Hi,

I'm working on an NFC, 13.56MHz, project where I'm reading several tags at the same time using anti-collision detection. I'm using the RSSI parameter from the tags to determine which tag is physically closer to the antenna. I'm using a PCB coil antenna.

The problem that I'm having is that the EM field coming from the antenna is not uniform. For example, on top of the coil PCB traces the EM field is stronger than in the middle of the antenna giving me "false" RSSI readings. I would like to have a "fairly" uniform EM field everywhere just on top of the antenna. I'm hopping that my explanation is clear enough. Is there a 13.56MHz antenna pattern that I can achieve what I want? Is there a 13.56MHz antenna design other than the PCB coil antenna with a more uniform EM field distribution? Any comments are greatly appreciated.

Thanks,

Robert

 

NFC means Near Field so these are not really classic radiation antennas, they are more like coupled tuned inductors. The reactive near field distribution is heavily influenced by any source of induction near the 'antenna' tuning coil(s).
https://www.antenna-theory.com/definitions/nfc-antenna.php

 

Thanks, that helped.

 

Texas instruments has a wealth of information relating to NFC and antennas for it.
https://www.google.com/search?client=firefox-b-d&q=texas+instruments+NFC+antenna

 

Texas instruments has a wealth of information relating to NFC and antennas for it.
https://www.google.com/search?client=firefox-b-d&q=texas+instruments+NFC+antenna

The ST appnote does a great job of showing NFC coil magnetic field coupling.
https://www.st.com/resource/en/appl...or-st25-nfc--rfid-tags-stmicroelectronics.pdf


https://www.rfwireless-world.com/Articles/NFC-basics.html

NFC devices work based on inductive coupling. Induction is the production of electric current by passing a wire through a magnetic field(H). As we know NFC devices have coils built into them. A magnetic field from a NFC device generates power in these coils, which initiates the transmission of data over the radio waves. Both devices (target and initiator) share this power.

I guess they're called antennas because it sounds sexy.

 

In our lab we are applying a lot of ICP plasma coils, what is 3 to 4 turn coreless coil of diiameter of 15 to 45 mm fed by 27.12, 40.68 or any other frequency. The wattage is about 200-500 W.

I personally had measured the need for safety shielding. Yes, the shield may cause the slight interference problem. But when there are no any shield, the few cm from "lamp" the field is withing order of several kV/m; about 10 cm it already falls into guidelines for human health normative (ICNIRP-97) and after one meter the RF leaking is so small that it is in-recognizable from the environmental RF noises - few microwatts per meter.

Thus, I guess, the problem why on the Nordic NRF-24L PC-antenna coil is well radiative but our coils not, is set ONLY by the impedance matching. When the coil have characteral impedance near to vacuum impedance 276 Ohms then it is good antenna, and vice versa - when it is far away from that wondervalue, then radiation is strucked and weaned in half-cry. Our coils uses 5...10 A at 3000 V what after ignition falls down to about 30-50 V, thus the generator impedance is roughly 50/5=10 Ohms.

 

About NFC. The NFC is defined by two criterions. First, the distance to antenna is smaller than 10 heigth of antenna mast. So, if Riga TV tower has 369 m high means the near zone is 3.7 km in radius. Second criterion is wavelength, the near zone ends in 4 lambdas.

If lowest frequency is Radi-Riga MW 1602 kHz alias 300/1.6=200 meter*4=0.8 km in radius. As the Zaķusala have dimensions about 1/2 k in radius, I used to walk with measurer there. In that zone the EACH STEP changes field value from mahimum to minimum, for example, from 100 W/m2 to 1 W/m2 and have minimum midway to Zaķusala bridge where the signal is reflecting thus the near zone of bridge again is the maximums.

The same time at the doors of antenna tower the field intensity is virtually nil, few miliwatts per m2. Att the dreadful values starts within about 200-300 m off the tower, then along the megawatt underwater power cable from HES to tower, and then near the reflecting bridge. Ah ya, the local millioner, no billioner, bought the all land in that island and each year tries to prove there is no EM field at all, and build there the skyscapers full with an exclusive ++ rate rent-flats for less successful people having full ass with the money. What to say, thats an Natural Selection by Darwin. Stays alive the smartest not richest.

PS: Good article about NFC, discussing more stricter criterions how to guess the NFC size, is given here https://en.wikipedia.org/wiki/Near_and_far_field

 

In our lab we are applying a lot of ICP plasma coils, what is 3 to 4 turn coreless coil of diiameter of 15 to 45 mm fed by 27.12, 40.68 or any other frequency. The wattage is about 200-500 W. I personally had measured the need for safety shielding. Yes, the shield may cause the slight interference problem. But when there are no any shield, the few cm from "lamp" the field is withing order of several kV/m; about 10 cm it already falls into guidelines for human health normative (ICNIRP-97) and after one meter the RF leaking is so small that it is in-recognizable from the environmental RF noises - few microwatts per meter. Thus, I guess, the problem why on the Nordic NRF-24L PC-antenna coil is well radiative but our coils not, is set ONLY by the impedance matching. When the coil have characteral impedance near to vacuum impedance 276 Ohms then it is good antenna, and vice versa - when it is far away from that wondervalue, then radiation is strucked and weaned in half-cry. Our coils uses 5...10 A at 3000 V what after ignition falls down to about 30-50 V, thus the generator impedance is roughly 50/5=10 Ohms.

Each of those cans uses a 3KW 13.56MHz generator to generate up to 80KV of voltage at the tip of the resonator. You can see the SWR meter on the input to the can from the feed coax. The coil is adjusted in length inside using SF6 as the HV insulator gas.