Telemetry Antenna

Question

I am developing a telemetry system for a solar car. I am going to be using xbee chips to communicate from the base-station/chase-car to the solar car. The xbee chips are 900MHZ, or 2.4ghz. I am thinking about using high gain omnidirectional antennas on both ends, with lengths about 3.6ft, and a gain of 15DB. I need to be able to reliably transmit data line of sight from 2 miles away.

Do you think this will work? Should I go with the 900mhz or 2.4ghz xbee?

Thank you for your help!

Answer 1

I don't think XBee radios, even the higher power XBee-Pro which has a maximum power output of +18 dBm (North America only), will be able to reliably communicate over a 2 mile link in all conditions.

One of the XBee modules with "long range" capability is the XStream which claims:

Indoor/Urban range up to 1500 feet (900 MHz model) Outdoor line-of-site range up to 20 miles (with high gain antenna)

Now it is probably safe to assume that actual results in the field will not be substantially better than the claims being made by the manufacturer, and the above ranges are under ideal conditions. Assuming there will be times when your car will be driving in less than ideal conditions, such as an urban area, there will be times when their urban range of 1500 feet may be much closer to what you observe than 2 miles.

When designing a data link between your cars the best way to start is not with the radios themselves but rather with a link budget analysis.

A link budget is the accounting of all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication system. It accounts for the attenuation of the transmitted signal due to propagation, as well as the antenna gains, feedline and miscellaneous losses. Randomly varying channel gains such as fading are taken into account by adding some margin depending on the anticipated severity of its effects. The amount of margin required can be reduced by the use of mitigating techniques such as antenna diversity or frequency hopping.

A simple link budget equation looks like this:

Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses (dB)

For a data channel you will also need to determine what is the required capacity ( do you need 10 MBits per second or is 300 BAUD OK?) and an acceptable bit error rate. In other words there will be times when data gets garbled in transmission and you need to deal with this either by re-transmission, redundant transmissions, check digits or some such means.

Here is an Intersil Tutorial on Basic Link Budget Analysis.

Bringing this back to what is expedient, the easiest solution would be to simply use higher power radios, if possible. You do not say if this is a school, for profit company or what. Educational projects often use ham radios, specifically APRS for projects like this.

ADDENDUM: One alternative you might like to investigate is TI's CC1120 development kit;

TI claims "More than 10 km out-of-the-box with development kit (139-dB link budget) and 65-dB adjacent channel rejection"

Answer 2

Last year we have used similar system for solar car races. 15dbi antenna (connected to an access point) in the middle of the race track, 7dbi antenna on car, 4dbi in PIT. We reduced access point's data transter rate to 1 mbit. It worked OK.

Access Point

Center Antenna

Car Antenna

Race Track

Network protocol tracks the error, using hash algorithm and package IDs. Sync -> Control -> ID -> Message -> Error Check -> End of Message.

The message is sent with an error code. And reciever takes the message and error code. It processes the same hash algorithm, and produces a new error code. It compares 2 error codes. If they are not same, it sends the package ID to the transmitter, and wants the package again.

You might want to be participate in World Solar Challange. And short range communication may not be the best option. GPRS should be in the box.

I hope it helps. I wish you luck in the race =)

Edit: Here is an example software design. It is called Strategy Pattern. You can change communication easily even at runtime. Electronics guys usualy don't use Object Oriented Software Princibles. You can achieve same behavior without OOP. But as a software developer, I prefer that way. Design Patterns remembering tip: While playing a shooter game, you change your weapon depends on situations (like distance). It is your game "strategy".

#include <iostream>
#include <string>

class ISendable
{
    public:
        virtual void Send(std::string data) {};
};
class Network: public ISendable
{
    public:
        void Send(std::string data)
        { std::cout<<"Send over Network: "<<data<<std::endl; }
};
class GPRS: public ISendable
{
    public:
        void Send(std::string data)
        { std::cout<<"Send over GPRS: "<<data<<std::endl; }
};
class Car
{
    public:
        Car() { sendBehavior = new Network(); }
        void Send(std::string data)
        { sendBehavior->Send(data); }
        void ChangeCommunication(ISendable *communicationType)
        { sendBehavior = communicationType; }
    private:
        ISendable *sendBehavior;
};
int main()
{
    Car *c = new Car();
    c->Send("data 1");
    c->ChangeCommunication(new GPRS());
    c->Send("data 2");
}

Output:

Send over Network: data 1

Send over GPRS: data 2

Answer 3

Just a couple of considerations about choosing the antenna (the shape, not the power, since Jonny has already covered this topic).

You don't need an omnidirectional antenna, but eventually only in the horizontal plane; so if you can achieve a good vertical directionality (check for broadside antennas, like the ones used in radio transmission) you save a lot of power, and that means higher range within the legal limits.

This is the radiation diagram od a broadside array

Then you can make another consideration: is the base station in the center of the track, or in one side? If you are in the second case, you can suppress the radiation behind the antenna (outside the track) with a screen: again, you would save a lot of power (the system becomes more directive) and avoid transmitting where is not needed.