PASSIVE BROADCAST BAND RECEIVERS
(CRYSTAL RADIOS)
Comments, Facts, Suggestions, and Opinions
Modern crystal radio sets can outperform some superhets, either in tone quality or sensitivity, or both!
High Performance: Attention to everything matters; there is no room for sloppy work when you build a
passive set!
Antennas: The ones that work the best have a vertical component. An Inverted L is a good choice, but
a Tee (top-loaded vertical) may be better. An 80 Meter dipole used as a top-loaded antenna will work as
a Tee with the center conductor and shield tied together (be sure to un-ground the shield). Do NOT
forget lightening protection!
Ground: Since the ground system makes up the other half of the antenna system, careful attention is
needed. Just a ground rod alone will give mediocre results for DX. Good grounds include radials and
multiple ground rods. Bonding to metal cold water pipes will help. Tying all these together will move you
toward an effective system.
Tuned Circuits: A good set requires two sets of tuned circuits (front-end and detector) for improved
selectivity. A DX set will be double-tuned plus have an antenna matching circuit. Also, in areas with
strong local stations one, or more, wave traps are needed to notch out interference. Wave traps are also
needed for notching strong DX signals when they interfere with weaker signals.
Variable Capacitors: Use well-built units, preferably with fingers that provide electrical connection for
the rotor to the frame. High Q is the order here. Generally ceramic or glass insulation provides higher Q
than phenolic. Silver-plated plates tend to increase the Q, also.
Coils: Air core and ferrite core coils are both used successfully. Generally, the largest gauge wire that
will provide the needed number of turns is used to increase Q and reduce ohmic losses for both types of
coils. Litz wire that has been designed specifically for the AM Broadcast Band is very effective in building
higher Q coils with low distributed capacitance. Most ferrite core coils use toroids, although ferrite bars
have been used with good results. Amidon #61 mix is preferred. Useful sizes are: FT82-61, FT114-61,
FT114A-61, and FT240-61.
Detector: A good set requires a sensitive detector, either a good homemade galena detector or a good
germanium diode. Germanium diodes are standard because they turn on between 0.2 and 0.3 volts. Hot
carrier (Schottky diodes) have been used successfully by some, especially with applied DC bias. Silicon
diodes require about 0.7 volts to turn on and are useful only if biased.
Headphones and Transformers: Generally, the higher the impedance, the better to reduce the
impedance mis-match and loading of the detector. Crystal earphones are used by many for Hobby Class
sets. Magnetic earphones with a matching transformer can work quite well. Some have had good
success with homemade piezo electric headsets. However, the most sensitive type of headphones is the
balanced armature designs, especially those used in military sound-powered (SP) phones. SP phones
usually have impedances ranging from 600 ohms to 1200 ohms per element. Also, AT&T/Western
Electric handset elements beyond 20 years ago make good elements for headphones. Regardless of the
type of headphones used, an efficient audio transformer is need for impedance matching the diode to the
headphones for best performance. The T3/AM20 (100 ohms to 100K ohms) found in many old military
sets works well. The Bogen T725 PA transformer also works well and provides several taps for matching
versatility.
Other Components: Reduction drives (verniers) are needed for precise tuning, especially for wave
traps. The old Velvet Verniers are a favorite of DX set builders.
Set Design: A modified band-pass approach is used by many. This design approach may be used for sets using
either air core or ferrite core coils.