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There is an article on wikipedia about loop antennas.

It shows like an AM radio's antenna with the ferrite rod and says this:

This greater conductance channels thousands of times more magnetic power through the rod, and hence through the loop, allowing the physically small antenna to have a larger effective area.

Now if I imagine the coil without the ferrite but I multiply the area by 1000, that will be a huge area, how can it be 1000 times more power?

If I place another radio next to it, will it literally zap half the power from the other radio?

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  • $\begingroup$ I would think maybe 4 times more or something like that. $\endgroup$
    – pgibbons
    Aug 25 at 12:01
  • $\begingroup$ Even with that 1000-fold increase, the power "grabbed" by the antenna is still only a tiny fraction of what's passing through the space in its vicinity. $\endgroup$ Aug 26 at 5:39
  • $\begingroup$ yes, this title isn't great, but the title edited in by @user3486184 was worse, and posed a question that the answers don't answer. $\endgroup$ Sep 10 at 10:58
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Now if I imagine the coil without the ferrite but I multiply the area by 1000, that will be a huge area, how can it be 1000 times more power?

Because, as the article says, the magnetic permeability of the ferrite rod is much higher than that of air, and thus it concentrates the magnetic flux from a large area around the antenna.

enter image description here
[Ferrite Antennas for A.M. Broadcast Receivers, Laurent and Carvalho, 1962]

This has the same effect as making the loop physically bigger (neglecting losses in the ferrite, which aren't large at AM broadcast frequencies).

If I place another radio next to it, will it literally zap half the power from the other radio?

I don't know what "literally zap" means, but yes, any antenna can absorb electromagnetic energy from the space around it, making that energy unavailable for other receivers. It does require that the antenna be terminated in a load that will convert the electromagnetic energy to another form, such as a resistor converts electrical energy to heat.

If you were to optimize for this energy capture, perhaps putting this antenna very close to the transmitting antenna with the objective of capturing all the transmitted power such that nothing else can receive it, you will have made a transformer.

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  • $\begingroup$ If the coil is 1 square inch in size (without core), I pick up 1 square inch worth of energy, let's just say that. If I now install a ferrite core and it says it will increase the energy by 1000, that means 1000 square in of area. I just don't see how it is 1000 times more but by everybody's feedback it sounds like it's legit. If I use a large enough ferrite I should be able to steal the RF of my whole neighborhood lol. $\endgroup$
    – pgibbons
    Sep 5 at 17:52
  • $\begingroup$ @phil frost, If you wrote a book on antenna theory, I would buy it and read it from cover to cover. Are you an author? $\endgroup$ Sep 7 at 5:02
  • $\begingroup$ @pgibbons I suspect no matter how large the ferrite, the antenna can't be made to have an effective aperture much greater than that of a half-wave dipole because at some point the magnetic field lines being "pulled in" are no longer in phase. However, at the relatively low frequencies where this type of antenna is used a half-wave dipole is very large compared to the physical size of a loopstick antenna. $\endgroup$ Sep 7 at 18:17
  • $\begingroup$ @nielsnielsen no, that sounds like a lot of work :D $\endgroup$ Sep 7 at 18:20
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Nearby antenna elements do affect each other. Consider a Yagi-Uda. Pointed in the "wrong" direction, the director and reflector elements will reduce the EM power that the driven element receives from the impinging RF field.

However, a parasitic loop can often increase the power to a nearby small radio antenna, rather than steal (or "zap") power.

The magnetic field concentration caused by a multi-turn loop inductance, whether with a ferrite core or not, concentrates the EM field lines not only inside the loops of the coil, but in the total virtual aperture area or volume nearby. All nearby EM field lines (in the entire neighborhood) will be distorted towards the loop inductor and/or volume of higher relative permeability.

There are inductor coil antenna products that take advantage of this. You merely put a tuned passive loop antenna near your small receiver's internal antenna to increase the received signal level. Here's an example of one by Kaito, a "Tunable Passive AM Radio Loop Antenna", but there are other vendors of similar products: https://www.amazon.com/gp/product/B001KC579Q/

If you are wondering where the "added" power comes from, consider it stolen, not from nearby receive antennas, but from the transmitter's antenna, due to more efficient inductive coupling between the two stations. All the EM field lines will be pulled closer, not just the ones near the receive antenna ferrite core.

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Remember, an antenna does not amplify power. It just gathers it. If it has gain (or equivalently, directivity), it gathers more from a specific direction and less from others. In this case, the core increases the virtual aperture so it gathers energy from a larger volume of space.

The antenna is not 1000 times more powerful. It gathers energy from 1000 times more volume. And that may seem like a lot, but think of it as being 10 times larger in 3 dimensions, and that isn't that much larger. (Actually, it doesn't increase volume evenly in all dimensions, so this is an oversimplification.)

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  • $\begingroup$ Thank you for recognizing that 1000 times more seems like a lot. When you bring in the 3rd dimension and say it's only 10 times more, it makes a lot more sense to me now :) I gave all 3 answers an upvote because they were all good. $\endgroup$
    – pgibbons
    Sep 5 at 17:57

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