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@valerio,

Improvement in neatness perhaps, but don't expect better signal strength/sensitivity compared with the plain wire antenna.

That's not what I call a helical antenna. In a real helical antenna, the coil spacing is λ/4 and the circumference of one turn is λ approximately. That antenna doesn't look as if it meets that specification.

See http://www.antenna-theory.com/antennas/travelling/helix.php

The only advantage I can see in your cheap bit of curly wire is it's less likely to poke your eye out.

That's a normal mode helical antenna. (and, as mentioned above, has relatively poor performance.)
In the normal mode or broadside helix, the dimensions of the helix (the diameter and the pitch) are small compared with the wavelength. The antenna acts similarly to an electrically short dipole or monopole, and the radiation pattern, similar to these antennas is omnidirectional, with maximum radiation at right angles to the helix axis. The radiation is linearly polarised parallel to the helix axis. These are used for compact antennas for portable and mobile two-way radios, and for UHF television broadcasting antennas.

As opposed to the antenna Robert described:
in the axial mode or end-fire helix, the dimensions of the helix are comparable to a wavelength. The antenna functions as a directional antenna radiating a beam off the ends of the helix, along the antenna's axis. It radiates circularly polarised radio waves. These are used for satellite communication.

Excerpted from: https://en.wikipedia.org/wiki/Helical_antenna

ok, so it seems that it's not a good improvement, so I think I will use it only if I will have problems to reach the base and nothing else works! :)

thanks for the help!

@valerio,

You don't have to guess if you are using RFM69CW modules. The received signal strength is indicated in the packet trace as the last figure in brackets. The RF noise floor is typically -90dBm or better, so if the simple wire shows -70dBm or better, that is enough signal to noise ratio to decode most incoming packets successfully.

You can try the helical stub if neatness is the aim, but expect the signal strength to drop between 2-5 dBm if it is tuned correctly (more loss if not).

I replaced the antenna on the transmitter side by this model: MR77 from Diamond. I already had this antenna in my spare boxes...

I gained 10 dB, jumping from -80 to -70.