The Radiometrix NTX2 is the main radio transmitter module in use for UK HABing - it is available in 434.075 and 434.650MHz variants. The NTX2 is a 3V Narrowband FM module and primarily chosen for its data input characteristics - which are analogue rather than the more usual digital. This enables narrow frequency shifts and pulse shaping to be obtained. Output is 10mW. Current draw is 20mA when transmitting and 0.6mA in standby. The input voltage to frequency characteristics of the NTX2 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.

The regulatory requirements for airborne operation on this frequency are that the channel bandwidth must be less than 25Khz and less that 10mW ERP (Effective Radiated Power). There are no duty cycle requirements in this frequency band.

The NTX2 temperature stability isn't great - in addition to drift the frequency shift tightens up with decreasing temperature.

NTX2 Adjustments

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.

The TX3H is a Radiometrix 869.5MHz module which provides 450mW of output (up to 500mW is allowed for airborne use in this frequency band). The TX3H is a 5V Wide-band FM module which like the NTX2 has analogue input on the data pin. The input voltage to frequency characteristics of the TX3H are shown in the graph below - and is approximately 11kHz/V about the 2.5V input centre point. The graph shows frequency offset (kHz) against data input voltage over the range 0 - 5V.

TX3H temperature stability is worse than the NTX2 (as might be expected of this higher frequency module) - also noticeable heat is generated by the module due to the higher power and lower efficiencies at higher frequencies.

The regulatory requirements of airborne operation in this band are a channel bandwidth of less than 25KHz, less than 500mW ERP and less than 10% duty cycle. The latter requirement reduces the heat dissipation issues.

The HX1 is a Radiometrix 2m module which provides 300mW of output. The HX1 is a 5V Narrow-band FM module. Deviation in the Audio Frequency range does not appear to be very linear. Below is a chart of the relative audio frequency response of the module. More information concerning the testing set up can be found here: https://k7nvh.com/wordpress/?p=123.