How to hand-make a ~15 cm loop antenna for STEVAL-25R200SA?

[MelioNB]

Hi,

Follow-up to my earlier thread:
https://community.st.com/t5/st25-nfc-rfid-tags-and-readers/st25r3911b-disco-for-rfid-mid-range-in-a-box-with-constraints/m-p/894801#M13962
where a similar hand-wound loop approach was discussed, I tried to reproduce it but it won't work. On the GUI I get a score of 2 on the RF field measurement.

I'm a real beginner in RF. I'm using the STEVAL-25R200SA and I want to build my own ~15 cm diameter loop antenna by hand for ISO 15693 tags.

What I've done so far: I wound a loop with enamelled copper wire, scraped the enamel off both ends to get a clean copper contact, and connected it on the Flexible Antenna daughterboard. Photo of my setup attached.

Is the flexible antenna the best route for a custom antenna, or should I use the 50 Ohm interface board instead?

Could someone give me a step-by-step on how to build and tune the loop myself (number of turns, geometry, target inductance, matching)?

Thanks!

[AScha.3]

Hi,

building an antenna is not difficult, but you have to measure, what you build.

So for 13.56 MHz - i suppose , you didnt tell frequency - you have to check, what you puzzle together.

A spectrum analyzer+tracking generator or a network analyzer is needed, maybe just a cheap one:

- then you can check, what the antenna is doing:

....where is the main resonance .

Without any measurement...its just rolling the dice.

btw

Your free flying wires to the box are a no go. At 13M your in the short wave radio range...and the rules of rf design apply.

Here example from Ali , 13M, 

34x40 mm ; 4 Wdg. -> about 0.7...1 uH.

so at 150 mm : 2 wdg. -> 1.3 uH --- too big.

minimum requirement is to get similar inductivity , and have connection wires short and close together, not wild flying.

try yourself, to get something around 0.9 .. 1 uH:

https://www.df7sx.de/luftspule/

.... 110 mm dia. + 2 Wdg. could work...try...

[Brian TIDAL]

Hi,

Informations about antenna design for ST25R200 can be found in Application Note AN6065 Antenna design for ST25R200 and ST25R100 devices.

Rgds

BT