Crystal Radio Detectors

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

Detector as Rectifier

A detector may be viewed as a rectifier. FIXME

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.

Detector as Inductive Converter

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

   AC In                       AC+DC Out
        |  ::  |      |  :::: 
        |      |      |
        +--||--+      +---||---+
               |               |
              ---             --- 
               -               -

Detector as Mixer

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

Detector as Demodulator

The detector also functions as a demodulator.

Detector Characteristics

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 Is4)
Package Part TypeIF(AV)VRRated VF*VF@20mA,25ºCMeasured VFCT
(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
DO-7 Double-black Band 1N60 “UNIZON” Ge 0.050 20 1.420 1.130 0.303 1
Opaque Black 1N60-odd Ge 0.050 20 1.420 1.130 0.291 1
DO-35 Red 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
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 @IF(AV), TJ=125ºC
  • †Measured with cheap multimeter, probably at about 1.15 mA
  • ‡Observed to be similar to 1N34A.

Current vs. Voltage (I/V) Curves of Tested Diodes

Detector Modeling

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.

1) In fact, ideally the peak is pi times the average. overline{I}={int{0}{pi}{I sin(t)}dt}/{2 pi} = -I{{{cos(pi)}-cos(0)}/{2 pi}}  = -I{{(-1-1)}/{2 pi}} = I/{pi} right I = pi overline{I}
2) The most sensitive detector I have tested is the “UNIZON” germanium point-contact 1N60, available in quantity from many sources as of 2007-12. It is easily identified by its glass DO-7 package with two wide black bands.

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