# 13.56MHz RFID Antenna design

I am currently in the process of building an RFID access system for my house + car. I've worked out the software side (I am using an arduino) but am having difficulty with the hardware side. I bought one of these for the transmitter but the read range is terrible - the access dongle has to be touching it for it to register.

I'm thinking about cutting the traces on the card and building a new antenna to amplify the read range. Unfortunately I don't know how to do this. I've read through the antenna cookbook and the antenna design notes from TI, but I'm a hobbyist and a lot of it seemed very general and went over my head - it would be great if I could get a more prescriptive answer.

I would like to have the antenna read from 5-10cm in most orientations if possible.

How important is material choice (wire/tape/tube)? Do I need an additional power supply on the antenna end or is the 5V from arduino enough? Can I build a square antenna into the flashing around the card window or will the metal create too much interference?

• What kind of antenna do you have on the receive side? – Yuriy Nov 30 '13 at 13:29
• Currently the stock dongle that came with the card: imall.iteadstudio.com/im120808003.html Looking to transition to something small later on though. – cjdbarlow Dec 1 '13 at 5:33

Unfortunately, it is not easy to design a good 13.56 MHz antenna. In general, a good antenna is at least 1/4 of the operating wavelength (of ~22m), so any practical 13.56 MHz antenna would be a bad antenna.

The main problem with electrically small antennas is that radiation resistance is tiny (less than 1 Ohm), making it almost impossible to match it to the 50 Ohm. One technique for matching would be to introduce a small series resistance (e.g. 2 Ohm) and then match to it with capacitors and inductors. However, this approach is difficult without access to a vector network analyzer.

I noticed that the module has a UFL connector. One way to improve the antenna would be to plug in another antenna you buy off the shelf and disable the current one (will probably need to cut traces of the old antenna)

To answer your questions: "How important is material choice (wire/tape/tube)?" - not important at 13.56 (as long as it is metallic)

"Do I need an additional power supply" - not if you are using the same chip

"Can I build a square antenna into the flashing around the card window" - would need more info on this.

• Thanks for the tips. I had a look for antennas on eBay and a few online retailers, but they are all quite expensive and I'm not sure what sort of quality they would be. In regards to the car window, what I envisaged was crafting a square loop antenna which would sit in the rubber around the drivers side window (with the window forming the centre of the antenna). The theory being that I could then touch the dongle to the window to unlock the car. I wasn't sure how well this would work if the antenna was shielded by the card door though. Maybe a linear one would be better? – cjdbarlow Dec 1 '13 at 5:42
• Yes, a car door would significantly lower the magnitude of the signal. I would try to find an RFID chip with a bit more range. It will inevitably use more battery but may give you a better range. Messing with antennas is nearly impossible if you don't have the right equipment (which costs \$) – Yuriy Dec 1 '13 at 15:06

I think you may be out of luck on this device. It takes its power from the near-field transmitted by the TAG reader and probably (said with a little caution) responds with a small data stream that modulates the received field.

There's enough modulation for the TAG reader to recognize that it's transmit waveform is being subject to "counter modulation" and it decodes this modulation and recognizes the TAG's address or ID.

Firstly, the receive coil on the TAG (I don't like to call it an antenna because it is a near-field magnetic coupler and not an EM wave antenna), if you made it bigger to "grab" more magnetic field at the same distance (or the same field at a bigger distance), the "counter-modulation" would be proportionally weaker at the TAG (as a percentage) and much therefore weaker at the reader but, it would be twice weakened because the distance has increased.

Even increasing the Q makes it work at a bigger distance but harder to impregnate the field with a sensible response.

I hope I'm wrong but that's how it looks to me. The better TAGs are those that use the near-field just for power and transmit on a completely different frequency.