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RFID Signal Collision Issue

Posted on 2013-12-17
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Last Modified: 2013-12-30
I am not sure if I can get any help here on this one but i am going to throw it out there.  I time races using an RFID system.  Last week we timed our first race with an overhead gantry system.  It was a 20' span over the starting line that we covered with 6 evenly spaced antennas.  The first 4 antennas were on one reader and the last two were on another reader.  We tested all of our equipment prior to the event and all was working.

During the race we only had a 75% read rate with the vast majority of our reads coming from the  outside antennas and almost none coming from the antennas between them.  The antennas were in a line and aimed downward.

Does anyone have the expertise to tell me why this might have happened?  Our RFID system routinely reads well over 99% at the finish line (using antennas embedded into mats that the runners run across) so this 75% rate is totally unacceptable.
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Question by:Bob Schneider
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d-glitch earned 334 total points
ID: 39725611
This is probably a range issue.  Are you using shoe tags?
Shoes and mats get very close, the gantry is much further away.
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by:Bob Schneider
ID: 39725624
No they are disposable chips on the bib numbers.  Probably 3-4 feet from the mats and 6-7 feet from the gantry.  It is my understanding that the range of our system is in excess of 25 feet.  We have verified that when we used side antennas...it was picking up runners well away from the finish line area.
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by:d-glitch
ID: 39725625
Do you have the specs for the equipment you are using?
Can you post a link?
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by:d-glitch
d-glitch earned 334 total points
ID: 39725650
Antenna orientation could also be an issue.  Maybe the bibs have a horizontal radiation pattern?
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Author Comment

by:Bob Schneider
ID: 39725653
We use the Ultra system here: http://rfidtiming.com/  And yes we have to orient the antenna correctly, which we did.
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by:Craig Beck
Craig Beck earned 166 total points
ID: 39726302
I've used tags such as ChampionChip for mass-participation events and never seen an issue with ankle-tags and mats.  One event that I worked on regularly used ankle-tags for 55,000 participants and the only tags which weren't read were the ones which didn't cross a split or the finish line, or damaged ones.

The lace or ankle-tags work well because the antenna is in the mat, so there can be no objects between the tag and the mat and the proximity is only a few inches.

It could just simply be dud tags though.  I've seen that a few times on some events I've worked on.
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by:d-glitch
d-glitch earned 334 total points
ID: 39726987
>>   20' span over the starting line      How high was it?

>>  the vast majority of our reads coming from the  outside antenna
That certainly suggests that there was something wrong with the central antennas.  

Are the side antennas at ground level or at altitude?  

Is the gantry made of metal?  A metal ground plane can interfere with antenna operation.

Is it possible that a connection came loose after the race started?  That could happen if the pack bumps the gantry.  

When did you notice the problem?  Did you test the system after the event as well?  
Can you test the system during the event?  Maybe run some dummy tags through at sever lane locations.

Do you test the antennas one at a time?  Can you tell if the timer picks up a tag on more than one antenna?
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Author Comment

by:Bob Schneider
ID: 39727139
The antennas are 10' above the ground and, yes, they are on a metal pole.

Highly doubtful that a connection came loos after the race started but I suppose anything is possible.  

All equipment did test out ahead of time and it is my understanding that my partner did test all equipment individually.  I am not sure if the system was tested in it entirety.  We will definitely do that before our next race.  

We did NOT test the system after the race.  

We do get a text file containing all reads while the system is running so we can determine if multiple readers are picking up the same chip, although I can't say that I looked for that.  I will look again.

Thanks so much.  Any further thoughts would be appreciated.
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by:d-glitch
d-glitch earned 334 total points
ID: 39727283
There is some additional material here:
   http://www.skyrfid.com/RFID_Gen_2_What_is_it.php

Check out the sections on Gen 2 Readers and the Gen 2 Tags.  They do mention range and orientation issues.  They also mention metal shielding effects for the tags, but it can be a problem for the antennas as well.

Checking the photos for the Ultra System.  They use individual TNC cables for each antenna.  These should be extremely reliable if they are assembled according to spec with the right tooling (barrel strippers and crimpers).  If you are making them yourself with common tools, it could be trouble.  It is easy to make a cable that looks good with an ohm meter but doesn't work at all at 900 MHz.  This is officially in the UHF band ( which stands for Ultra High Frequency).

One final note.  A good way to get some redundancy would be to use two readers with four antennas each.  And interleave the antennas, 1 3 5 7 on one reader, and 2 4 6 8 on the other.  Hopefully every tag would register on each machine.
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by:Craig Beck
Craig Beck earned 166 total points
ID: 39727317
Is there a specific reason why you're using antennas in the gantry and not just regular mats?
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Author Comment

by:Bob Schneider
ID: 39727788
We don't have enough mats to do a start and a finish when they are not in the same place.  And, with Nordic Skiing mats are not an option.
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