I need to construct an antenna for a reader that will pick up a 134.2 kHz RFID tag.

The antenna will need to be in "tunnel/culvert" under a road. The tunnel is 2 metres wide, so antenna needs to be lie across that distance. The animals carrying the tag are frogs so only a few cm normally above the antenna (they normally just "crawl"). However, Frogs do jump sometimes too so it may have to read a bit higher than that. It sounds to me like even ten cm is a bit of a stretch for these....true? false?

Any advice on what an antenna of this length (it would have to go across and back as a loop of course) would need to be made of and how to tune it to give me a starting point for further research would be appreciated.


Similar, not identical, see RFID Antenna Array Also search site for RFID.

Also see Microchip AN710

Polling multiple readers would be easy.

At 132 kHz, switching antenna loops would be "easy enough" using eg MOSFETS but it is unlikely to be needed.

A long loop antenna tuned to 132 kHz should be easyish.
At high frequencies a distance of 2 metres would be large compared to the wavelength of the signal being used and building it could cause problems. However, at 132 kHz this is not the case. ie
Wavelength in metres is given by: wavelength = (300 / Mhz) metres So here wavelength = 300/0.132 ~= 2300 metres = 2.3 kM
so the wavelength is very long compared to the coil length.

so producing a resonated coil should be easy. Probably a few turns 2m long and forming a rectangular loop perhaps half way up the tunnel height = at the tunnel max width if this is a round pipe, so that the frogs travel under it at a pipe radius or less away.

  • \$\begingroup\$ Hi Thanks very much for your response. <br> Being new to this, your last sentence @russel "....wavelength if 132...." doesn't mean much to me in reality. Not sure what the 2.3 kM is and how it relates to a few turns 2 m long. How does stretching a loop into a narrow rectangle affect the antenna? I have searched through the suggestions given by the responses to my query.....thank you all so much. I'm sure I may have other questions as I go along. This is a wildlife project and as such always very short of funds - i.e. I will have to build it myself. Cheers, Joan \$\endgroup\$
    – Joan
    Mar 1 '12 at 8:52
  • \$\begingroup\$ @Joan - I've expanded my comment re wavelength. Where are you based? \$\endgroup\$
    – Russell McMahon
    Mar 1 '12 at 10:50
  • \$\begingroup\$ thanks for that. I need to do more research into antenna construction...I am not sure how the coil length vs wavelength plays out. Do you have any hints of places to get good info on this for a new kid on the old block? Sorry not used to this format so it gets away from me and "sends". \$\endgroup\$
    – Joan
    Mar 3 '12 at 4:35
  • \$\begingroup\$ again...The antenna would need to be on the ground if possible since we also have bears using the tunnel and they would potentially break anything if it was off the ground. I was thinking of a pipe lying on the ground so they have to crawl over it (tags are in their back). I am on the west coast of Vancouver Island Canada. Cheers Joan \$\endgroup\$
    – Joan
    Mar 3 '12 at 4:40
  • \$\begingroup\$ Sorry about the formatting...I don't seem to have it figured out - I thought I was following instruction to get paragraphs etc to make it easier to read...even more learning! \$\endgroup\$
    – Joan
    Mar 3 '12 at 4:45

Take a look at the RFID tag systems used for timing sporting events like running and biking ( eg http://www.chronotrack.com ). These may provide hints to form factor. These sorts of systems support multiple tags and the distance from the mat to the tags during an event is comparable to your worst case.

  • \$\begingroup\$ Great idea! If the OP is a scientist or researcher and not an engineer, just buying one of these systems would probably be quicker, cheaper, and easier than trying to hire a consultant to build a custom, large, complicated, outdoor system like this. I think that ChampionChip does in fact use 134.2kHz tags, but other manufacturers sometimes use higher frequencies. \$\endgroup\$ Feb 29 '12 at 20:18
  • \$\begingroup\$ @kevin Thanks We already have the tags (expensive) and they are 134.2 kHz so are stuck with that...they read shorter range but penetrate flesh well for reading imbedded tags. \$\endgroup\$
    – Joan
    Mar 1 '12 at 8:56
  • \$\begingroup\$ 134.2 is used in a lot of the sports tags. Mats are typically rented from regional companies by race organizers. Depending on the length of your study, this could still be the most economical approach. \$\endgroup\$
    – spearson
    Mar 1 '12 at 16:37
  • \$\begingroup\$ Your link has gone dead and there isn't much content left in your answer. \$\endgroup\$
    – Kellenjb
    May 27 '12 at 22:37
  • \$\begingroup\$ Added a new link to a comparable system. \$\endgroup\$
    – spearson
    May 28 '12 at 17:53

You should start by knowing the reader itself. Most readers have an inductive / capacitance (LC) circuit that the antenna will have to tune to in order to achieve resonance. The larger the antenna , less wire wraps , more capacitance. For instance, a 12" pipe may have 20 turns and 270 uf capacitance in series with the reader.


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