Set 4
With this receiver I took part in a crystal receiver
contest, here in The Netherlands, in 2003. The contest was named "Back To The
Future" (BTTF).
In the contest we had to receive during 7 days as many stations as possible per
day. My total was 460 stations (about 65 per day), with which I won the contest.
The number of different stations was 113, coming from all over Europe and North
Africa.
Go to the list with received stations
The receiver has 2 tuned circuits, the antenna circuit and
the detector circuit. Both must be tuned to receive a station.
The coupling of the antenna to the tuned antenna circuit is made variable by
means of a variable capacitor and taps on the coil.
During the contest I mostly used the tap "50%".
The variable capacitor in the antenna wire must be set to a low value (e.g. 50
pF). Then the selectivity of the receiver is high, and adjacent stations are
better separated.
The distance between antenna coil and detectorcoil is adjustable, a distance of 12 cm is a good value. A smaller distance results in less selectivity.
The coils are spiderwebcoils wound on corrugated cardboard
formers, with 0.2 mH induction, the
Q-factor is 250.
At 1 MHz the parallel resistance of the coils is 314 kΩ.
The load resistance of the audio transformers and the choice of the diodes are
matched to this value of 314 kΩ
for maximum power transfer.
I used 4 transformers with a primary impedance of 80 kΩ
each in series, so with a total impedance of 320 kΩ.
The output impedance of one transformer is 16
Ω. By
using a combination of series and parallel connection the total of 4
transformers also is 16
Ω.
The loudspeaker is a driver unit
from a horn-speaker which is used on sports grounds etc.
Normally these driver units are connected to a horn, but I don't use the horn
and hold the unit near my ear while listening.
The efficiency of the driver unit is 112,5 dB/watt/meter which is a very high
value.
The driver unit is much more sensitive than normal headphones or a crystal
earpiece.
The photo above shows the driver unit, laying near the receiver.
When receiving strong stations it is important that the
primary audio-impedance of the transformers (320 kΩ)
and the DC resistance of the transformers have about the same value. When this
is not the case, strong distortion may occur.
Because the DC resistance of the transformers is very low (some kΩ’s),
I added a 330 kΩ
resistor.
Parallel to the resistor a capacitor is connected, allowing the audio current to
pas through, so that there is no loss of audio signal across the resistor.
Across the 330 kΩ
resistor we can measure a DC voltage, indicating the strength of the received
station.
A high impedance voltmeter should be used here (e.g. 10 MΩ).
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During the contest I used this
antenna. Antenna wire lenght: 22 meter. Highest point of antenna wire: 18 meter. The earth connection is made by a 48 mm diameter 3 meter long metalpipe driven into the ground.
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