Inductor Antenna for 125kHz NFMIC System: FCC Part 15

[¡MR.AWESOME!]

I am building a 125kHz Near-Field Magnetic Induction Communication system and so will be using ferrite cored inductors for the transmitter and receiver. I am worried about RF radiation as it pertains to the conformance of FCC part 15. I haven't been able to get any good info on how an inductor/solenoid emits electromagnetic radiation. I have read things that allude to the inductor being quite poor as a typical antenna. I have also read things that allude to the reason for it's poor performance is because NFMIC systems generally use lower power than an RF system. I read a bit on helical antennas and an inductor, at 125kHz, would definitely be a normal-mode type, radiating in all directions.

I don't know much of the equations and math involved and am hoping to get a quick and dirty answer. If I can't get an answer without learning the math, then so be it, but I want to see what you guys have to say.

Thanks,
Andrew

 

[skeptic2]

I used to work with near field communications in the 200 to 400 kHz range. We transmitted about 50 mW using rather large ferrite rods as both transmitting and receiving antennas and got about 30 meters of range. Our receivers had a sensitivity of about 100 nV.

How much power were you planning to use?

Think of near field radiation as an air core transformer with the transmit antenna as the primary and the receive antenna as the secondary. Without a receive antenna most of the inductive field will be reabsorbed by the transmit antenna and be seen as inductance. As you bring a receive antenna closer and closer to the transmit antenna, more and more of the inductive field couples to the receive antenna, less and less is reabsorbed by the transmit antenna and the apparent inductance of the transmit antenna decreases.

There is a band from 160 to 190 kHz in which you are allowed to transmit up to 1 watt.

§ 15.217 Operation in the band 160–190 kHz.

(a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed one watt.

(b) The total length of the transmission line, antenna, and ground lead (if used) shall not exceed 15 meters.

(c) All emissions below 160 kHz or above 190 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measurements at the intentional radiator's antenna output terminal unless the intentional radiator uses a permanently attached antenna, in which case compliance shall be demonstrated by measuring the radiated emissions.

 

[¡MR.AWESOME!]

I'm only planning on a range of 2-3 meters, but I'm hoping o use off-the-shelf inductors so they will be much smaller. I definitely won't be using anywhere near 1 watt. I would like to be able to use a declaration of conformity and not have to pay for testing. I think I read that if the radiated limits are low enough, you can do that. I'll have to look again, though.

 

[¡MR.AWESOME!]

It turns out I can only use a declaration of conformity for unintentional radiators.

But there's better news. Give this a read through and make sure I didn't miss anything, if you would.

§ 15.201 Equipment authorization requirement.
(a) Intentional radiators operated as carrier current systems, devices operated under the provisions of §§15.211, 15.213, and 15.221, and devices operating below 490 kHz in which all emissions are at least 40 dB below the limits in §15.209 shall be verified pursuant to the procedures in Subpart J of part 2 of this chapter prior to marketing.

§15.209 says that between 9kHz and 490kHz, the field strength must not exceed 2400/freq(kHz). So 2400/125=19.2µV/m at 300m. -40dB of that is .192µV/m. Right?

Now I'm not too sure about the field strength drop off rate. Even a distance of 300m is still in the near field at 125kHz. How would I calculate the maximum allowed field strength at the source?

And if anyone has any info on my original question, I am still very much interested.

 

[skeptic2]

I'm confident you'll be under the FCC limits. Have you seen this provision?

§ 15.23 Home-built devices.

(a) Equipment authorization is not required for devices that are not marketed, are not constructed from a kit, and are built in quantities of five or less for personal use.

(b) It is recognized that the individual builder of home-built equipment may not possesses the means to perform the measurements for determining compliance with the regulations. In this case, the builder is expected to employ good engineering practices to meet the specified technical standards to the greatest extent practicable. The provisions of §15.5 apply to this equipment.

 

[skeptic2]

And if anyone has any info on my original question, I am still very much interested.

Was your original question about how the electromagnetic field radiates? I think when we're talking about near field radiation we're talking about just the magnetic field, not the electric field. Without a receive antenna or other conductor, the near field is reabsorbed into the transmitting inductor and does not radiate.

 

[¡MR.AWESOME!]

Yea I have seen that provision. As long as the system ends up working the way I want it to, I am actually going to incorporate it into a device that I am going to sell. So FCC regs are a concern. Thanks for the heads up, though.

I get how the magnetic field works, I'm wondering if the coil and the ferrite changes the E field or the whole EM field strength and the way the field propagates compared to a "more standard" antenna. I'm not too sure about all that shtuff and that's what I was originally wondering.

 

[skeptic2]

Yes, a magnetic field is different from an electromagnetic field. Generally in order to get good coupling between the transmit and receive antennas, the magnetic lines of flux must pass lengthwise through the ferrite rod.

 

[levnu]

Hi,
how should i setup and measure the transmitted power?
BR
Arye

 

[skeptic2]

Do you have an RF power meter? That would measure power into the antenna. You could calculate power out of the antenna by measuring the field strength and converting that measurement into power. In order to get good field strength readings you should be in an open field or at least away from all metal.

If you want advice about how to get a declaration of conformity, the best place to do that is from the FCC. Their helpline number is 888-225-5322.

An alternative albeit more expensive method would be to send it to a testing lab. They can produce all the measurements needed to get it properly certified.