CRYSTAL SET DESIGN
1998 - Alan R. Klase
- All rights reserved.
Crystal Set Design 102.
It is assumed that you've already completed the 101 level course
and know, at least, something about electronics in general and crystal
radios in particular. If not, hit the books. The stuff in
library will get you started. The early chapters of
Amateurs Handbook" are especially concise and approachable.
a little disappointed
at the lack of meaningful crystal set technical information on the web
and in current literature. This is my attempt to at least
remedy this situation. This work is the result of about ten
part-time investigation of crystal radios from an engineering
Serious development of passive receivers pretty much came to an end
the introduction of reliable vacuum tubes around 1920. A lot
sets, both commercial and home brew, have been designed in the interim,
but most are mediocre performers. So the mission turned out
one of rediscovering the the secrets of the age when spark was king.
My approach is to build crystal
radios out of quality
vintage radio parts. They are available in great profusion at
radio "hamfests" and antique-radio meets (and in my garage).
don't have access to these sources, I'd suggest you start at Radio
Buy their Crystal AM Radio Kit, 28-177, for $6.99. This is
not a bad crystal set, and it contains a coil, a variable capacitor, a
germanium diode, and, most importantly, a reasonably sensitive
ear phone. You're also going to need an antenna.
means wire up in the air. The attic may be the next logical
Apartment dwellers may be in trouble unless you're near an AM radio
or can arrange a "stealth" antenna of some sort.. Again, if
(yet) have a junk box, get Radio Shack's Outdoor Antenna Kit, 278-758,
for ten bucks. Wire has always been expensive, and hard to
so keep your eyes open for bargains. Another RS item, that's
indispensable, is a set of mini-alligator jumper cables, 278-1156, 10
$3.99. These are how you make temporary "breadboard" hookups
experimenting with new circuits. You'll also want
hand tools and a soldering iron.
|| The simplest
radio you can build is just a
diode detector and a headset. With a reasonable antenna and
you will hear the strongest stations, albeit all a once. This
not much of a radio. but it will give you some indication that you have
enough signal strength to continue experimenting.
The primary problem
with the above set is that it offers no selectivity. We'll
this problem by building a tunable filter. This will
of a coil and a capacitor forming a tuned circuit. Either the
or inductor, or both need to be variable so circuit can be tuned to
stations. The classic values are 250 uH and 365 pF to tune
band. This is the basic crystal set schematic you'll find
everywhere. It works better than just a diode, but has some
short comings that are easily remedied.
THE IMPORTANCE OF
In a crystal set, all the audio
power that arrives
at your ear drum came from the distant transmitter. If the
is hundreds of miles away, the amount of power captured by even a good
antenna is reckoned in nanowatts . At each point in the set
strive to transfer at least a reasonable percentage of the available
to the next stage. Perfect impedance matching is not
don't fret over a 2 to 1 mismatch, but let's eliminate as many of the
to 1 and 100 to 1 mismatches as we can.
|| In the case of the simple set in
the last example, lets
do two things: Tap the antenna input "down" on the
impedances of the antenna and tuned circuit will vary with frequency,
it's a good idea to provide multiple taps and a selector switch or
jumper. As a starting point tap the coil at about 5%, 10%,
50% of the total number of turns from the ground end of the
Secondly, connect the detector to the 50% tap. This does two
both of them beneficial: It provides a better match to the
when it's connected to the usual sort of crystal set headphone that has
an impedance of about 10K ohms, resulting in a louder signal.
also reduces the loading on the tuned circuit, increasing it's Q and
These improvements result in a better
crystal radio. With 50-75 feet of wire up in the air, you
hear daytime 50KW stations out to 40-50 miles, and night-time skywave
will come in form hundreds of miles away. This is essentially
circuit I arrived at for my Cub Scouts a few years back. (See
Den Two Crystal Radio in Crystal Set Projects
published by The
Xtal Set Society) You'll also notice it's almost exactly the
circuit used in the Radio Shack set
Another effective way to match the antenna
to the tuned circuit is to use a variable capacitor as shown in the
A cap in the 300-500 pF range is appropriate. Tune in a
change the coupling and retune and see if there's an
Some sets have the coupling capacitor in the ground lead instead of the
antenna lead. Electrically it's the same thing, but sometimes
more convenient for mounting and grounding and eliminating hand
|| With the previous circuit,
connected to a good sized antenna,
you'll find you can set the tuning capacitor to it's minimum value, or
even remove it completely and still get good performance. To
how this "series tuned" receiver works, I've inserted the equivalent
of the antenna in the drawing. Any Macroni antenna less than
wavelength long appears to be a capacitor in series with a small
known as the radiation resistance of the antenna, and an RF voltage
This is almost always the case as a quarter wavelength, even at the top
of the broadcast band, is 187 feet. Our aim is to make as
current as possible flow from our antenna. When the series
of the antenna and tuning capacitors and the inductor are
the frequency of interest, the inductive and capacitive reactance's
out, leaving only the DC resistance of the inductor. This
in maximum current flow in the tuned circuit, and maximum voltage
to the detector. A receiver of this sort will usually require
inductor to cover the entire broadcast band. In such cases, keeping the
detector connected to the optimal point on the coil presents
|| One classic
variation on the series tuned
receiver, known as the "two-slider tuner", dispenses with the tuning
entirely. Instead a sliding contact on the coil varies the
in the antenna circuit. A second slider connects the detector
the best point on the coil. Total inductance should probably
better part of a millihenry.
|| Yet another,
historically common circuit, connects
the detector in series with the variable inductor. This is
a great idea from the standpoint of impedance matching, but it is
Series tuned sets all have difficulties with short antennas, because
capacity is low.
Single-tuned crystal sets, whether series or parallel tuned,
a lot to be desired in terms of selectivity. Yes, the nose
can be improved by increasing the Q of the tuned circuit, but the skirt
selectivity remains hopelessly broad. The obvious solution is
use more than one tuned circuit. The point of diminishing
is between 2 and 3 circuits for a crystal set due to cumulative losses
and tuning difficulties.
Another way to implement the series
tuned set, that improves the match to the detector, is to connect the
and headset across a second coil in the series circuit. For
band, this coil has a fixed value of approximately 80 uH. The
capacitor is optional, especially if the larger inductor is
|| The classic
solution is the "two circuit"
tuner. The antenna circuit is series tuned by a variable
and an inductor, while the detector circuit is connected to a parallel
LC circuit. The amount of mutual inductance, or coupling,
the primary and secondary circuit is generally made variable.
allows light coupling to be used to obtain the sharpest tuning, while
the coupling increases sensitivity at the expense of
This is essentially the same architecture used to great effect
the communication receivers of the wireless era. If you're
better performance for your crystal set this increased complexity is
There are several way to
coupling in the two-circuit set. The navy style "loose
used a secondary coil that telescoped inside the primary.
schemes include variable taps on the low end of the secondary coil,
coupling between the two coils, or the use of a small variable
common to both circuits.
schemes for two-circuit tuners.
inductance will want to have a
maximum value of about 500 uH to reach down to 530 KHz.
a lesser value in needed to reach the 1600 KHz end of the
I've been building coils with five or six evenly spaced taps.
lets you tune around for an optimum match to the antenna.
Install a DPDT switch to allow
circuit to be operated in a parallel-tuned mode: The input
the primary capicator is grounded, and the antenna attached to the top
of the coil. This allows effective operation with short
If a non-fixed detector is
used in a double-tuned
circuit, it's a good idea to include a buzzer to generate a local
to adjust the detector. The circuit is a low-voltage
a battery, a push-button switch, and a one-or-two-turn link to the
coil. A low voltage relay with it's normally-closed contact
in series with the coil is a good substitute for a buzzer.