For almost twenty years,
I've been holding on to an old ham radio magazine that contains an
article describing the construction of a Lyonodyne style crystal
radio. Finally, I got around to building it.
Construction
A 3
gang capacitor is called for in this circuit. One gang is used to resonate L1,
while the remaining two gangs provide capacitive coupling to ground. All three
gangs are around 365pf. You could try and make do with a dual gang unit, (i.e.
only 365pf max to ground from the tank coil.) but I found that there was a loss
in sensitivity at the low frequency end of the band with that arrangement. In a
pinch, you could probably use one dual gang from tank coil to ground and one
half of another dual gang for the tank itself. Tuning will be a little
difficult but not impossible. The gang I used came from an old MW/SW radio.
The coil is 30 turns of 1mm enamelled copper wire wound on an 11 slot
rook former (one over, two under) about 120mm in diameter. It
measures 200uH. It's tapped
every sixth turn. The coil is held together using wax lacing string and is held in place with glue from a hot glue
gun.
I used a pair of OA90 diodes in a
special switching arrangement that allows for three detector
configurations.
The first setting is a floating voltage
doubler with high selectivity for use above around 1200kHz.
Position two is a grounded voltage
doubler for use over the whole band.
Position three is a standard single
diode configuration which is less selective but has more volume than
the other two positions.
The trimmer cap (C2) feeding the
detectors is used as a selectivity control when switch S1 is in
positions one or two. A small value variable unit (30pf max)
is used here.
I
built the set into a nice vinyl covered box that I found in an Army Disposal store.
The complete set fits into a (170mm x 215mm x 175mm HxWxD) box with a
hinged lid.
As always, good
mechanical/electrical connections are a must with these sets. The gang I used
does not rely on the connection through the bearings for the rotor. Rather it
uses silver fingers to connect to the rotor shaft. Note that each finger is
soldered to the frame of the capacitor making a good connection.
Headphones.
The
original 1940's style bakelite headphones I used measured around 4000 ohms DC.
These sounded quite good and the set performed well but I felt it could be
better. I ended up using a set of very sensitive military 'phones coupled to the
set via a high quality audio transformer with a multi-tapped primary from 30
ohms to 1000 ohms and a 4800 ohm (160 ohm DC) secondary. I mounted the
transformer in a utility box and drove the crystal set into the secondary and
selected the appropriate tap from the primary to drive the headphones.
It
really is worth the effort to try and match the output of your crystal set to
your headphones.
Loose Coupling.
I followed up with a second
identical Rook coil connected in Lyonodyne configuration loose
coupled to the original set. The antenna and ground now connect to
this antenna coil and the set now acts as a tuned detector coil
feeding the headphones. The best detector configuration is in switch
position 3.
L2 is a copy of L1 but is tuned by a
dual gang 365pf capacitor. The two sections of the capacitor that
were connected to ground from L1 have been removed from the diagram
for clarity but still exist. Because they are no longer connected to
ground they play very little if any part in the operation of the set.
Coupling between the two coils is adjusted by moving the two coils
closer or further apart. The closer they are, the louder the signals
in the headphones. There comes a point however, where moving the
coils closer together results in a decrease in sensitivity and
selectivity. The trick is to find this point where the coupling between the two coils is optimum. For my set, this distance
is one coil diameter (200mm). I mounted L2 in a
matching vinyl box.
The change
in performance brought about by going to loose coupling with a tuned
detector coil AND the addition of a quality audio transformer and sensitive
headphones have
been impressive. This level of improvement makes it easier to
use a crystal radio in the city when you're surrounded by high power
stations.
References
Based on an article published in "Amateur Radio
Action", Vol. 4 No. 13 - p29-33, - 27th April 1981.
From an
original article by R.M. Tuggle published in "DX News",
bulletin of the National Radio Club Vol. 44, No. 25, p 5-8, 23rd May
1977.
I found these sites very
informative during my experimentation.
