woensdag 24 oktober 2018

Special transmission of SAQ

More than 20 years ago I built a active loop antenna for VLF.
I listened from 100 kHz down to 20 kHz.

Today there was a special transmission in CW of SAQ from Grimeton in Sweden on 17.2 kHz. The transmitter is a generator that makes a power of 200 kW with a frequency of 17.2 kHz. The station has a huge antenna. The station was used 100 years ago to send messages in CW to North America.

My loop antenna is made of a cable with a length of 5.5 meters with 12 wires. The wires are connected in series. This inductance is tuned with a capacitor. The signal of the loop is picked up by a FET, followed by a bipolar transistor, that form a follower.

Placing the loop in the shack was easy. But then I had to remember how to connect the wires of the loop to the loop amplifier. Luckily I found the schematic in the article, that I wrote for the spring issue of 1997 of SPRAT. With the schematic I connected the 3 wires of the loop to the loop amplifier.

The Databuch 1980/81 Integrierte Schaltungen für die Unterhaltungs-elektronik of Siemens gave the information on the SO42P mixer, that I use in the frequency converter. After more than 20 years I could not remember which pin I used as an input. But with the book and a magnifying glass, I could connect the coax between the amplifier and the converter to the right pin.

Loop amplifier and frequency converter
Then I tested the frequency converter. On the oscilloscope I could see, that the oscillator was running and I could hear the carrier on 8 MHz on my receiver. So now it was time to tune the loop. I can drive the loop gently into oscillation to determine the frequency with the oscilloscope. With a capacitor of 47 nF my loop was oscillating on 25 kHz. I calculated that I would need a second capacitor of 47 nF in parallel for 17.2 kHz. But in my boxes of capacitors, there was only 56 nF, so with the extra C the loop was tuned to 16.6 kHz and I started listening. A quick look at the clock showed the transmission of SAQ had already started.

At first I heard a strong crackling (gekraak), but when I tuned the receiver I heard a weak CW signal. With the very strong crackling I could only copy a few letters.
I only managed to copy:  ork  gor  ba  l c pe c n  
But then I realized, that this noise source must be very close, because of the strength of the noise signal. Well my loop ran less than an inch from my Led lamp, that is placed on the oscilloscope. 

I disconnected the LED lamp and the band was quiet. Now I could copy the last part of the transmission about the QSL policy of this transmission. The transmission ended with QRU DE SAQ  SAQ  SAQ  SK

The next transmission of SAQ will be on the 24th of December 2018. If I start in time, to build up the circuit, I will be able to tune the loop precisely.  And I will have time to adjust the Q of the loop for the best reception. hi.