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From what I've learnt so far, the longer the wave frequency on which you want to transmit or receive, the longer the antenna needed to do so. However my portable receiver other than SW can also receive LW and MW, and is receiving those using its internal (very short!) antenna.

How is that?

How is that even possible that the short internal antenna does its work better than the much longer telescopic antenna that the radio uses for SW? (my radio automatically switches to the internal when on LW MW...)

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A ferrite stick antenna is a kind of small loop antenna, "small" here meaning small retaliative to wavelength. These kinds of antennas can most easily be understood by electromagnetic induction:

  • The loop acts as a "net" for magnetic flux.
  • As the magnetic flux changes, an electromotive force (EMF) is induced in the loop.
  • This force pushes the electric charges in the loop around, making a current through the loop and a voltage across the feedpoint which the receiver can detect.

This is the same operating principle behind an electric generator. However, in the case of a loop antenna the changing magnetic field comes not from a nearby moving magnet, but instead from a distant radio transmitter.

Electromagnetic induction is formally described by Faraday's law of induction. It says EMF ($\mathcal{E}$) is proportional to the rate of change of magnetic flux ($\Phi_B$).

$$ \mathcal{E} = -{{\mathrm d\Phi_B} \over \mathrm dt} $$

There are several ways we might increase the magnetic flux through the antenna, thus increasing the EMF and making it easier to build an efficient antenna.

We could make the magnetic field strength higher, by increasing the power of the transmitter or getting closer to it.

Or, we can make the loop bigger. A bigger loop captures more flux, like a bigger net.

We can also add more turns around the loop. It doesn't matter that both turns are capturing the same area of space. The EMF in each turn adds together, so an antenna with two turns captures twice the magnetic flux as an antenna with one turn of equal diameter. A ferrite stick antenna might have 60 turns, so it captures 60 times the magnetic flux as one turn would.

The last thing we can do is to stick a ferrite rod in the loop. The ferrite rod has some interesting magnetic properties that make it work.

All ordinary matter has some magnetic properties. For example, every electron is like a tiny bar magnet, or a magnetic dipole. In most matter, these magnetic dipoles are arranged in random directions, and so at macroscopic scales there are no interesting magnetic properties.

In ferromagnetic materials some of these magnetic dipoles can be reoriented. For example, if a piece of iron is placed in a strong magnetic field, then its magnetic dipoles tend to align with that field, just like a compass aligns with the Earth's magnetic field. The ability of the magnetic dipoles to align is called permeability.

When the external field is removed, some of the magnetic dipoles tend to "stick" in their orientation, and you are left with a permanent magnet. This tendency for the magnetic dipoles to stick is called coercivity.

The interesting property of the ferrite material used in a ferrite antenna is that it has low coercivity and high permeability. So their magnetic dipoles are good at aligning with an external field, but they tend not to get stuck, like a compass needle.

So when you stick a ferrite rod in the loop, whichever way the magnetic field from the transmitter is going (remember it's oscillating with the transmitter's frequency), the magnetic dipoles in the ferrite are aligned in the same direction. The fields of the magnetic dipoles add to the external field, and the result is that the magnetic flux through the loop is stronger.

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  • $\begingroup$ Then why my telescopic antenna is not a telescopic ferrite loop antenna with 10000 turns (when all extracted? $\endgroup$ – jj_ Mar 13 '16 at 18:20
  • $\begingroup$ @jj_ The short answer is it would be less efficient. 10000 turns of wire would be a very long, thin wire, with significant resistive losses. Also, ferrite stick antennas only work at low frequencies since hysteresis losses in the ferrite increase with frequency. If you want the long answer, it would make a good new question. $\endgroup$ – Phil Frost - W8II Mar 14 '16 at 1:04
  • $\begingroup$ Ok, but just to be sure I fully understood your comment please allow me to be a bit pedantic: in my example 10000 was just some arbitrary number I used to describe the covering of the entire telescopic antenna with wire turns. What I really wanted to say with 10000 turns is the right amount of turns necessary to cover it. So would it still be less efficient that way? $\endgroup$ – jj_ Mar 14 '16 at 7:33
  • $\begingroup$ @jj_ I'm not sure what good covering the telescopic antenna in wire would do. But yes, small wires, and longer wires, have more resistance. Resistance converts current into heat. If it's making heat, then that must be less electrical energy making it to the receiver. $\endgroup$ – Phil Frost - W8II Mar 14 '16 at 12:02
  • $\begingroup$ @PhilFrost, Would it be correct to say that the loop antenna receives the magnetic portion of the electromagnetic wave, and that a typical dipole or telescopic antenna would receive the electric portion of the electromagnetic wave from the transmitter? I always enjoy your answers since they seem very much like a university professor's explanations. $\endgroup$ – Craig K Mar 14 '16 at 16:18
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Electricaly short antennas seem to work fine on RX at these low frequencies .The humble AM car radio is case and point .Sure the little antenna produces less signal but it also produces less noise in proportion.So the Carrier to noise ratio coming in to the radio is roughly the same for both cases.Any low frequency RX that is worth its salt will have enough gain to compensate for this and its internal noise is much less than atmospheric noise .This is why increasing the length of the antenna often does not give a observable reception improvement .Things will actually get worse on some cheap solid state recievers because of poor overload performance.

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  • $\begingroup$ "Electricaly short antennas seem to work fine on RX at these low frequencies ." that's what I said in my question, I asked how. $\endgroup$ – jj_ Mar 13 '16 at 6:20
  • $\begingroup$ Well signal voltage is down due to extremely low radiation resistance .This low resistance is inconveniant to match efficiently.So less signal and less atmospheric noise. $\endgroup$ – Autistic Mar 13 '16 at 6:41
  • $\begingroup$ AM antennas typically have coils of wire that have a near wavelength amount of wire in them, such as was designed at ccrane.com/!ZWZH0veEn1U6mfPDHx13MQ!/… $\endgroup$ – PearsonArtPhoto Mar 13 '16 at 12:56
  • $\begingroup$ The "7 turns of wire to make a coil" described in the linked article, are no way near to a wavelength in MW and LW! $\endgroup$ – jj_ Mar 13 '16 at 18:26
  • $\begingroup$ @PearsonArtPhoto What makes coils of wire work has nothing to do with the wire being a wavelength long. Consider that all of the turns are magnetically coupled by virtue of sharing the same (possibly air) core. This high degree of coupling means the current in each turn is essentially equal. This is true however many turns there are, and whether or not the total length of the wire is related to wavelength in any way. $\endgroup$ – Phil Frost - W8II Mar 14 '16 at 1:26
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Antenna efficiency is far more important for transmitting than for receiving. If the small AM antenna in your transistor radio produces only one thousandth the signal that a 300-foot vertical with 160 radials does, the deficiency is easily made up by the amplifier. It's nowhere near as easy to increase the power of a transmitter by a thousand times, which is why great care is taken to make AM broadcast antennas as efficient as possible.

The internal AM antenna in your radio works better than the telescoping stick antenna because the internal antenna is either a wire loop with many turns, or a ferrite stick with many turns of wire around it. The many turns of the internal antenna 'amplify' (in a manner of speaking) the received signal.

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  • $\begingroup$ "The many turns of the internal antenna 'amplify' (in a manner of speaking) the received signal.".. That's a bit too vague. $\endgroup$ – jj_ Mar 13 '16 at 6:24
  • $\begingroup$ Whenever magnetic flux passes through a coil of wire, a voltage is generated. (An RF signal has both a magnetic field and an electric field, but this kind of antenna works with the magnetic field component.) For a small wire loop antenna, if the wire loop has 100 turns then it will pick up 100 times the signal that it would if it had only a single turn. $\endgroup$ – rclocher3 Mar 14 '16 at 17:49
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How is that even possible that the short internal antenna does its work better than the much longer telescopic antenna that the radio uses for SW?

let me try to answer it differently: the "short internal" antenna, even when the summary length of the coils do not even approach the length needed of a stretched dipole, is efficient due to "L/C resonance"

for every frequency you can find (multiple) coil/capacitor combinations which will be resonant for that frequency. And will work "better" as an antenna compared to a small telescopic antenna for receive.

This is also why certain HAMs which want to Tx on such bands are using "magnetic loop antennas" which have the same principle, it is a resonant L/C circuit.

HTH, Edwin

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