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I'm in the middle of testing a Z-Wave device I created with a PCB antenna. I want to make sure that the antenna has a good range and the signal strength generated is strong enough and comparable to similar devices in the field.

How can I properly range test my device?

  • 4
    This seems like more of an electrical engineering/DIY question than a question specific to the IoT. – ArtOfCode Dec 6 '16 at 17:41
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    @ArtOfCode if you're antenna doesn't work, you don't have an IOT device. I've seen this come up in the industry all the time. Yeah, there's overlap with EE, but if we want to talk about creating devices at all, there will be overlap. – Dom Dec 6 '16 at 17:44
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    Dom, this site isn't as much about the deeper engineering challenges that go into designing and building these devices. This site is more oriented towards end-users with questions about IoT, solutions, security concerns, and making their devices work. There will always be a bit of a cross over (industry issues, trouble shooting, optimization, and repair, for example), but engineering questions that just happen to be about IoT design are a better fit for the experts at our engineering/electrical sites. – Robert Cartaino Dec 6 '16 at 17:57
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    @RobertCartaino the definition phase had several questions the were development focused like "Which protocols we can use for device to device(D2D) communication in IOT, apart from MQTT?" and "I'm building a system that includes a mobile app and a bluetooth enabled hardware device. How do I secure communication between the devices?". If we want just consumer level questions then yes this doesn't make sense here, but if we to have questions about the development and process of making an IOT device then we need questions like this. – Dom Dec 6 '16 at 18:11
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    Dom, Looking at the top example questions (defining the target audience for this site), I believe you are simply asking in the wrong arena — especially considering that we created several expert electrical/engineering sites targeted specifically to answer your question authoritatively. – Robert Cartaino Dec 6 '16 at 18:20
15

You can use a spectrum analyzer to make sure that:

  • The antenna is emitting at the correct frequency (range).
  • The output power is the expected (theoretically calculated) output power (or it is close enough).

A higher end version of a spectrum analyzer (including bit error rate analyzer) could help you further on making sure that not only the antenna but also the encoding is correct.

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    Not really. There is a lot more to antenna characterisation than this. – Sean Houlihane Dec 7 '16 at 11:17
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There are two main approaches to characterising an antenna and its effectiveness in-system, free space and in-situ. The first will give you a best-case baseline, is less susceptible to measurement noise (especially if you have access to a large anechoic chamber), and probably works best for selecting between candidate antenna variations. In-situ measurements are much less definitive, but allow you to be specific about a possible working scenario, and rule out silly mistakes (like forgetting that the final device has a conductive casework).

Both approaches require a transmit-receive pair. Either one or both can be real devices, propagation is commutative. Take care to use representative power supplies and autonomous operation on real test device. If you connect power or serial cables, these become part of the antenna system.

A good alternative to a spectrum analyser is a Software Defined Radio. Certain USB-DVBTV dongles can be used at very low cost. You shouldn't be as concerned about signal fidelity in this scenario, simple signal strength should be enough.

Ideally you will be close enough in that uncorrected error rate is not a useful metric. Independently, you should understand how signal strength, uncorrected error rate and system performance are related to work out what operating point you are aiming for.

For free space measurements, ideally you will be in an open field, free of power lines and reflective structures for many 10s of metres. Place the DUT on a wooden stand, and observe it from a distance of 5-10 metres (several wavelengths at least). Rotate it in all directions, and measure. Repeat with a comparative device, repeat with any other variations you care about. A directional antenna at the frequency of interest is normally used at one end of the test range, this removes some variation due to the test location (and your presence near this end)

For in-situ measurements, it is important to trial as many realistic deployments as you can. Ideally you would run device-to-device, with signal quality being logged on the device. Look for the worst-case scenarios that you can find - end users are likely to find even worse ones.

If in doubt, consult an EMC test facility. They are often able to consult on, for example, validating your approach with some spot measurements. With a custom antenna, you'll probably need their services later anyway.

8

In order to properly test the antenna at minimum the following items are required

  • A source antenna with the known pattern and transmitter such as a RF signal generator create known signals
  • The test antenna with a receiver system measure and monitor the receive RF signal. In this case the z-wave device could be used receiver system.
  • Antenna under test (AUT) needs be test as function of angle. A positioning system need to be used to rotate the AUT with respect to the source system.

Below is a simple diagram to illustrate the above.

Test Antenna Setup
Click on image for a larger version of the image.

The above setup will help test the AUT both a transmitter and receiver.

Next set of ranges appropriate for the application need to be selected. The most common ones are

Elevated Range (Free Space)

Free space testing is sometimes further divide into outdoor and indoor testing. For indoor testing an anechoic chamber will work very well.

Elevated Range
Click on image for a larger version of the image.

Compact Ranges

It is good to consider preforming compact range testing. In the simplest from a parabolic reflector is used to create a plane wave appropriate the make the necessary measurements. Below is an elaborate example.

Compact Range
Click on image for a larger version of the image.

After the equipment and possible test setup have been determine the fundamentals of the Antenna need to be evaluated. Following is a list to consider

  • Radiation patterns (gain and efficiency)
  • Impedance (Voltage Standing Wave Ratio)
  • Bandwidth
  • Polarization

References:

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