fm crystal radios

7/23/2019 FM Crystal Radios

1/12

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 M radios. In fact, it sounds !high"fidelity!.

This picture shows the #olomon FM $rystal #et 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. #o, it should wor' without any batteries or $ power. n FM


2/12

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 receier is an ama!ing per"ormer# $t has crystal clear reception %p&n intended'( good

sensitiity( b&t only "air selectiity# This set )as a discoery "or me# $ started o&t by designing and

b&ilding the normal *M sets# Then one day )hile testing the +Mystery+ set %see my other )eb

lin,s'( to my s&rprise( in addition to the e-pected panoply o" *M stations( $ heard a ery "aintsignal that $ co&ld not t&ne o&t# *t "irst( it seemed too )ea, to identi"y# .hen $ t&ned o&t all the

*M stations( $ )as astonished to hear the anno&ncement +/iiM FM( #1+ This is a co&ntry

m&sic FM station here in T&cson# $t )as all oer the dial( &nt&nable( b&t the m&ch lo&der *M

signals mas,ed it )hen they )ere t&ned in#

$ set mysel" the tas, o" trying to improe the FM reception# $ tried some simple circ&it

modi"ications that did not seem to improe anything# Then $ connected a dipole antenna instead

o" the *M antenna $ normally &se# S&ddenly( the FM signal )as m&ch clearer( altho&gh still

)ea,# 3y &sing the a&dio o&tp&t and so&nd system ampli"ier( $ )as een more ama!ed that "o&r

di""erent FM stations came in lo&d %or rather medi&m' and clear# $ "o&nd that changing the

telescoping antenna length and position $ co&ld t&ne the stations in and o&t# They )ere /R4(

/L5( /iiM( and /78T all local FM stations )ith transmitters nearby# Their reception )as

also a""ected by the length and position o" the a&dio o&tp&t cable#

*"ter doing some research( $ discoered that there )as a physical theory that claimed that FM

reception )as possible and een probable &sing the same circ&it as an *M receier# The theory is

called +slope detection+# So( $ set o&t to "ind circ&it improements# * )eb search

yielded little( mostly theory# 3&t there )as eno&gh in"ormation that $ tho&ght $ co&ld ma,e some

modi"ications to the *M circ&its to ma,e them more t&nable to FM signals and less t&nable to

*M# Since FM operates at higher "re9&encies( all $ had to do( $ tho&ght( )as ma,e the coil and

caps smaller# *"ter m&ch +tin,ering+ $ arried at the c&rrent circ&it#

The circuit loo's identical to a classic M crystal circuit but is even simpler to build. The components

were reduced in dimension to resonate at higher fre(uencies. This was done by experimenting with

smaller and smaller coils and capacitors. The antenna is also much reduced in si)e %from that of M&

to resonate at higher fre(uencies %the antenna is crucial&. The air variable capacitor I used has two

trimmers in it which should be ad*usted for best reception.I have found that a commonly available

vernier dial and 'nob will fit the capacitor nicely. #ee end of article for a picture of the variable. $+ is a

ceramic capacitor of - pf, but may be anywhere from to /pf. detected FM signal is converted toM due to an effect called slope detection that modulates amplitude.

This FM $rystal #et wor's best near the transmitter %I have not tested it beyond about miles&.

#econdly, the sound level is (uiet, especially without an amplifier. (uiet 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. 0owever, in addition to listening with high impedance earphones %crystal or otherwise&, the set

can be connected directly to an audio amplifier1s low level magnetic input which can then play


3/12

amplified through a sound system at any volume "" sounds 234T. 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.

%5owadays, many receivers don1t even have a phono input6& That way you can cran' up the volume,

which ma'es it more li'ely to find the FM stations. If no signals are detected, I also recommend

connecting an external !rabbit ear! antenna or hanging a short wire %7 inches or so& in various

positions next to the internal antenna. The variable length of rabbitt ears can help to tune in stations.

5o additional wiring or antenna is necessary %the antenna is optimi)ed in length for FM.&

8 " 9 turns :- copper or silver wire, 7mm inside diameter, tapped at 7./ turns

nt " ; inches of :- bare copper wire

$ " - pf ceramic capacitor

$7 " / pf air variable capacitor

< " 5+9 diode or roc' crystal

3 " /= 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! :- W2 wiring is used

without any insulation. It is important to 'eep the components physically close together. The

component specifications are the same as in circuit :7. The coil is silver rather than copper, but

copper does *ust as well. I thin' that the contrast of the silver and copper is beautiful. The coil was

wrapped around a #harpie >ermanent Mar'er, then slipped off and expanded slightly. The wooden base

is made from lac(uered, polyurethane padou'.


4/12

I consider this set a wor' of art as well as science and thin' 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 blac'

vernier dial, the white and transparent variable capacitor, and the subtle colorings on the resistor, the

diode, and the lucite base. ?et the circuit is so ridiculously simple that some will not believe it is

possible without building it themselves. 5o shielding is necessary, and there is no problem with handcapacitance. 0owever, the output cable position may affect reception sensitivity.

>hotos of wired circuit

hand is included in this photograph to show scale. 5ote the military style wiring, diode, and antenna.

I wanted the wiring to create a modern design similar to a Mondrian painting. 5ot only is this set

beautiful, it wor's6 5o power and no long antenna6 It loo's li'e a wor' of fiction.


5/12

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 thin'. There is only one resistor and

one fixed capacitor.


6/12

The inside of the tuning capacitor and the phono *ac'Aoutput can be seen here. $an you spot the fixed

ceramic capacitor@ 5ote the polished edge of the face plate and the reflection in the wooden base.


7/12

(uarter"inch piece of lucite was fitted under the tuning capacitor to anchor it. 5ote the two tiny

trimmers on the bac' of the tuning capacitor. Brass screws were used to enhance appearance.


8/12

The vernier dial is large to accomodate ease of tuning, and the vernier ma'es it easy to separatestations. Two golden %brass& wood screws fix the face plate to the base. 0oles for the face plate were

made with special plastic drills, but ordinary drills may be used if drilled very #8OW8?. The 'nob is

removable.


9/12

FM Crystal Circ&it :2

8 " / turns W2:- bare copper or silver wire, 7mm inside diameter, tapped at 7./ turns

< " 5+9 or roc' crystal diode

$ " -7 pf capacitor

$7 " - pf air variable capacitor

$+ " - pf capacitor

3 " /= resistor

The following photographs show the circuit wired with the handmade #aturn


10/12

The #aturn dial and 'nob were fashioned from a !doll1s head! from Michael1s rts and $rafts, a pieceof lucite cut with two circle cutters, and a brass paper fastener. The 'nob 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 !#aturn1s rings!. This must be done very carefully and slowly. The inside

edge should be cut slightly undersi)ed 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.


11/12


12/12

The air variable capacitor may be obtained from 4lectronix 4xpress at httpCAAwww.elexp.comA. >art

number 9D$3F"7->. The - pf side is recommended for the second circuit, contacts 7"+. $ontacts and + were used for the first circuit %/pf&.

O#$C /"/E pf

5TC /"97 pf

O#$ and 5T Trimmer pf range

0ome "" If you want to contact me go to the home page and go through the Music $liniclin'.

fm crystal radios

Documents