The Radiometrix NTX2/NTX2B is the most popular radio transmitter module in use for UK HABing due to its simple operation and robust construction. The original version was available in 434.075Mhz and 434.650MHz variants. The newer NTX2B is more stabled fitted with a TCXO but Uputronics sell a modified, frequency agile version.
Default output of the module is 10mW (Radiometrix do a 25mW HP version). Current draw is 20mA when transmitting and 0.6mA in standby. The input voltage to frequency characteristics of the NTX2/NTX2B are reasonably linear at around 1.9kHz/V about the 1.5V input centre point - see graph below. The graph shows frequency offset (kHz) against data input voltage over the 0 - 3V range.
NTX2 modules can be powered directly from the batteries having its own internal 2.7V regulator. The manufacturers datasheet is here NTX2 Datasheet
The regulatory requirements for airborne operation in Europe on this frequency are that the channel bandwidth must be less than 25Khz and less that 10mW ERP. There are no duty cycle requirements in this frequency band. Use of this module in the United States requires operation under an amateur license.
The original NTX2 temperature stability isn't great - in addition to drift the frequency shift tightens up with decreasing temperature. Adequate insulation is recommended, ensuring the module isn't exposed to rapid temperature changes is essential. The newer NTX2B with its TCXO is much more stable.
Removing the sticker from the NTX2 package reveals 3 holes in the metal casing - 1 test point (I haven't checked what it is) & 2 adjustments.
With the module oriented such that the holes are facing you & the pins facing downwards, the upper adjustment screw is frequency adjustment - perhaps adjusting loading on the crystal. Testing with a 434.650MHz NTX2 I was able to push the frequency about 15KHz either way before the oscillator became unstable.
The lower adjustment screw seems to be a pot adjusting the deviation. With the voltage driven FSK implementation we use, this conveniently allows for fine tuning the shift.
Radiometrix are understandably unable to offer technical support on these modules when used in high altitude ballooning therefore please direct all technical support queries regarding the operation of these modules to the UKHAS mailing list or preferably the #highaltitude IRC channel.
There is a guide on Linking an Arduino to a Radiometrix NTX2 Transmitter here.