Short Wave Coil

[Mike Watterson]

steel is a good conductor

Actually it's very poor, though RF has a skin effect, so a polished and varnished coated surface or silver plating makes a huge difference. The higher the frequency, the thinner the better layer needs to be. The reason for Litz wire is partly to avoid skin effect (many separately insulated strands), it's also used because it's easier to "wave wind", less slippy and more flexible.

Magnetic materials also have a deleterious effect on RF circuits and can increase losses.

"Q" is "quality factor". Parallel resistive leakage in capacitors, series resistance in capacitors or coils, magnetically induced eddy current losses, dielectric losses (anything that heats up in a microwave oven has likely high dielectric loss, and plastic/ceramic/glass that stays cold is low loss) all reduce Q, which is bad. If Q is too high (you deliberately want losses, wider band etc) then a series or parallel resistor is added, depending on circuit.

Thus 4,000 Ohms headset not only gives more level than a 600 Ohms on a crystal radio, it gives sharper tuning. A crystal earpiece may need 10,000 Ohms across it to fix frequency response as well as provide a DC load for the diode / crystal detector. The Valve or ZN414/MK484/TA7641 IC or JFET has sever megaohms input impedance so is little load (High Q) on the tuning coil, whereas an ordinary transistor might be 2,000 to 20,000 Ohms input impedance and thus Q is very low.

This why a valve superhet with only 2 IFTS can perform better than a four IFT transistor set. Also the older IFTs are high Q and the miniature pot core IFTs may be very much lower Q.


Surprisingly despite fashion for it, Gold isn't that good a conductor though better than steel, it's advantage is lack of corrosion, though if there is any dampness it increases corrosion of other nearby metals, especially tin. Very many metals are better than Iron, Steel, Nickel etc. Tin is far better than most steels but so much worse than copper, that only copper power and audio lines should have tin plating, copper for RF should be silver plated, or polished and varnished. Gold plating copper PCBs is bad practice for RF, due to the intermediate plating required. Usually silver is used with a protective coating.


Poorer Q "damps" the response, wider band, less sensitive. Higher Q gives narrower tuning and more signal. High Q is like a frictionless pendulum in a vacuum chamber. Very low Q is like a rusty pivot for the pendulum in an oil bath. A compass needle in air is high Q, takes ages to settle. One in a liquid is "damped", low Q.

Slinky springs are used as MW or LW crystal radio coils because they are cheap, readily available and un-insulated so fixed capacitor and a metal strip as a "tap" can tune. However a copper coil on a glass bottle with the enamel rubbed off with emery paper/ nail board/wet& dry will work better. Or 1.5mm solid copper wire, polished, varnished and then the connection area sanded will also be far better.