100mm Long 9.5mm Diameter Ferrite Rod

This is a 10cm long, 9.5mm diameter ferrite rod of standard MW LW permeability grade.  Originally, these were all specified as being 3/8 of an inch, hence the odd 9.5mm metric measurement.  The ferrite material is magnetic, but unlike solid iron, it has a low electrical conductivity.  The permeability factor of the material magnifies the magnetic field within the windings.  This means that you can use fewer turns to get a certain inductance, and fewer turns means less signal loss because a smaller amount of copper wire is used.  It also allows you to make the coil smaller, so that the finished set is portable.  There is some small loss of signal within the ferrite material, but the reduction in wire resistance more than compensates for this.

The 30 turn coil is connected to aerial and earth.  The 50 turn coil is connected into the parallel variable capacitor and the rest of the circuit.  The combination of the inductance in the 50 turn coil and the variable capacitor is called a resonant tank.  You can imagine it to be somewhat like a swinging pendulum.  Separating the aerial and earth from the main tank allows you to couple it to the tank less by moving it away on the rod.  If you have a good aerial, this can help you to separate strong stations on adjacent frequencies.  If your aerial is indoors, or not well separated from various earthed structures, the capacitance between your aerial and earth could have a damping effect on the resonant tank if it were to be connected directly to it.  The separate aerial coupling coil used here helps to avoid that.  It is essential in many cases.

The number of turns on each coil does not have to be exact. 

More turns on the main tank will increase the inductance and tune lower MW frequencies, as will moving it towards the center of the rod, and vice-versa.

If you want to try to get that last bit of sensitivity, you could try to improve the quality factor, or "Q" of the main tank coil.  One way to do that is to separate the turns by about one wire width.  This decreases the capacitance between each turn and make a more pure inductance.  I've not found an easy way to do that, but you might.  The coils here are shown with the fixing tape only at the very ends, not over the coils themselves.  This is deliberate.  You want to avoid putting damp, gluey material over the coils as this can reduce the Q. 

Q:  Can I used a piece of metal pipe instead of the paper formers?
A:  There's no such thing as a stupid question.  No, the metal will completely short out any signal by forming a very short circuit single turn.  Try it by sliding some copper tube over the end of the rod on a working kit.

Q:  Can I use some cut-up PTFE drinks bottle plastic instead of the paper formers?
A:  Yes.  Anything that is completely dry and non-conductive.  Watch for sharp edges on cut plastic. 

Plastic hosepipe, thin overflow pipe, or anything else like that which is not too larger diameter than the ferrite rod will do.

Q:  What is permeability?
A:  Usually expressed as permeability relative to using no core at all other than fresh air (actually called an air-cored coil), it is the magnetic multiplying factor of the ferrite material.

Q:  what about LW (Longwave) reception in Europe, and SW (Shortwave) reception elsewhere?
A:  About 200 turns will tune you LW, though the wire supplied is less effective on LW.  You'd like to use fancy "Litz" wire.  If you're near Droitwich in the UK, 200 turns will work for Radio 4 on 198 kHz.  The ferrite core is less effective at SW frequencies.  I'll discuss this more thoroughly elsewhere.

Navigate Up

My primary email which is checked daily including spam folders is: