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For reception in the medium wave range (500-1600kHz) a ferrite rod (loopstick) has a measured inductance of Lc=2mH and a resistance Rf=5 ohms, whereas an RF choke with Lf=2mH has a resistance Rc=8 ohms.

Given that a choke is much smaller than a ferrite rod, why do medium wave (AM) radios use a ferrite rod rather than a choke to tune into radio stations?

Context: I am using a choke (4mH) and a capacitor (2.2nF) in parallel in a tank circuit with a resonant frequency of 50kHz, and I am wondering if I should be using a loopstick instead of a choke. I am using a choke because I need to minimise weight.

EDIT1: The following text and image were added for information after answers were received. From the comments below, it seems to matter that the choke has ferrite caps. Not sure if this is relevant, but I am trying to receive (not transmit) RF signals.

enter image description here Figure 1. Picture of a (very) spare choke, with the plastic coating removed (I managed to break the wires inside in the process of removing the plastic). The material at both ends (caps) of the coiled wire is magnetic (ferrite?).

EDIT2: This edit has also been included as a new question, so please put any responses there.

The circuit in Figure 2 was used to test predictions made in answers to this question.

The inductor was either a) a choke with Lc=0.54mH (R=3.2ohms) (as in the figure 1), or b) a loopstick with Ll=0.6mH (R=8.3 ohms)) (Figure 3, from a radio clock).

A sinusoidal 1V (p2p) signal from a signal generator was injected across the whole circuit. The frequency was adusted to maximise the voltage across the tank circuit, which corresponds to the resonant frequency, measured as f0=40kHz (for both the choke and the loopstick).

As an aside: The measured p2p amplitude across the tank circuit at f0 was 57mV, so that

57mV/1000mV = Z/(Z+20k),

so, solving for Z yields Z = 1200 ohms at f0.

Finally, I disconnected the scope from the circuit, and used the scope to generate an RF signal at f0 through a long wire antenna. I then measured the amplitude V induced across the tank circuit at a short distance from the antenna.

Result: With the loopstick, Vl = 4mV, whereas with the choke, Vc = 3mV.

This seems to suggest that both the choke and the loospstick are almost equaly good at converting the RF signal into voltage. What am I missing?

enter image description here Figure 2. Circuit used to estimate resonant frequency f0 and impedance Z of tank circuit at resonant frequency f0.

enter image description here Figure 3. Loopstick used to replace choke in tank circuit.

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    \$\begingroup\$ Might be good to wind your own on a core designed for the application. fair-rite.com/product-category/inductive-components/… \$\endgroup\$
    – John Doty
    Feb 12 at 15:43
  • \$\begingroup\$ You may want to ask your latest edit in a separate question; expecting more from existing answers, or new different answers, is veering dangerously close to "changing the question". \$\endgroup\$ Feb 15 at 12:09
  • \$\begingroup\$ James, If we are done here, please take note of this: What should I do when someone answers my question. If you are still confused about something then leave a comment to request further clarification. \$\endgroup\$
    – Andy aka
    Feb 15 at 14:11

2 Answers 2

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why do medium wave (AM) radios use a ferrite rod rather than a choke to tune into radio stations?

For long/medium wave reception what "we" want is a diameter that is as large as is possible. Clearly, space limitations prevents a 1 metre diameter ferrite rod so we compromise and pick a diameter of around 10 mm.

But, we also want a long ferrite rod as per this image below: -

enter image description here

Image taken from this answer and you should be able to see that a bigger diameter rod will attract more flux lines from the prevailing radio wave you are trying to detect.

So, dimensions are important if we want to detect a signal. It's all about concentrating the magnetic flux and inducing as much voltage as we can in the coil wrapped around it.

I am using a choke because I need to minimise weight

You might get away with a choke but smaller is definitely not better and also, the choke needs to be open to external magnetic field influence i.e. it can't be a closed-form core like a toroid or similar. A barrel/drum inductor will work but, if the dimensions are small, the received signal will also be small.

Drum/barrel inductor from here: -

enter image description here

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    \$\begingroup\$ You drawing is misleading: magnetic fields don't quite work that way. The drum core, in particular, makes a poor antenna. The easiest way to see this is with the reciprocity principle: apply a current to the coil and see where the field lines go. To get the best transmission, the field lines should make big loops as far as possible from the coil. By reciprocity, that's also the best for reception. A long rod makes big loops, but a drum core concentrates the field just outside the windings. Good for avoiding crosstalk from a choke, not so good for an antenna. \$\endgroup\$
    – John Doty
    Feb 12 at 15:41
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    \$\begingroup\$ Why do you think reciprocity is invalid? You need something like Faraday rotation to make nonreciprocal electromagnetic systems. That's not present here. \$\endgroup\$
    – John Doty
    Feb 12 at 18:59
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    \$\begingroup\$ Yet big microphones make more signal, just as you'd expect from reciprocity. Small microphones need more amplification. Similarly, a short dipole is a fine receiving antenna for LF, with appropriate amplification. \$\endgroup\$
    – John Doty
    Feb 12 at 20:28
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    \$\begingroup\$ The only thing that shouldn't be reciprocal here, is the application: you would use a loopstick where the poor gain is acceptable for some reason. In this case, because atmospheric noise dominates at such a level that signals must be that much stronger and therefore little receiver gain is required; the converse is not true, a transmit antenna must be high efficiency to radiate such power. But this is not the usual meaning of "reciprocity"; I would use "asymmetry" for this. \$\endgroup\$ Feb 12 at 21:02
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    \$\begingroup\$ @Andyaka Do not attempt to hide expletives with alternate characters. They are not allowed, either spelled correctly or with alternate characters. This will be your only warning. \$\endgroup\$
    – Null
    Feb 12 at 23:42
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That coil serves two purposes: antenna and resonator (the resonator may require an associated capacitor). A choke is a terrible antenna, but may have lighter weight than a loopstick's ferrite.

Andy's answer shows how a ferrite rod collects AC magnetic fields, aiding its purpose as antenna. If you wish to limit weight, use a long rod with small diameter. Such a rod is fragile, because ferrite is brittle.
Stacking rods end-to-end is theoretically possible, but very difficult to do practically, because magnetic coupling from rod-to-rod involves an inevitable air-gap.

The alternative to ferrite rod antenna uses a large-diameter solenoid. The large diameter makes the resulting inductor a fairly efficient antenna with directional characteristics similar to a loopstick. However, with no ferrite, a great length of wire is required, which is also weighty. For example, a 1m diameter coil of 50 turns has about 5mh inductance.

A choke inductor might have 5mh inductance, but is likely designed to confine magnetic field to itself, so that it doesn't radiate - opposite of what you want for an antenna.
Some inductors inside a radio are used with a capacitor as resonators and not as antennas. In this case, they tend to be choke-like, with carefully-confined magnetic fields. Small size is a design objective.

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