For an antenna ground plane to work efficiently with a vertical (monopole) antenna, the ground plane must be good at reflecting electromagnetic waves.
if I solder "ground plane" from, for example, piece of textolite board, to another end of coax cable (which is doesn't soldered now) - antenna should work better?
I'm not familiar with textolite but it seems it doesn't always have a metallic surface?
Metallic surfaces are best (copper, aluminium, steel, iron, etc..). Salt water is surprisingly good for low frequency waves, but I don't think it works well for near-GHz frequencies. Plain, rocky ground (or snow) is very poor at reflecting waves, especially at near-GHz frequencies, so your best bet is to provide an artificial ground plane in the form of a copper, steel or aluminium plate right where the antenna starts:
quarter-wave vertical with a round, copper-clad PCB as the ground plane. You could also solder your helical onto the inner-conductor of the coax instead of going straight up, but I think a straight vertical will have a slightly better range.
You could also replace the ground plane 'sheet' with 4 ground radials that are about 8.1cm long each (for 915MHz). Something like this:
When these radials are bent down about 45 degrees as shown in the picture you will also have a better impedance match with your coax feedline and the radio will be able to output power more efficiently. With better matching, the length of the cable also doesn't so much anymore (but keep in mind the losses purely due to the length of the cable).
mount your 1/4 wave monopole upside down with the ground plane at the top
Interesting.. Is this purely because the slope (falling ground) messes less with the radiation pattern on that side? I know it doesn't work if the ground is level.
but I wonder - is the BNC also good for 915MHz (I remember that I used BNC for old computer local network) ?
Hah, that shows your age a little. Yes I used them too in networking, many moons ago..
BNC is fine for up to 4GHz, so don't worry about that.
Would you recommend where to buy dipole antennas for LORa 915Mhz ? Or DIY manual for creating it?
If the antenna is going to be mounted outside in the harsh environments of cold, rain, snow and sun I would definitely go for something rugged. Google 'sleeve dipole' antenna as it doesn't require a ground plane and can be mounted all vertically with no bits sticking out. Also google 'radiation pattern' for the antenna you are interested in so you have an idea where you can get the best reception.
Btw, what sort of range are you hoping to get with these radios? How many radios? All at the same level/altitude? Knowing some of these factors will help members on the forum assist you with the best possible antenna setup for what you're trying to achieve.