ARLA/CLUSTER: Esquema de um beacon para VLF (8,76 kHz)

[João Gonçalves Costa]

VLF earth mode beacon TX schematic

Roger G3XBM has made available the schematic of the 5W QRP VLF beacon transmitter used in his successful 6km 8.76kHz VLF earth mode (through the ground) tests.

The keyer IC is from K1EL's website and gives 10wpm, QRSS3 and QRSS30 beacon messages. This keys the frequency stable 4060 crystal oscillator-divider that feeds the TDA2003 amplifier. The output of this is matched to the earth electrode pair using a tapped toroid transformer with a number of step-up taps between 10-160 ohms.

Receiver is a pair of earth electrodes in the ground into a low input impedance tuned preamp feeding a PC running Spectrum Laboratory (free) software set at 11mHz bandwidth. The whole system is far from optimized.

This week Roger hopes to repeat the test at 1.095kHz and possibly 0.547kHz. He expects stronger signals as he goes down in frequency as long as the mains hum is OK. Earlier tests suggest 0.838kHz signals were up to 6dB stronger than those at around 8.76kHz at 5km range.

9 kHz Schematic at
https://sites.google.com/site/sub9khz/earthmode/vlf-xbm-blog

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