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
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
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.
© Henry J Walmsley 2012