The detectors used for crystal radio sets include many rare, unusual, old, and even homemade devices. Most common are the few germanium diodes still in production and the remaining stock of old germanium devices, though some modern Schottky rectifiers and low-threshold MOSFETs are gaining in popularity. In addition, the traditional catswhisker mineral detector still has its uses, and other even stranger detectors can be pressed into service to demodulate radiofrequency signals.

• Cat's-whisker and Perikon Mineral Detectors
• Germanium Diode
  • Tunnel and Back (Backward) Diode
• Thin-film Metal Oxide (Razor Blade)
• Low-threshold MOSFET
• Zero-bias FET
• Schottky Rectifier
• Light-emitting Diode (LED)
• Electrolytic Aluminum Rectifier

A detector may be viewed as a rectifier.

A more selective detector will have higher impedance at the lowest current sufficient to power your headphones; a line on an I/V graph will have a shallow slope near the origin.

Now, I measured 30 µA (average) being drawn from a strong local station on a mediocre antenna. The peak current of a sine wave through an ideal rectifier is about three times the average.¹⁾ Since detectors are less than ideal and enough current must remain in the tank to keep sufficient voltage oscillating, 100-200 µA seems a reasonable upper limit to the peak current through the detector. (With a better antenna, the maximum average current is as much as 150 µA at an average voltage of 0.4 V for 60 µW using a modern germanium 1N60.²⁾ With a vintage black-band 1N34A (or a “mystery” yellow-band diode), 100 µA at 0.6V is still 60 µW at higher impedance.)

The shape of the curve will also effect the overtones produced by the detector when it demodulates the signal, but a sharp cutoff or a flat curve is not necessarily the best shape to cleanly demodulate the signal. If the I/V curve is flatter at lower voltage (concave upward), the resulting RF pulse will be more rounded off by the higher resistance at the beginning and end of the rectified half-wave being shaved off of the top of the original signal. This will reduce the high-frequency overtones produced. However, if the curve is too sharp the leading and trailing edges can be rounded too much, the half-wave become attenuated until it begins to resemble an impulse train, and overtones may even dominate the signal.

Another useful way of looking at the detector is similar to the function of a charge pump or a resonant transformer.

• Resonant Mode Converter Topologies

   AC In                       AC+DC Out
   (~)--+--UU--+-->|--+--UUUU--o
        |  ::  |      |  :::: 
        |      |      |
        +--||--+      +---||---+
               |               |
              ---             --- 
               -               -

One other way to consider a detector is as a radiofrequency mixer.

• Electronic mixer
• Frequency mixer

The detector also functions as a demodulator.

• Envelope detector
• Product detector

Even for modern devices, the data relevant to their use in unpowered radio sets is sparse and scattered. Perhaps the information provided here will help others in their pursuit of this rewarding crystal radio hobby.

Hypotheses to test:

• Is a detector's selectivity related to its average impedance to a typical crystal radio signal? Can this be predicted from the Shockley equation?³⁾
• Is a detector's sensitivity related to it's saturation current I_s⁴⁾

Package	Part	Type	IF(AV)	VR	Rated VF*	VF@20mA,25ºC	Measured VF†	CT
			(A)	(V)	(V)	(V)	(V)	(pF)
DO-7 Green Band	1N34A	Ge	0.050	75	>1.00	>1.00	0.382	0.8
DO-7 Red Band	1N34A	Ge	0.050	75	>1.00	>1.00	0.360	0.8
DO-7 Radio Shack	1N34A	Ge
DO-7 Black Band	1N34A	Ge	0.050	75	>1.00	>1.00	0.305	0.8
DO-7 Yellow Band	“Mystery”	Ge	–		–	–	0.293	–
DO-35 Red	1N34A	Schottky					0.307
	1N60 “UNIZON”	Ge	0.050	20	1.420	1.130	0.303	1
	1N60-odd	Ge	0.050	20	1.420	1.130	0.291	1
	1N60P	Schottky	0.500	45	0.840	0.340	0.279	2-6
DO-35 Orange with Green Band	1N60	Schottky	0.030	40	0.320 @1mA	–		2
DO-7 Gray Band	FO-215	Ge					0.276
DO-7 Brown, White, Red, Blue Bands	1N192	Ge					0.350
DO-7 Black Band	1N198	Ge	0.500	100	1.00	>1.00	0.278	low‡
DO-7 Black Band	1N277	Ge	0.500	110	1.00		0.335
DO-7 Opaque Gray	1N277B	Ge					0.310
DO-7 Black Band	1N270	Ge	0.325	80	1.00	<0.5	0.285
Large Glass Red and Green Bands	1N452	Ge					0.450
DO-7 Double-black Band	“Unknown”	Ge	–		–	–	0.364	–
DO-7 Opaque White, Black Band	OA47						0.320
	OA90
	OA91
	OA95
	AAZ15
DO-35 Red	BAT45	Schottky	0.030	30	1	0.320	0.263	1.1
DO-35 Blue	BAT46	Schottky	0.150	100	0.800	0.510	0.254	6-10
	1N5817	Schottky	1	20	0.450	0.230	0.165	125
	1N5819	Schottky	1	40	0.450	0.230	0.205	110
	21DQ06	Schottky	2	60	0.550	<0.320	0.298	120
	MBR1100	Schottky	1	100	0.680	<0.420	0.299	35
DO-17 Gold	BD-3	Ge Back	-

• *Usually @I_F(AV), T_J=125ºC
• †Measured with cheap multimeter, probably at about 1.15 mA
• ‡Observed to be similar to 1N34A.

The purpose of collecting this data is to learn how to build a better crystal radio. To this end some mathematical analysis may be helpful.