FM Crystal Radios?
copyright © 2006 by
Larry J Solomon
I have heard, even from a physicist, that it is impossible
to build FM crystal radios. On the other hand some experimenters
claim that they have built them. This argument intrigued me to try and
build an FM crystal radio, which I have done successfully. To
my surprise, the result is an astounding performer, pulling in four
local stations in Tucson. When connected as a receiver to a good sound
system the sound fidelity is as good or better than more expensive AM
radios. In fact, it sounds "high-fidelity".
This picture shows the Solomon FM Crystal Set in an acrylic display
case. I made the set specifically to fit inside this case (the case
came first).
My definition of a crystal radio is one that
is not powered, except by the radio transmission itself and employs a
crystal detector. So, it cannot have any batteries or AC power. An FM
crystal radio must
be able to detect and receive FM signals well enough to be heard in
earphones without any such extra power.
The circuit looks identical
to a classic AM crystal circuit but is even simpler to build. The
components were reduced in dimension to resonate at higher frequencies.
This was done by experimenting with smaller and smaller coils and
capacitors. The antenna is also much reduced in size (from that of
AM)
to resonate at higher frequencies. The air variable capacitor I used
has two trimmers in it which should be adjusted for best
reception.I have found that a
commonly available vernier dial and knob will fit the capacitor nicely.
See end of article for a picture of the variable.
C3 is a ceramic capacitor of 18 pf, but may be anywhere
from 10 to 50pf. A detected FM signal is converted to AM due to an
effect called slope detection that modulates amplitude.
This FM Crystal Set works best near the transmitter (I have not
tested it beyond about 10
miles). Secondly, the sound level is not very loud; a quiet room
is
best for listening.One must be willing to move the set around to find a
location for the best reception of signals. However, in addition to
listening with high impedance earphones (crystal or otherwise), the set
can be connected directly to an audio amplifier's
low level magnetic input which can then play amplified through a sound
system at
any volume -- sounds GREAT. No additional wiring or antenna is
necessary (the antenna is optimized in length for FM.
L - 4 turns #18 copper or silver wire, 12mm
inside diameter, tapped at 2.5 turns
Ant - 7 inches of #18 bare copper wire
C1 - 18 pf ceramic capacitor
C2 - 50 pf air variable capacitor
D - 1N34 diode or rock crystal
R - 150K resistor
I consider this set a work of art. The diode is tapped
directly to the antenna. The vernier dial fits directly on the tuning
capacitor. The antenna parallels the perimeter of the acrylic face
plate. "Military style" #18 AWG wiring is used without any
insulation. It is important to keep the components physically close
together. The component specifications are the same as in circuit #2.
The coil is silver rather than copper, but copper will
do just as well. The coil was wrapped
around a "magic marker", then slipped off and expanded. The
wooden base is made from lacquered,
polyurethane padouk.
I consider this to be the most elegant crystal set I have created. Yet
the circuit is so ridiculously simple that some will not believe it is
possible without building it themselves. No shielding
is necessary, and there is no
problem with hand capacitance. However, the output cable position may
affect reception sensitivity.
Photos of wired circuit
A hand is included in this photograph to show scale. Note the military
style wiring, diode, and antenna. I wanted the wiring to create a
modern design similar to a Mondrian painting.
This shot shows the elegance of the FM set best, I think. There is only
one
resistor and one fixed capacitor.
The inside of the tuning capacitor and the phono jack/output can be
seen here. Can you spot the ceramic fixed
capacitor here? Note the polished edge of the
face plate.
A quarter-inch piece of lucite was fitted under the tuning capacitor to
anchor it. Note
the two tiny trimmers on the back of the tuning capacitor.
The vernier dial is large to accomodate ease of tuning, and the
vernier
makes it especially easy to separate stations. Two golden (brass)
wood screws fix the face plate to the base. Holes for the face
plate were made with special plastic drills, but ordinary drills may be
used if drilled very SLOWLY. The knob is
removable.
FM Crystal Circuit #2
L - 5 turns AWG#18 bare copper or silver wire, 12mm inside diameter,
tapped at 2.5 turns
D - 1N34 or rock crystal diode
C1 - 82 pf capacitor
C2 - 80 pf air variable capacitor
C3 - 18 pf capacitor
R - 150K resistor
The following photographs show the circuit wired with the handmade
Saturn Dial. and knob.
The Saturn dial and knob were fashioned from a "doll's head" from
Michael's Arts and Crafts, a piece of lucite cut with two circle
cutters, and a brass paper fastener. The knob is fixed to the tuning
capacitor with a small machine screw that fits in the hole below the
brass fastener. The most difficult part of this was fashioning
"Saturn's rings". This must be done very carefully and slowly. The
inside edge should be cut slightly undersized and then sanded with a
drum sander to fit snugly. The outside edges can be sanded with fine
sandpaper and polished with a plastic polisher.
The air variable capacitor may be obtained from
Electronix Express at
http://www.elexp.com/. Part number 14VCRF10-280P.
The 80 pf side is recommended for the second circuit, contacts 2-3.
Contacts 1 and 3 were used for the first circuit (50pf).
- OSC: 5-59 pf
- ANT: 5-142 pf
- OSC and ANT Trimmer 10pf range
Home -- If you want to contact me go
to the home page and go through the Music Clinic link.