of things you need to take into consideration with your design. I am not trying to criticize your attempts in any way only want to help.
1. You have positioned your antennas with a horizontal polarity. Not sure what the best polarity would be but I think the
Tello is vertically polarized. The repeater's antenna is vertically polarized.
2. Your driven element is facing down and the reflector element is reflecting the RF energy down towards the ground not towards the drone.
3. The driven element should point the desired direction (towards the drone) with the reflector behind. The reflector element is placed in such a way so as to redirect the RF energy coming off the driven element going in the undesired direction back froward in the desired direction. This is what gives this antenna gain over a simple dipole antenna.
4. Wood is a terrible sub-straight to use as it is very lossy at 2.4 GHz. A lot of the RF energy is being absorbed and lost in the wood. It will also change the impedance of the antenna away from the desired 50 ohms. Your printed frame should be better. Take your printed holder and place it (plastic only no antenna wire) in the microwave for a couple of minutes. If the plastic gets warm (absorbing energy) it is not a good material to use. If it remains cool would be a great material to use. Free space (no sub-straight) is the best.
View attachment 2729
The illustration shows the a vertical driven element in front of the reflector in a vertical plane. If you rotate the antenna 90 degrees it will then be in a horizontal plane. Both planes should be tested for best signal level.
I believe when measuring the signal strength a lower number is a better stronger signal. Even if you are looking at signal to noise. Lets assume the receivers threshold is somewhere around -100 dBm then a measured signal of -90 would be 10 db above the noise. A signal of -70 dB would be 30 dB above the noise. In any case using the WiFi signal analyzer in your phone will be your best tool for determining actual gain. When making your measurements be sure to rotate the phone 90 degrees and get a measurement in both the horizontal and vertical polarization as we are not sure what plane the antenna in the phone might be.
You may want to experiment with only one antenna as spacing antennas for diversity is another problem all together. You can have 2 perfect antennas placed the wrong distance apart and totally screw up the antenna pattern.
I am looking forward to following your progress.
~Bill