Detector unit 1

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Here a description of a detector unit, this can be used in combination with a antenna unit for a double tuned receiver.
On the output a high impedance audio transformer must be connected, like my transformer unit1.

Detector unit 1	Under the tuning capacitor, the two trimmer capacitors are visable.

Schematic of the detector unit.

Coil L1 is a coil made of litzwire 660x0.04 (660/46 AWG) and is wounded on a polypropylene former.
This coil is described here as coil L13.

The tuning capacitor has silvered plated, and polyethene insulators, and is described here under the name C2b.

The switches S1 and S2 are reed-switches, which can be switched by placing a small magnet near it.
These reed-switches with glass body give low dielectric losses.
When the reed-switch is opened, the capacity between the contacts is about 0.2 pF.

With trimmer capacitors C2 and C3, the frequency range of the detector unit can be adjusted.

Diode D1 is a double SMD schottky diode type: HSMS282K, the two diodes in the package are parallel connected.
This diode has also a internal screen between the two diodes, the connection pins of this screen are not connected.
After soldering, all solder flux is removed from the diode, because this can reduce circuit Q.
More information about diodes you can find here.

Also a second diode can be connected (D2), so there can be switched between D1 and D2.

C4 is for removing RF signal behind the diode, it's value is variable so I can do experiment with it.

The output of this detector unit must be loaded with a high impedance, preferable more than 1 M.Ohm.

The detector unit is made of polyethene sheet, this gives low dielectric losses, more information about this, you will find here and here.
There are only copper screws used, no iron because of the lower losses.

Detail of the reed-switches, these are the glass tubes. The magnet can be shifted from left to right, so one or the other reed switch is switched. Diode D1 is placed on the lower side of the left reed switch, but is almost too small to see.

Tuning dial of the detector unit. The frequency can be directly read from the dial. Between the knob and the tuning capacitor there is a 1:5 drive. The frequency dial is connected to the tuning capacitor, and will also turn 5 times slower than the knob.

The Q factor of the detector unit is measured, one time without the diode connected, and one time with the diode (2 parallel) and a load resistor of 1.5 M.Ohm.
Also the frequency shift is measured, which the diodes gives.
(These measurements are done with 1.12 Volts peak-peak across the LC circuit at resonance.)

With the formula Z= 2.pi.f.L Q the impedance of the unloaded circuit can be calculated (the inductance of the coil is 230 uH):

Z= 1.1 M.Ohm at 600 kHz  
Z= 1.57 M.Ohm at 900 kHz
Z= 1.93 M.Ohm at 1200 kHz
Z= 1.97 M.Ohm at 1500 kHz

First the tuning range of the unit was 530.65 to 2050 kHz, this was measured with the diode connected.
The lowest tuning frequency was too close to the lowest MW frequency of 531 kHz, so I want to make it lower.
Also if another diode with a lower capacitance will be used in future, I couldn't reach 531 kHz.
The highest tuneable frequency I want to reduce to about 1700 kHz, this gives a better spreading of the stations over the dial, so the stations are not so close on each other.
Here in Europa the highest MW frequency is 1620 kHz, but above this up to 1700 kHz some pirate stations are active, which can also be interesting to hear.

To reduce the frequency range, I placed a trimmer capacitor across the tuning capacitor.
The maximum capacitance of the trimmer capacitor is 13.8 pF.
With the trimmer at maximum capacitance, I measured the Q of the unloaded circuit.
600 kHz  Q=1224
900 kHz  Q=1210
1200 kHz Q=1090
1500 kHz Q=898

We see the trimmer capacitor has little influence on circuit Q.

There are two trimmer capacitors used (C2 and C3), these are placed behind the reed switches, so every diode has it's own trimmer.
Now it also becomes possible to tune away differences in diode capacitance, so switching from one diode to another will not give a frequency shift.

With the trimmer capacitors, the tuning range of the receiver is adjusted to 522 - 1710 kHz.

With the detector unit I can receive the following stations, in this case there is no external antenna connected, only the coil of the detector unit is used as a small loop antenna.

These stations are received late in the evening.
A * in the last colomn indicates the station can also be received at daytime.
The detector unit was placed on a wooden table, this gives a small reduction in circuit Q.

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