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 should work without any batteries or AC power. An FM
crystal receiver must
be able to detect and receive FM signals well enough to be heard in
earphones without any such extra power.
This
FM receiver is an amazing performer. It has crystal clear reception
(pun intended), good sensitivity, but only fair selectivity. This set
was a discovery for me. I started out by designing and building the
normal AM sets. Then one day while testing the "Mystery" set (see my
other web links), to my surprise, in addition to the expected
panoply of AM stations, I heard a very faint signal that I could not
tune out. At first, it seemed too weak to identify. When I tuned
out all the AM stations, I was astonished to hear the announcement
"KiiM FM, 99.5"! This is a country music FM station here in
Tucson. It was all over the dial, untunable, but the much louder AM
signals masked it when they were tuned in.
I set myself the task of trying to improve
the FM reception. I tried some simple circuit modifications that did not seem to improve
anything. Then I connected a dipole antenna instead of the AM antenna I normally
use. Suddenly, the FM signal was much clearer, although still weak. By using the
audio output and sound system amplifier, I was even more amazed that four
different FM stations came in loud (or rather medium) and clear. I found that changing the
telescoping antenna length and position I could tune the stations in and out.
They were KRQ, KLPX, KiiM, and KHYT all local FM stations with transmitters
nearby. Their reception was also affected by the length and position of the
audio output cable.
After
doing some research, I discovered that there was a physical
theory that claimed that FM reception was possible and even probable
using the same circuit as an AM receiver. The theory is called "slope
detection". So, I set out to find circuit improvements. A web search
yielded little, mostly theory. But there was enough
information that I thought I could make some modifications to the
AM circuits to make them more tunable to FM signals and less tunable to
AM. Since FM operates at higher frequencies, all I had to do, I
thought, was make the coil and caps smaller. After much "tinkering" I
arrived at the current circuit.
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 antenna is crucial). 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 quiet, especially without an
amplifier. A quiet room
is needed for listening with earphones. 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. In fact, I recommend starting tests with
the FM crystal set by connecting it to the low-level phono inputs of a
receiver or preamplifier. (Nowadays, many receivers don't even have a
phono input!) That way you can crank up the volume, which makes it more
likely to find the FM stations. If no signals are detected, I also
recommend connecting an external "rabbit ear" antenna or hanging
a short wire (12 inches or so) in various positions next to the
internal antenna. The variable length of rabbitt ears can help to tune
in stations.
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
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 does just as well. I think that the contrast of the silver and copper is beautiful. The coil was wrapped
around a Sharpie Permanent Marker, then slipped off and expanded slightly. The
wooden base is made from lacquered,
polyurethane padouk.
I consider this set a work of art as well as science and think it
is the most elegant crystal receiver I have created. I love the
contrast of the silver coil, the copper antenna, the clear acrylic
faceplate, the black vernier dial, the white and
transparent variable capacitor, and the subtle colorings on the
resistor, the diode, and the lucite base. 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. Not only is this set
beautiful, it works! No power and no long antenna! It looks like a work
of fiction.
Is this thing imaginary -- science fiction? Well, imagination did
play a part, but it is definitely not science fiction. 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 fixed
ceramic capacitor? Note the polished edge of the
face plate and the reflection in the wooden base.
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. Brass screws were used to enhance appearance.
The vernier dial is large to accomodate ease of tuning, and the
vernier
makes it 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.
It is perhaps not as visually striking as set No. 1, but it works just
as well. In fact, this set was the original version. Notice that all
the wiring and coil are copper.
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.