6
\$\begingroup\$

I want to build a compact 1 MHz receive coil (antenna for very near field). For that I need to make a choice on whether to build a simple RF coil only consisting of wire and capacitors or using a coil with a ferrite core. I found materials that have a high real permeability and very low imaginary permeability even at 1 MHz. For example, this NiZn ferrite material.

How do I compare the sensitivity of the magnetic core antenna to the sensitivity of the antenna without the core. Does the core introduce noise beyond what is described with the complex and real permeability?

\$\endgroup\$
1
  • \$\begingroup\$ It sounds like both designs you're talking about (with and without the core) are small ("magnetic") loop antennas, you're not comparing a magnetic loop to anything else. \$\endgroup\$
    – hobbs
    Commented Aug 1, 2020 at 4:21

2 Answers 2

Reset to default
8
\$\begingroup\$

It's pretty common to use a ferrite core for signals around 1 MHz. Consider the standard ferrite rod coil in a regular AM receiver: -

enter image description here

Picture from here.

It uses a ferrite rod because it improves the signal amplitude received by concentrating the transmitted magnetic field part of the prevailing signal through the core and therefore inducing more signal voltage into the coils. Given that the coil is also an inductor, that property can be used to give a tuning capability when used with a capacitor.

The longer the rod the more field it will concentrate through the coil and the bigger the signal.

It's still pretty good if really compacted. For instance, when using near-field power transfer, even a mere hint of ferrite material in the right place can significantly improve the received power.

The Fair-rite material 61 you linked is very good in this area and I have used it several times for improving power transfer between coils: -

enter image description here

Does the core introduce noise beyond what is described with the complex and real permeability?

Given that this technique is used in radio receivers (that deal with very low incident powers from distant transmitting antennas), I don't see a problem and I've personally never seen one. I've even pushed the envelope with type 61 material and used it in a data transformer for bit rates of circa 600 Mbps and it proved a useful and beneficial addition.

Given that the core will cannot be a continuous loop (like a toroid) in this type of application, there is no danger of core saturation or significant non-linearities.

\$\endgroup\$
3
  • \$\begingroup\$ Thanks! That is very encouraging. Do you think Fair-rite material 61 is already the best choice? I'm also considering this material PC200. \$\endgroup\$
    – Fulix
    Commented Jul 31, 2020 at 12:31
  • \$\begingroup\$ @Fulix at 1 MHz PC200 is fine - what you have to remember is that with an open slab or core of ferrite, the actual real (or effective) permeability is a fraction of the graphs due to the air gap. I'd prefer to go for PC200 at 1 MHz because it will "concentrate" the field more due to having much greater initial permeability. The losses will be incidental. \$\endgroup\$
    – Andy aka
    Commented Jul 31, 2020 at 13:24
  • \$\begingroup\$ @Fulix if you are done with this Q and A session please select an answer as per this guideline \$\endgroup\$
    – Andy aka
    Commented Sep 17, 2021 at 15:27
5
\$\begingroup\$

No, the ferrite does not introduce noise. But it can lead to nonlinearity. If you want to build compact, then a ferrite is the way to go.

\$\endgroup\$
1
  • \$\begingroup\$ Nonlinearity in what sense? What if at the operating frequency the real permeability is much higher than the imaginary permeability, does that still introduce unwanted effects? \$\endgroup\$
    – Fulix
    Commented Jul 31, 2020 at 12:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.