The RFMAXMAPPER: Use in Railroad Car Application

We recently had a customer inquire about our power field mapper, the RFMAXMAPPER. See the question below and the response from the creator of the device.

The Questions: 

How do you turn the -7dB switch on?  And, How do I measure RF power within 3 feet of the RFID Antenna so that maintenance technicians need not back up 10-15 feet away to test?  Specifically, how do I do this in a Railroad car application?

The Answers:

You probably have a good large linear tag on the rail car, set vertically. The reading antenna can be somewhat troublesome to get right; I will clarify:

Now, this may not be what you want to hear but you will get excellent results using a large vertically oriented, linear patch antenna set at 30 degrees to the side of the carriage. This is so that the tag is in the UHF bean for the longest period possible. Experimenting with this angle is a good place to start.

Because most tags can be read easily at 5 meters, sometimes moving the antenna further away will improve results. Again this puts the tag in the beam for longer.

With this application you should be able to read the tag easily at a slow speed; however, when the train is travelling at a high speed then the reader hasn’t got time for the data slicer to settle to the right level and so results are quite random. For this reason, it is best to place the tag below any windows and at the back of the carriage (or middle for bi-directional carriages), this gives the reader's data slicer more time to settle to a constant steady level.

Obviously, it is important to set the ‘Q’ value of the reader to 0 or 1, you don’t want a dynamic ‘Q’ as you need the lowest value of ‘Q’  possible.

Also, reducing the number of data bits in the ePc you are collecting will dramatically improve results. Moving a truncated version of the TID into the ePc is one way or simply using the reader's masking capability.

Let's take a look at the worst case. A circular polarized antenna pointing directly at the tag at close range. Even if the antenna is at exactly the correct height, it will only have a few milliseconds to acquire the tag; also, the backscatter will be many orders of magnitude large than the tag data;  especially after the carriage gap.

For this problem, a greater distance between the tag and the reader may help as the tag spends more time in the reader's narrow beam.

Normal patch antennas have a beamwidth of 70degrees; however, you can get antennas with wider bean angles of 100 degrees. You may even find that a simply grounded monopole that has 180 degrees may be better than a high gain patch. (much less power but much wider beam angle.)

The calculation in my notes says that with a  70 deg antenna, 3 meters from the carriage; and a train going at 60 MPH you have only 6.4mS to capture the tag; if the reader is pointing directly at the tag. !!!

A good reader can capture 200 tags per second which is about 5mS per tag. And so, at 60 MPH everything else needs to be perfect to read the tag in one attempt. And so, It’s not going to be reliable, especially in rainy weather or snow.

The Power Mapper is essential as it will allow you to analyze your antenna and also the environment.

To analyze the reader turn the dial fully clockwise to calibrate the meter, and switch to 0dBm mode. Turn on the reader and if possible switch it to constant CW mode. If you can't do this switch the power mappers detector to slow.

Now with your fingers and body behind the meter move the meter backwards from the center of the antenna until the meter reads 0dBm.

The distance between the meter and the antenna will be about 3 meters if you have your reader set to a maximum. This is 4W EIRP in the USA. If you reading 0dBm at less than 2 meters something is wrong! Check that the reader is capable of 4W and that its internal attenuator is set to 0dB.

At the distance where you find the meter reads 0dBm move the meter to one side until the meter reads -3dBm and mark this location. Do the same on the other side. The resulting angle between the two markers and the antenna is the beam angle of the antenna. This will probably be about 70 degrees. You want this angle to be as large as possible. But be careful as some countries, over in Europe for example set the Max power level down by 50% if you use a wide-angle antenna.

Now we can use the Mapper and get some data about the reflection from the train. Set the Power Mapper to map mode and the fast-slow detector to fast.

With the reader still on constant CW; place it 3 meters from the carriage and the Power-Mapper just behind the antenna (dial facing you). As the carriage moves slowly past watch the meter needle change as the carriage changes the backscatter at windows or carriage gaps.

The map mode is about 9dB more sensitive than the 0dBm mode; so you should see a small movement of the needle when the carriage goes past.

You can gather real data using a Bluetooth voltmeter attached to the back of the meter; you will get a plot or the backscatter as the train moved past. You need to position the tag so that this backscatter is stable before the tag reaches the antenna.

I hope our experience is useful and answers questions about your application and your Power Mapper.  The Power Mapper can do much more than the above; we hope you find it very useful.