Problem with 433Mhz module antenna

I have a problem with the little 433 Mhz module (FS1000A) transmitting data. I've connected an antenna (a wire 17.3cm long, roughly), and i'm trying to transmit some numbers with an attiny85 connected to it. I'm receiving with an USB RF analyser just to see if it's working or not, and i found out that the little module is transmitting only when i touch the middle upper part of the insulated antenna. Otherwise, if don't touch anything or i touch the middle lower part i don't see any signal at all.
Can you help me with that problem, and more important, can somebody explain me why is it having that strange behaviour?
Maybe the antenna is not the correct length?

Interesting problem,
Have you tried powering the transmitter with 12 volts for maximum power output?
Is your Arduino program continuously sending data?
Did you SOLDER the antenna wire to the circuit board terminal?
Paul

Some thoughts on the matter.

Perhaps you do not have a proper ground plane.

For the 433 MHz band the wavelength is 299,792,458 / 433,000,000 = 69.24 cm. Half of this is 34.62 cm and a quarter is 17.31 cm.

If you had measured the wire to being a full wave and you touch the middle of the wire to find it works. My guess is that you are either serving as a ground plane or a tuning capacitor.

Are you using 50 ohm cable?

I'm powering the transmitter with the little 12 volt battery, the one used commonly in RF remote controller. Although the voltage drops down to 10 volt when transmitting, i don't think that's the problem.
Yes, my program continuosly sends data, and yes, i've soldered the antenna wire to the board.

I've measured the antenna to be a quarter wave length, 17.3 cm as you said.
I'm not using a 50 ohm cable, anything fancy, just a normal cable (1mm in diameter with the insulation).
What you mean by having a proper ground plane?
Thanks

An internet search will revel what 50Ohm cable is. An internet search will tell you the specifications and requirements of the devi ce you are using, which will explain about the use of 50Ohm cable.

As to what is a ground plane, The internet is a great way to do your own research and find out on your own.