UHF RFID Antenna in Timing Systems
Hi! Today I want to talk to you about UHF RFID Antennas, because of all components, I believe these are the most critical because they are the component that people know the least about and not all timing systems use the same types of antennas.
Why are the UHF RFID antennas the most complex part of the timing system?
Ultimately, with the proliferation of UHF timing systems, a lot of people have thrown themselves into developing and manufacturing timing equipment without the knowledge or experience needed to do it.
The UHF antenna is the most complex component. Programming an Impinj reader with the libraries provided on .Net and then putting it onto an adjacent industrial PC or similar is relatively simple. Even so, not everyone does it. We buy standard grommets, we empty them and stick an Impinj antenna inside them and start timing… right? Nothing could be further from the truth.
To begin with, you need to be knowledgeable about all timing system components; for example, the cables. The Impinj reader accepts more than 128 different configurations of the GEN 2 protocol and I guarantee you that there’s one that’s better than the rest. We worked for 4 months, with a lot of training sessions and several timed races in order to detect it. Moreover, accurate timing is no longer enough. Now there’s also a need to communicate very widely, very clearly and, above all, very fast. Sending push messages to each runner, posting on each runner’s Facebook wall and Twitter timeline, Live Timing, photos, newsletters… these are just a few things that we now offer thanks to Sportmaniacs.
Even with all this, the antenna is still the most complex element because the average person doesn’t know how to design one. There isn’t a single antenna on the market that can operate well for timing races from the ground, and side dishes don’t allow for certain very normal widths on popular races.
Every manufacturer with a decent timing system has manufactured its own antenna or has contracted another company to manufacture its antennas. Here are the solutions that other manufacturers have adopted:
For the Chronotrack timing system, Impinj manufactured the antenna; specifically, a guy named Ronald Oliver designed it. I find the design of this one simply incredible. The work this guy has done is exceptional and, after studying his antenna extensively, I continue to be amazed by it. I am no expert in antennas. So you might say, “well then, we’ll buy the same Impinj antennas, and problem solved!” But no, it turns out that Impinj provides a special edition of these antennas to Chronotrack so that they operate well in their timing conditions. There is no way to modify a commercial antenna to turn it into a Chronotrack antenna. Believe me, I’ve tried it. It’s too complicated to make one from scratch and, without the tools, the necessary equipment and the right knowledge, you might think that two antennas are exactly the same and one will work and the other one won’t.
In Mylaps timing systems, I see some things that are really good, where their experience is clear, and others that aren’t so good because I think they had to rush it. For example, their ground antenna. It’s a patch with a plane on the back and a dielectric in the middle. I haven’t been able to disassemble one, but I’m convinced that this is what it is. It’s a much simpler antenna than Ronald Oliver’s, and that means that Mylaps have a timing chip with a huge protective foam layer. So huge that it’s annoying. In reality, in my humble opinion, the better or worse performance of Mylaps is due more to having such a thick foam layer than to the antenna itself.
There are other timing system manufacturers that have made a similar antenna to the Mylaps one, some with less luck because they use two timing chips per shirt number.
How have we made timing antennas at Timing Sense?
At Timing Sense we’ve developed our own antenna based on a similar model to Impinj’s, but adapting it to our timing systems needs.
The first aspect is that our antenna is made of steel to give it a good resistance to passing vehicles. This is not the case of every timing system antenna.
The excess weight of the antenna is compensated for by making the grommet a little lighter so that the unit has an ideal weight.
Our antenna is a little bigger than the Impinj antenna and our grommets are around a metre wide. I’ve never liked the 90cm width of the Chronotrack system’s grommet. I think that to cover 4 metres with 3.6 metres of grommets is feasible, but to cover a 12-metre starting line with 12 grommets measuring 10.8 metres is a problem. The consequence is you have to use an extra device to power the thirteenth grommet and its antenna so as to cover 11.7 metres. Our grommets are much closer to one metre. They measure a little less because, for example, if the finish line arch has an 8-metre opening, and if it’s made of metal, it would be hard for us to install 8 one-metre grommets in that space. I think it’s better for the grommet to be between 2 and 4 centimetres less than a metre to give you the clearance you need to be able to work without having a very significant loss in an assembly of 12 or 16 grommets.
The UHF antenna is designed to open its beam a little wider and cover those extra centimetres without a problem.
And now comes the technical stuff…
What type of antenna is it exactly?
The UHF RFID antenna we use, which is from the same family as the Impinj antenna, is a slot antenna with a cavity. The slot is where the signal enters very close to it and cuts the flow of the current. For this reason, the current has to travel around the slot. As it has a specific size, this antenna operates on Lambda and not Lambda/2, generating an electric field and some currents that provoke radiation.
This type of antenna has a peculiar feature in that a duality is produced with the dipoles. In broad terms, slot antennas behave in a similar way to a dipole, which is the simplest antenna, but with an inverse polarisation. And this is as far as I wanted to go…
End of technical stuff…
Why does the Mylaps system put the timing chip horizontally while we do it vertically?
Because of the type of timing antenna, and here the radiation pattern of the chip antenna also comes into play. The chip radiates in a way similar to this image from antenna.theory.com:
In the image, the chip would be vertical and aligned with the Z-axis. If we turn this chip 90º and put it parallel with the ground, the pattern also turns and would be in the same position as a car wheel and and with a polarisation parallel to the chip and the Z-axis. Therefore, if the chip is like the axis of the wheel and its radiation pattern is the wheel, we only need an antenna with an equal polarisation, horizontal on the ground. Well, this is what Mylaps has done.
Basically, their antenna is a metal square of a very specific size, with another, bigger metal square below and plastic in between. The polarisation depends on the side where the signal enters and where it exits. It is modifiable. They have developed a very simple antenna and the chip is on a total maximum since the red part is what would be pointing at the ground antenna.
Our antenna, as mentioned above, behaves in the inverse manner to a dipole in terms of polarisation. Therefore, although the slot is perpendicular to the direction of the race, as it is a slot, the polarisation is inverse. It isn’t horizontal but vertical. This is why the timing chip is also placed vertically on the shirt back. Those of you who have followed me so far will be thinking the same thing I did. “OK, yes, the ground antenna radiates vertically, but my chip has a zero just when I pass over the ground antenna.” What doesn’t radiate anything, according to the image above, is the greenest almost blue part. That is, it doesn’t radiate there.
Why then does our timing system operate so well if the chip is on zero radiation?
Because both antennas radiate in a wheel shape and transmit to each other moments before passing right over. In reality, the ground antenna is more complex and there’s more happening, but we won’t go into that here.
The timing chip receives energy several metres before passing over the grommet and, normally, it is read quite a few times before it gets there. In fact, I think that it does radiate just when it’s over it because of the meanders that chips have, perhaps because of the loop that comes out of the chip and always bearing in mind that the runner is moving and changing the exposure of the chip to the antenna.
It is important for anyone studying the different timing systems with a view to purchasing one to understand that the UHF RFID antenna is the most important component for an accurate reading. In spite of the technical nature of this post I hope you’ve enjoyed it and I invite you to ask me questions; you can ask them here on the post or privately to: firstname.lastname@example.org