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communication:basics

Since the radio path to a balloon is “Line of Sight” (LoS) radio communication is governed by some physics called “free space path loss” – here are some basic rules which will help in selecting a radio communication system.

Free Space path loss is governed by the frequency and the distance between the transmitter (Tx) and receiver (Rx).

• Each time the frequency of communication is doubled the path loss increases by a factor of 4 (6dB) – i.e. double the frequency and a quarter of the power will be received - all other factors (like the transmitter power and antenna gain) being equal.
• Each time the distance between the Tx and the Rx is doubled the path loss increases by a factor of 4 (6db) – meaning that 4 times the power is needed to achieve double the distance – all other factors (like frequency and antenna gain) being equal.

Thus there is a balancing effect between frequency and distance - i.e. half the frequency and get double the distance. So for example:- a 10mW 173.7Mhz radio system will go 2.5 times the distance of a 434MHz system (given the same transmitter power and antenna gain etc.)

Some of the loss due to frequency can be compensated by increased antenna gain – doubling the frequency of communications halves the physical dimensions of an antenna – meaning that an antenna with twice the power gain (3db) can be built in the same size. Apply this to both the transmitter and the receiver and the frequency effect can be negated. However as the gain of an antenna increases so does its directivity (the Rx and Tx antennas must be pointed at each other more accurately). A balloon will almost certainly want to use an omni-directional Tx antenna (avoiding the need to steer the antenna) and there will be an upper limit of the directivity of the receive antenna that can be used.

Another variable that can help is bandwidth – the bandwidth of a communication channel not only governs the data rate that can be passed over it but also amount of channel noise taken in. Halving the bandwidth will halve the data rate that may be sent over that channel (all other things being equal) – but will also halve the received nose taken in – thus quartering the bandwidth (and hence 1/4 data rate) will give an effect equal to doubling the distance of communication.

So in general for long distance communication think:

• More power
• Higher gain antennas (but more accurate pointing needed)
• Lower frequency
• Narrower bandwidth (but lower data rates result).

RocketBoy's path loss calculator

This spreadsheet calculates received noise margin for a particular frequency, path length, transmit power transmitter and receiver antenna gain. Just fill in the green cells - results are calculated in the pink cells. Some cells need to be filled in from data in the graphs included.

Also includes a handy maximum line of sight distance calculator for calculating the slant distance to a balloon for a given altitude on the horizon (i.e. the maximal distance a comms link will have to work over.

Testing of standard 1200baud Ham Radio Packet equipment with 10mW 434MHz license exempt modules shows that data communication with high altitude balloons is possible at distances of tens of kilometers. Test setup:

• Radiometrix NTX2 434MHz 10mW license exempt module - specially set up to send AFSK with +/- 3KHz peak deviation
• quarter-wave vertical transmit antenna
• 7 element Yagi receive (claimed 10dBd gain)
• 2 x 1200baud TNC-X packet radio TNCs
• 10Km Line of sight path + attenuators

the tests suggest that the limit of line of sight communication is of the order of 40Km with such a set-up - certainly greater than 20Km and definitely a lot less than 100Km. This indicates that communication at distances in excess of 100Km are possible with 300 baud 200Hz FSK equipment (HF packet Radio) - in fact the MiHAB 2 radio system achieved the equivalent of over 280Km with such a system.

Error Correcting Codes

Another factor that can increase communication distance (for a given power) is the use of error correcting codes. These codes add additional data to the transmission which are used to correct any errors introduced by “noise” on the communication channel. The codes can be used to reduce the amount of transmit power needed to achieve reliable communication. One of the most famous examples was the use of the Reed Solomon codes by the voyager spacecraft to get pictures of neptune back at over 57,000,000Km using just 20 watts of transmit power. Reed-Solomon (RS) codes can typically reduce the amount of power needed to achieve reliable communication by a factor of 4x (6dB) or so (depending on error rate needed and size of the transmitted data block) - so the use of a RS codes can effectively double the communication range without increasing power requirements.

A more recent coding scheme is “Turbo Codes” (used in 3G mobile phone systems) - these have a better performance that RS codes - improving on them by a factor of about 1.5x (2db) in terms of power - equivelent to about 20% further in distance. Turbo Codes can come close to the theoretical maximum for radio system perfomance.

The experince gained on un-corrected balloon radio links would suggest that a range of 1000Km is not infeasable using a 10mW 434MHz licence exempt transmitter (and modest recieve system) if Turbo Codes were used on the transmission link.

Note: The above makes some fairly gross asumptions about underlying modulation schemes and other factors.