Q: What Does The Ferrite Rod Do in the Crystal Set?

A: I was asked recently what the ferrite rod does in the crystal radio kit.  Here's my answer.

1)  To tune mediumwave frequencies you need a certain capacitance and a certain inductance and you need high Q for sharp tuning and low signal loss.  High Q coils need low resistance in the windings.  A higher inductance and a higher capacitance tunes lower frequencies.  In the crystal radio kit the main tuning coil is made of about fifty turns of fairly ordinary enammeled copper wire.  The ferrite has a higher magnetic permeability than air so the inductance of the coil increases with it fitted without adding more turns.  Fewer turns means less wire, less wire means less  resistance, and a higher Q than would otherwise be possible for that inductance.

Yes, there is some also some power loss in the ferrite and different grades are made for different frequencies, but that is definitely one thing that the ferrite rod does for us here.  (This is a simple explanation and I've glossed over many finer points, probably including things that I don't know about)

2)  The ferrite rod links the antenna input coil to the main tuning coil via the magnetic field through the rod more loosely than a direct wire connection would.  It does this without adding any real resistors into the circuit.  Whenever you add a resistor, there is some signal loss which we can't afford in the crystal set.  We need this looser coupling to keep the Q of the main tuning coil high.  The antenna might "look like" about 2K Ohms impedance and will probably be capacitive overall due to forming a capacitor with surrounding buildings, other bits of fence etc.  We want much more than 2K for the main coil tank, more than 100K if possible. 

Let's digress for a mechanical analogy:  If you have a grandfather clock pendulum, you need it to swing back and forth a few inches for it to work properly.  (Take this on trust)  If you wanted to drive it with an electrical actuator you might have trouble finding one which could move more than say 2mm, but that actuator could probably create quite a strong force.  The obvious answer is to link the high force actuator right at the top of the pendulum rod just under the pivot where the movement is only 2mm.  The small distance, strong force a the top creates a low force big movement at the bottom.  The same kind of thing is happening electrically with the looser magnetic link between the lower impedance antenna coil and the main tuning coil.  The antenna is the actuator, the main tuning coil and tuning capacitor is the whole pendulum, the voltage signal on the main tuning coil is like the large movement of the pendulum.

Unless you have an aerial 50 metres long high up in free open space you Have to do this.  As a kid I spent months trying to make standard kits work, finally getting one to work with a slightly different method using a second variable capacitor in line with the aerial.  We can't afford two variable capacitors and this method is better.  Because...

3)  The aerial coupling coil has some inductance.  Most aerials that you can make from fences, existing wires near to the house or an aerial wire a few meters up in the garden will have more capacitance with the surrounding bricks and ground than inductance.  The inductance in the aerial coil partly cancels with the aerial capacitance, and the rest of the tuned circuit which the aerial coil 'sees' reflected back from the tuning capacitor.  The whole system becomes more resonant, better matched, and works better.

4)  If you have a really good antenna, the rod provides a physical means of sliding the aerial coil away from the tuning coil to reduce the coupling and hence get finer tuning, and sometimes even some more loudness.

5)  You can theoretically achieve an impedance 'match' by connecting the aerial directly to a point nearer the earth end of the tuning coil.  I've tried this, and yes it works theoretically, but you often get strong shotwave signals overpowering the MW signal that you are trying to listen to after dark.  The inductance of the aerial coil here works against that tendency.  If you want a SW crystal set, there are designs elsewhere.

6)  Using the ferrite rod allows you to tune to higher frequency stations by moving the main coil towards one end, and lower frequency stations by putting the main tuning coil in the centre;  It's an auxiliary main tuning control to get wider band coverage than our fairly small value tuning capacitor can manage.

Some further asides:

X)  In transistorised AM radios the ferrite rod has another function.  It increases the effective area of the loop formed by the tuning coil by a few times and you can find diagrams showing that on the web. This increases the ability of the coil to pick up the magnetic part of MW radio signals without winding a coil which is, say, a few inches in diameter, and without adding an aerial wire which no-one wants on a portable radio.  You can sometimes find those bigger coils in older valve radios before ferrite and the higher gain from transistors became available.  In this crystal set the magnetic pickup direct into the rod is tiny compared to the external aerial wire and plays no significant part.  I might try to do an experiment with iron filings to demonstrate this properly. 

Y)  Can I use a ferrite rod to make a smaller than usual, low power MW transmitter antenna? 

No, not a proper one.  The rod is a fair pick-up for the magnetic part of electromagnetic waves, and only the magnetic part.  You can make an induction field RF transmitter with a ferrite rod or larger loop aerial, but it only generates a mostly magnetic field, not an electromagnetic wave.  Magnetic fields on their own die off quickly with distance and any pickup on an external set will always reflect back into the transmitting coil in some small way;  That's the laws of induction, even if it is happening at radio frequencies.

So that about wraps it up for ferrite rods in this particular crystal set and a bit more.  The question of antenna matching, in fact the whole thing can be treated mathematically, but that's a fair summary for why this crystal set design uses a ferrite rod.  If you have a secret stock of fancy Litz wire and a nine-inch spider-wound multi-tapped high-Q coil that's great, but you probably won't want to carry it up a mountain.

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Henry J Walmsley 2012