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Is ferrite rod antenna (about 2 to 4 inches) a good choice, or best choice, to receive induction communication (both transmitter and receiver use coil antenna), given that the H (magnetic) field strength is 3 dΒμΑ/m at 10 meters at 1MHz? Directionality is no problem as antenna rod will be fixed and rotated for peak signal level.

Ferrite rod antenna (top left of photo) with LC parallel resonance tuning, has been used in medium wave radio (0.5 to 1.6MHz) since many years ago. Is that still the best choice nowadays, for radio and in my 1MHz application?

enter image description here

As receiver (small current), at 1 to 4MHz, what is the difference, in term of power derived from 3 dΒμΑ/m H field strength, between proper Litz wire and solid wire coil?

Litz wire

solid wire

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A ferrite rod does this: -

enter image description here

Generally, a ferrite concentrates the magnetic field that would otherwise have passed by the receive coil not yielding a signal. In effect it multiplies the cross sectional area of the receive coil several times. The effective flux density seen by the receive coil is significantly increased and it is a good ploy to use one in a receiver of around 1MHz. Ferrites are good for significantly higher frequencies too but their permeabilities reduce to avoid losses and their effectiveness at concentrating lines of flux are therefore also reduced.

Litz wire overcomes the problem of skin-effect that is a phenomena seen as frequency rises. If you are not taking significant power from the receive coil then it makes hardly any difference. Litz only comes into its own when current is drawn from the coil.

As an example of personally using litz wire, I had to induce a large current into a 1.5 metre diameter coil from a relatively small transmit coil (100 mm). I needed to pass 3 watts to the larger receive coil at a gap of about 40mm. The frequency was 600 kHz and the current in the transmit coil was about 15 A RMS (due to a tuned circuit) but in the end I had to go for litz wire on the transmit coil to reduce losses sufficiently to get the power across the gap.

Regards your example - it's in a tuned circuit and it is trying to maximize received signal (and hence Q) by using Litz wire. It's worth considering for your application but I doubt that conventional tags use litz wire because of the expense.

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  • \$\begingroup\$ Many thanks for great answer. In real-life medium wave radio (0.5 to 1.6MHz), ferrite core are either round circular or rectangular. Any idea how they differ in performance and why one is chosen over the other in a specific design (apparently, both use in real life machines)? \$\endgroup\$ – EEd Sep 6 '14 at 15:56
  • \$\begingroup\$ Given a H field strength of 3 dΒμΑ/m, 10 meters from transmitter, 1MHz, what will be the received signal, in micro volts in 1 kilo ohms (radio receiver chip input spec.), from air core and ferrite core? How much different is resonant and non resonant (may need to use just coil for multiple widely separated frequency)? Will the difference, likely, 30 to 100 times, as 'similar' or 'related' to Q (quality) factor of the LC resonance? \$\endgroup\$ – EEd Sep 6 '14 at 16:07
  • \$\begingroup\$ I don't have anything like exact answers but I will say that if the field strength is "X" at the receive coil, it's irrelevant how far from the transmitter it is - my previous answer detailed how you can calculate the signal without ferrite (and tuning) but with ferrite it's beyond my learning to simply pull a figure out - I think it's one of those scenarios where a little experiment may be useful. \$\endgroup\$ – Andy aka Sep 6 '14 at 17:09

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