Zigbee Antennas Part 1

When I started on my Zigbee oddysey I started out buying a few XBee’s and the first thing that struck me was the antennas and power outputs. Since I wasn’t sure what the difference was I bought some of each.

Now I’ve been an RF guy for quite a while. I was an amateur radio operator, the radio engineer at my university, and VP of R&D for an RF company. With all that, I still think antennas are pretty mysterious things. Worse yet, they are often the last designed and least designed parts of a Zigbee subsystem.

I’ve seen a bunch of antennas on these USB sticks and XBees and … You have to wonder if the engineering is any good or they’re just sorta slapped on there at the last minute.

I don’t know enough to answer this, so I asked an expert to write the Part 2 blog post. He requested I start with an intro since most people don’t understand the topic well enough to understand his explanations.

Types of Antennas

In this world there are three different styles of antenna.

• Chip Antennas – these use a chip with the actual antenna structure embedded in the chip somehow. They can be good or bad but at least have the virtue that usually the board maker doesn’t need to know very much.
• Planar Antennas – a planar antenna is built on a circuit board (a plane). They can’t be great but can be good, and they are very dependent on available board real estate and substrate (what the board is made out of).
• Whip or external antennas – many of these devices have the ability to have a whip (the xbee is notable there) or an external antenna via connector. With an external antenna, of course, you have all sorts of variety and generally these are optimal (but large). A whip is often a coiled piece of wire matched to the circuit. Note that a whip requires a ground plane to be effective == board real estate.

Circuit Matching

An antenna is a reactive device and it’s important to have the antenna and leadin circuitry well matched to the chip output. If that doesn’t happen you get distortion, poor power transfer on transmit/receive, and poor results. Efficiency is one measure of the circuit matching.

At these frequencies (2.4GHz for most of Zigbee) note that the copper traces on your circuit board are part of that reactive system. To keep it in perspective, 2.4GHz translates to a wavelength of 5 inches so if you have even a 1 inch long trace it will be ringing like a bell (actually it’s an antenna).

Read through TI’s documentation on the CC2531 USB stick and you can see how proud they are of the matching circuitry for their planar antenna (a balun, primarily). The 2530/31 has a balanced output so the balun lets them use an unbalanced (one side grounded) antenna.

This looks pretty simple but much of the magic is in the layout, the substrate and the selection of the passive parts.

I looked up a dipole (a balanced antenna) at a popular site just to be treated to a discussion of how the coax feeding it was symmetrizing the signal. Hahahaha.

Radiation Patterns

It gets even harder. You might think that you can evaluate an antenna based on one spec (efficiency?). Nope. If you consider an antenna to be a power sink for transmitting - even if the system is perfectly matched so all the power goes to the antenna where is the power then going in the air? The converse for receiving is equally true. So it has some gain – gain from where? And Gain implies directionality.

Radiation Patterns are how you can tell. And you really need a 3d radiation pattern chart to see what’s up. Here’s a great image from the EE Training Series. Note that the standard dipole pattern (bottom right) really does not encapsulate what’s going on with the antenna.

Questions To Ask

So what questions do I have about antennas on boards I buy?

• What’s the efficiency? How well matched is the circuit? This determines power consumption (you need more input power on transmit) and receiver sensitivity.
• What’s the radiation pattern? I may want directional and gain with a stick I can point. Unidirectional with little gain for a device in the walls randomly pointing. These also affect receiver sensitivity.
• Is the antenna matched to the right frequency and bandwidth? We want gain to drop off outside the 2.4GHz range so it becomes less sensitive to outside effects. In-band it should be well tuned and flat.

If I were building a board I would also ask

• How easy is it to manufacture consistently? Is it dependent on the substrate? Batches of substrates can vary wildly. Hard to manufacture because of line width/spacing requirements could mean some iterations are better than others.
• How much do the case and surroundings (copper in a wall and case shielding) affect the operation of the device.