Project Help - Large RFID Antenna or other ideas

Greetings Guys and Gals,

I am working on a project very similar to this one:

For those who don’t want to spend the time reading through that page, it is an automatic cat door. It remains locked unless a criterion is met, in this case an approved RFID tag.

I am trying to replicate this for my dog, a ~50lb Golden retriever. Since she is bigger than a cat, I am hoping to increase the RFID read range so my dog can easily/casually open the door (not having to rub up against the door or anything). I am hoping for a read range of 6-12" (15-30cm).

I am using the following materials:
SMAKN® RDM6300 125Khz EM4100 RFID Reader Module UART Output Access Control System for Arduino

125kHz RFID Tag (

24AWG magnet wire

I have tested out the materials, and they seem to work fine. I have even built some of my own antennas with the magnet wire and gotten them to work. However, when I try to build the antenna I plan to use with the actual door, I can’t get any reading. I am using a wooden frame. The interior of the frame measures 10x12" (25.4x30.5cm), while the outside of the frame measure 11.5"x13.5" (29.2x34.3cm). The wire for the antenna is wound in a 1/4" (<1cm) groove cut into the outside of the antenna. I will provide pictures if anyone wants to see it.

So here is where I am hoping for help. Does anyone know if/what I am doing wrong to not get my large antenna to read anything? And if anyone things this set up will not work, do you have any suggestions for alternatives, and any information as to why you suggest it, such as specs, cost, reliability, etc?

Thanks for any replies!

Train your dog to present her RFID chip to the reader. Eventually mount the reader on a post, easily accessible to your pets.

Have you used any design procedure when making your antennas, or simply wound a random number of turns on to your frame?

You need to ensure that your antenna is resonant at 125kHz.

Have a look at this RFID coil design guide.

Have you used any design procedure when making your antennas, or simply wound a random number of turns on to your frame?

I have mostly used that guide that I linked to in my initial post. It says 24 turns. When it didn't work, I tried adding and subtracting a couple of more turns to see how that worked (given that my antenna is slightly longer than the one used in the guide, it may make a difference). I have seen the RFID coil design guide, my own discredit, I haven't done any EM physics/math since high school and even then I didn't like/understand it.

I'll take a look at the equations again and see what I can figure out.

I have been trying to get the information to plug into the equations listed on the RFID coil design guide, when I noticed something that I find strange. I was trying to measure the current and voltage going through the antenna by measuring the pins of the RFID module. It read out upwards of 1V at first, but dropped and finally stabilized at 0.73V (on DC setting,based on my presumption that it's DC), and 8uA. However, when I decided to get more information to double check the math later, by measuring the current/volts with an antenna in use, I got the same voltage, but ~34uA. It's been a while since I have done this stuff, but I thought with Ohm's law when you increase resistance and maintain the same voltage, the current must decrease. Is that wrong, or is there something funny going on?

Additionally, while trying to investigate further I noticed the following:
Measuring the current is not symmetrical (!?). When I use the red probe of my multimeter on one pen and the black on the other, I get the 8uA reading mentioned before. However, if I switch the probes (so now the red probe is measuring the pin the black probe was measuring in the first instance) I get 3-4uA.

Fearing that this may be due to it being AC current instead, I switched my multimeter to measure AC current. The voltage did the same thing before; start at 1+V and work its way down to 0.7V. I didn't think/know if multimeters could produce both AC and DC values on the same given circuit.

You're dealing with complex resistances (real+imaginary part), with unknown effects and readings on multimeters. IMO DC supply current of the sender should increase with lower distance between antenna and the RFID tag, indicating power consumed by the tag.