Magnetic Fields – G3JKX

Magnetic Fields

Moving a wire in a magnetic field causes a current to flow in it. Keeping the wire still and moving the magnetic field has the same effect. Your receiving aerial does just this, converting moving alternating magnetic fields into alternating currents for your Rx to sort out. Giving a wire some current also causes a magnetic field to be produced. Your TX PA is an electron pump and, as your aerial is conductive, it also produces magnetic fields. Applying a voltage to a coil of wire produces a strong magnetic field. As this field builds, it induces another voltage into the coil, called the ‘back EMF’, which opposes the applied voltage. This means that the current flow starts off quite small and takes some time to reach a maximum. In other words a coil has inertia, which is called Inductance, the unit of which is the Henry or H for short! (For RF frequencies, a Henry is too large a unit. e.g at VHF, coils of a fraction of a microHenry are used). If you now switch off the applied voltage, the resulting collapse of the magnetic field causes another back EMF to be induced which tries to prop up the rapidly falling applied voltage and reinforces the falling current! These ‘back EMFs’ can be very high, especially if the magnetic field is very strong and can cause quite high reverse or overswing voltages to be generated. To prevent a relay coil from doing so when it is de-energised, a diode is wired across the coil to short it out when the reverse/overswing polarity appears, safeguarding nearby circuits which may be damaged if, say, a large negative voltage appears on the collector of an NPN transistor.

A coil and capacitance in parallel presents a high impedance (Z) to a signal at the resonant frequency. If wired in series, the Z is low, accepting or passing the wanted frequency) Remember, Z is a mixture of R and reactance. The efficiency or ‘Q’ must be kept high by having a very low RF resistance. This means using wire of sufficient diameter or otherwise the RF currents will be lower than they could be, so the signal passed on to the next stage will be low too. As we go higher in frequency another problems appear. There, the RF tends to travel on the outside of the wire. We get around this by silver plating the wire and/or increasing the wire diameter even more, increasing the surface area and thus improving performance. Some VHF/UHF coils use silver plated tubing. Yes, the plating oxidises in air, but fortunately the oxide conducts almost as well as solid silver! The silver plating liquid you can buy is quite effective, even at HF. Try it on your tin-plated ATU coils. Big RF currents flow in here so keeping the resistances down is important. So, while you have the lid off, check all the soldered joints and the mechanical chassis connections for tightness and corrosion too. A good idea ? You bet your sweet life it is ! VLF coils, some needing thousands of turns, have to use Litz wire to keep the R low. What is Litz wire? Imagine a bunch of very thin varnished copper wires, all twisted together and soldered together at each end. All the wires in parallel will result in a very low overall resistance but have a large overall surface area, keeping the Q up. Finally there’s the subject of L to C ratio. Oscillators tend to use a low LC ratio , i.e a small L and a big C, so that circuit capacitances, including strays and those of the active device,
(transistor) are swamped out. i.e any changes in C have less effect, so the oscillator frequency doesn’t drift so much. Most RF circuits in your Tx & Rx have high LC ratio to keep the losses low and the signal up!