[SOLVED] How can we measure magnetic field strength generated by an antenna?

[EltonBrasil]

NFC System with antenna which can provide from 1 A/m to 7.5 A/m signal strength

Hello Everyone,

I am developing a NFC system using Texas Instrument S4100 as RFID reader module and a power amplifier attached as well. I would like that the NFC system with antenna can provide from 1 A/m to 7.5 A/m signal strength. I describe below my questions:

1) How can I generate this strong signal strength (7.5 A/m)?

2) How can I test the signal strength? By spectrum analyzer?

Many thanks in advance.

 

[vfone]

Re: NFC System with antenna which can provide from 1 A/m to 7.5 A/m signal strength

The transmitter output power of the S4100 module is 200mW. To get more signal strength you need to use an external power amplifier. I assume couple of watts (maybe less) will provide your desired signal strength. You can do the math.
Check the net and you will find many PA examples for RFID frequencies, with various output powers.
You can use the spectrum analyzer to measure first the conducted output power, and then the radiated field strength. For radiated field strength you need a calibrated setup (TX and RX antenna gains, distance, etc.).

 

[FvM]

Re: NFC System with antenna which can provide from 1 A/m to 7.5 A/m signal strength

1.5 to 7.5 A/m is the standard field strength according to ISO 14443 which has also been adapted for NFC, active mode. It can be achieved by most reader chips on the market without additional power stage with matched reader coil according to the manufacturer application notes.

Field strength can be easily measured with a small sensing coil and oscilloscope or SA. Use only one or few windings to make the coil inductance negligible.

Measurement methods are also defined in ISO_IEC 10373-6 and ECMA-356.

 

[EltonBrasil]

Dear,

Thank you very much for your quick reply. I did not know about those measurement methods.

Many thanks for your quick reply. I have a spectrum analyzer, so I will do what you have suggested.

 

[EltonBrasil]

Hello Everyone,

I need to measure the magnetic field strength generated by an antenna. So it is a NFC (Near Field Communication) application. I describe below my questions:

1) Are there some magnetic field strength sensors to measure that? If so, do you have suggestions for?

2) I can use a spectrum analyzer, but there is any calculator or formula that is straight forward where I could calculate the field strength in terms of A/m?

Many thanks in advance.

 

[FvM]

The obvious solution is a sensing coil. Relation between magnetic field strength and induced voltage is derived in physics textbooks (Faradays law).

As previously mentioned, measurement of reader coil field strength is also described in NFC and similar RFID standard documents.

 

[biff44]

https://www.mouser.com/ProductDetai...MItLyhx5-35QIVipOzCh08IQdZEAQYBSABEgKUcfD_BwE


i would just get an antenna that has a published known gain, and use it hooked to the spectrum analyzer. The data sheet, if you find one, will have a conversion factor between received magnetic field strength, and the reading on the spectrum analyzer.

 

[FvM]

The said FXR.07.A "antenna" (IMHO the appropriate term would be coil or inductive coupler, but I know that "antenna" is commonly used in the NFC world) has no specified "gain" according to the datasheet. In so far a self elaborated coil with known winding geometry would be better suited.

 

[FvM]

The calculation of induced voltage is quite simple

Vpk = ω*n*Acoil*µ0*Hpeak

For single turn coil with 50 mm diameter you get 0.21 V at H=1 A/m.

 

[EltonBrasil]

The calculation of induced voltage is quite simple

Vpk = ω*n*Acoil*µ0*Hpeak

For single turn coil with 50 mm diameter you get 0.21 V at H=1 A/m.

All right. Just making sure ... ω is the radial frequency? What about n?

Many thanks in advance.

 

[volker@muehlhaus]

n = number of turns of the pickup coil