Set 4
 

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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 k
Ω.
The load resistance of the audio transformers and the choice of the diodes are matched to this value of 314 k
Ω for maximum power transfer.
I used 4 transformers with a primary impedance of 80 k
Ω 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 k
Ω’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 M
Ω).

 

During the contest I used this antenna.
The antenna wire can be seen on the left side of the mast, coming down from the top of the mast.

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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