Some thoughts on UHF Crystal Radios

Diode capacitance

Low capacitance diodes are a must for UHF crystal radios.

It follows that they are to be so wired that the capacitance between their leads is kept to a minimum.

This does not imply that the leads are to be cut to minimum length. In fact longer leads facilitate antenna-diode impedance matching.

Antenna-diode impedance matching

Stub matching is a must for maximizing the signal strength.

Dipole with a single diode

Long diode leads may be used to form the stub.

Dipole with coil and diode

A diode length of λ/4 gives a proper match.

Folded dipole

A circular stub is best for minimum capacitance between the leads.

Dipole with single-diode circular stub

Related post: UHF Crystal Radio
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Motor problem

It was just another day at the plant.

A colleague from the neighbouring department called to say he had a problem.A replacement three phase induction motor, drawn from the stores and wired, just refused to start.

Our exchange :-

Voltage okay? 
Yes, all 3 phases terminal voltage 415V okay.

Maybe an overload or single-phasing. Does the overload relay trip?
No.

In that case it has no winding!
You crazy?!

No!
Okay, I'll check.

A little later the telephone rang again.

How did you guess?!

It turned out that someone in the motor rewinding department had sent an unwound motor back to the stores instead of a rewound one!
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