Hi, I have a hc-05 module ofr my arduino, I want to add an antenna DIY, i know that i have to usa copper wire, but with what length? Thx
Whats the frequency of Bluetooth ?
Frequency: 2.4GHz
Try a 1/4 Wavelength at that frequency.
You will find on-line calculators.
Sorry but i don't know where search becuase i found a lot of sites but i don't know how use them, can you say me the length of the antenna that i have to use? I have to use 12 cm antenna?
2 antennas = Bad idea , you can replace existing one, antenna should be tuned to 2.4MHz.
Cut old antenna as marked, solder 15 cm copper wire let say 0.5mm then cut off 0.5cm to make antenna shorter, cut again and again until you have max range.
2.4 mhz field strength meter will make job easier.
I always do a Google search on 'frequency wavelength calculator' and find this site;
https://www.everythingrf.com/rf-calculators/frequency-to-wavelength
Easy to use and you do need to learn stuff like this.
A 1/4 wavelength is in theory the 'correct' length but in practice the effectiveness of the antenna can usually be improved by tuning.
I have to use 12 cm antenna?
Is 12cm 1/4 of a wavelength at 2.4Ghz, I dont think so.
merging your answers, I have to cut the main antenna on the module as 79galinakorczak said and then attach an antenna to it which is a quarter of the wavelength of 2.4Ghz (0.12m / 4 = 3cm) as srnet said? I repeat that I am ignorant on the subject 
Full length antenna is most efficient, rough comparison;
full length = 100%, 1/2 = 50%, 1/4 = 25%
Yes around 3cm.
Ideally adding 1/4 wave radials, as shown in this tracker that used a 2.4Ghz SX1280 LoRa device, the 1\4 vertical is the yellow bit of wire.
A 3/4 wavelength antenna will normally preform slightly better, but avoid lengths that are multiples of 1/2 wavelength, they are normally very bad impedance matches and can damage transmitters.
ok, so now i cut old antenna, and paste new antenna (3cm long) with copper wire
right?
?
so you replace 1/4 factory tuned antenna with 1/4 DIY untuned.
The antenna is part of the LC resonator circuit.
You have 2 capacitors below marked x = C, length of the wire = L,
f = 1 / (2 * π * √(L * C))
I still do not understand
Start learning about LC resonance, https://en.wikipedia.org/wiki/LC_circuit
this is your case,
L is your wire or yellow strip on PCB, C are 2 capacitors mentioned in previous post. google for tuning antenna.
Tuning antenna by length and diameter of the wire.
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