The radio system used on NOVA1 was intended to be a test of what could be achieved using off-the-shelf modules. The system is essentially a standard 1200 baud packet radio TNC connected to a 10mW 434MHz licence exempt transmitter module - a small circuit is used to interface the two. The antenna used was a coax dipole, which was directly connected to the transmitter output.
Figure 1 - NOVA1 radio system schematic
The whole system was built in a package that was about 70mm x 96mm x 30mm which contained the TNC, transmitter, interface circuitry and a lithium battery sufficient for about 20hours of continuos operation. All in weight (including battery and antenna) was 135g.
Figure 2 – NOVA1 Data Down-link Radio package
TNC’s are used by radio hams for data communication over narrow band FM audio channel. The TNC provides a radio modem capability – converting the data coming from the host computer interface into a format suitable for transmission over a radio channel. It takes data packets presented at the serial interface into the synchronous format used for transmission. Data is sent using a 1200 baud modem chip – which provides the signal shaping necessary to pass the data efficiently over the radio channel.
The TNC used is the TNX-X from Coastal Chipworks (see http://www.tnc-x.com/) - which is available as a kit or ready built. The TNC-X weighs 45g and measures 63 x 96mm.
The UK radio-licensing regime is such that only a few legal ways to transmit data from a balloon to the ground currently exist. Unlike the US the UK Ham Radio licence specifically excludes airborne operation and many of the licence exempt frequencies are barred form airborne use. Narrow-band 10mW licence exempt modules in the 434.04 – 434.79 MHz frequency range seem about the only viable option for High Altitude ballooning at present
The transmitter module used was the Radiometrix NTX-2. This conforms to the licence exempt transmitter specifications and outputs a power of 10mW. The NTX-2 was selected because of its linear input to frequency characteristics (in contrast to similar modules which are more digital in operation). This characteristic allowed the module to be used with the AFSK (Audio Frequency Shift Keying) output from the TNC.
The antenna used is a coax dipole – this is a very simple antenna where the outer braiding of the feeding co-ax is stripped away to expose a quarter wave of inner conductor at its end.
This type of antenna was selected due to its radiation pattern – when mounted above the payload the antenna has a maximum signal strength directed at 45 degrees below horizontal – which is close to ideal given the expected range of the modules.
A LM324 quad op-amp was used to interface the TNC-X to the NTX-2. One of the four op amps are used to amplify the audio output form the TNC-X to the level required by the NTX-2 – the circuit also adds DC bias to fully use the NTX-2 +/-3KHz FM deviation. The other op-amp in the circuit is used as an inverter and level shifter needed to drive the NTX-2 enable input at 3V.
Figure 3 - Interface Circuit Diagram
The power supply used was a 6V Lithium photo battery with a LE50CZ low dropout regulator (with appropriate de-coupling).
Ground-station Antenna – 7 element 434MHz “ZL” yagi (11.5dbd). A Narrow Band FM receiver was used – feeding another TNC-X which was then used to feed a PC running a terminal emulation program.
Accurate pointing of the antenna proved to be critical - especially when the balloon was at distance. In a subsequent flight the receive antenna was mounted on a camera tripod – which made the job of holding and pointing the antenna more pleasant.
Figure 4 - Antenna on tripod mount
The tested path distance was 33Km (27Km downrange 19.4Km up) at this range the system was approaching limit of reliable communication – with about 50% of packets being lost. At this point in the fight the transmitter completely stopped transmitting – in all probability due to a temperature effect of some kind.
A combination of these improvements might at best double the overall communication distance to about 60Km.