IMPROVED CRYSTAL RECEIVER
FOR THE BTTF 2004
(2004)

KLIK HIER VOOR NEDERLANDSE VERSIE


The improved crystal receiver. Instead of the traditional crystal telephone, a Philips headphone
model HS415 was used with the same or perhaps even better sensitivity and much better audio quality!

BTTF contest!
From December 25 to December 31, a contest is organized here in the Netherlands. The name is Back To The Future (BTTF). The challenge is to log as many medium wave stations as possible with a passive receiver.
Click HERE if you want to download the zipped bttf2004log.doc log of the stations I heard in December 2004 with this crystal receiver during that contest. There were 76 different medium wave stations identified, but some more unknown signals were heard.
Click HERE if you want to download the zipped bttf2005log.doc log of the stations I heard in December 2005 with this crystal receiver during that contest.
Click HERE if you want to download the zipped bttf2006log.doc log of the stations I heard in December 2006 with this crystal receiver during that contest.


Inside view

Improvements of the BTTF 2004 receiver compared to the BTTF 2003 receiver
This receiver also has the antenna tuner to improve the selectivity and sensitivity.
But it also has an extra L/C combination that can be switched (with S4) as a filter for extra selectivity or as a notch filter to suppress signals from strong broadcast stations. All coils do have litze wire now. The variable capacitor with reduction gear has a calibrated frequency scale, very useful to identify stations quickly.
It is also possible to select a germanium or a schottky diode (S8). Sometimes the germanium is better, sometimes the schottky diode. This experience was different from that with the BTTF 2003 receiver, in 2003 a germanium diode was always better....
The diode detector is not directly connected to a crystal telephone. Experiments showed that the sensitivity is better when an audio transformer is used and that the crystal telephone is connected to a tap of that audio transformer. It is also possible to use a low impedance headphone now. It can be connected to the secundary of the transformer.
During the contest, a Philips headphone model HS415 was used. Sensitivity and audio quality are better than that of the crystal telephone. Only one earpiece is connected, the impedance is 16 ohm and sensitivity 102 dB. Also very good is an Adastra driver unit model 952.207. But it is very heavy. For more information about the transformer used here and the very sensitive Adastra driver unit, see the nice site of Dick Kleijer about crystal receivers http://www.crystal-radio.eu/.


Diagram of the improved BTTF 2004 crystal receiver.
big diagram

Explanation
All coils are standard available coils for ferrite rods as used in broadcast receivers. L1 is a longwave reception coil, L2 and L3 are types for the medium wave. For the 8, 4, 2 and 1 winding taps, extra coils are wound with 0,3 mm copper wire.
L1 and C1 are the antenna tuner, tuning the antenna wire to resonance for maximum sensitivity and extra selectivity.
L2 and C2 are the extra filter. It can be used as a notch filter or as a filter for more selectivity (depending on the position of S4). I used it a lot as a notch filter (mostly the tap on 2 and 4 windings was used, sometimes the tap on 8 windings but the tap on 45 windings was never used).
L3 and C3 are the main selective tuning filter. Thanks to the very good variable capacitor with reduction gear and calibrated frequency scale, it was easy to identify the stations with use of a portable receiver with digital frequency read out. The diode detector can be connected to a tap (at 45 windings) or to the top of the coil. The tap is used above 1 MHz, the top connection below 1 MHz.
After the diode detector, the already mentioned transformer and crystal telephone or low impedance headphones can be found. As the transformer has almost no DC impedance, the 150k ohm resistor with the 0,1 uF capacitor are added so that the DC resistance is more or less similar to that of the AC impedance of the audio transformer. This is important for a distortion free reception. However, for weak signals the sensitivity is better when this DC load is switched off (S9).
Due to the loading of the detector and the antenna impedance, the Q factor of the coils will be reduced from more than 150 to somewhere between 50 and 100. Selectivity will be somewhere between 10 and 30 kHz.


Antenna during the BTTF 2005: to the tree right behind
and then back to the left to a tree with a height of 2 meters

Antenna
The total length of the antenna is 50 meters. The first 30 meters run in the west-east direction, starting at a height of 3 meters to a tree at a height of 6 meters. The last 20 meters are running from this point to the south, ending at a height of 2 meters.


Barefoot in the snow to mount the antenna!
Just as in the old days, when they also did run barefoot in the snow!

Selectivity
Measurements of the bandwidth:

Frequency-3 dB-6 dB-20 dB
600 kHz12 kHz19 kHz52 kHz
1000 kHz15 kHz23 kHz99 kHz
1400 kHz32 kHz50 kHz130 kHz

Sensitivity
A 10 mV rms source with a 25 ohm series resistance, 30% amplitude modulated gives an acceptable sound level. This a 1 microwatt RF signal at the input of the receiver. An RF signal of 100 mV rms gives a strong sound level in the headphone (100 microwatt RF signal).

Frequency scale
The scale that is used is the following table, calibrated in steps of 30 or 90 degrees:

0 = 450 kHz--
90 = 500 kHz--
180 = 531 kHz--
270 = 585 kHz--
360-0 = 645 kHz--
90 = 750 kHz120 = 810 kHz150 = 850 kHz
180 = 880 kHz210 = 920 kHz240 = 975 kHz
270 = 1040 kHz300 = 1110 kHz330 = 1180 kHz
360-0 = 1250 kHz30 = 1320 kHz60 = 1410 kHz
90 = 1500 kHz--
180 = 1800 kHz--

Index PA2OHH