How Cave Communications works

Due to the ongoing (on 20 august 2019) rescue operation in the Wielka Śnieżna Cave in the Tatra mountains we decided to write about cave communications.

Thailand Heyphone
Thailand Heyphone

Rocks and soil attenuate radio signals well. Despite of this some means of communications in caves and mines are needed.

The attenuation rises with frequency. Normal VHF communications gear can’t be used, sometimes the distance can deteriorate to meters, depending on the geology and shape of tunnels. Of course cell phones are totally out of the question too. Shortwave radio behaves only a bit better.

Much larger distances can be achieved using low frequencies.

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ULF 2970.1 reception record

Stefan DK7FC transmitted at 6 aptil 2019 20:25 UTC at 2970.1Hz. The TX power was 487W into a 1130m ground dipole antenna. The ebnaut transmission parameters were:

f = 2970.1 Hz
Start time: 06.APR.2019 20:25:00.3 UTC
Symbol period: 6 s
Characters: 7
CRC bits: 16
Coding 16K21A
Antenna current: 2.4 A
Duration: 02:04:48 [hh:mm:ss]

The signal was received by Jacek / SQ5BPF in Warsaw/ Poland KO02md, at a distance of 976km!

After the filtering and noise blanking was optimised, the signal could be decoded:

[email protected]:/stuff2/sq5bpf/vlf_stefan2$ vtcat -p < stefan_2970_1 | vtfilter -a th=4 -h bp,f=2970,w=2200 > ../vlf_stefan2/pliktmp6

[email protected]:/stuff2/sq5bpf/vlf_stefan2$ vtblank -v -d0.0007 -a20 -t 50 < pliktmp6 | vtmult -f 2970.1 | vtresample -r 240 | vtresample -r 10 |vtraw -oa | ebnaut -dp16K21A -r10 -c2 -v -k16 -S6 -N7 -PS -L 50000

vtblank: selected channel: 1 = -:1
vtblank: channels: 1, sample_rate: 48000
vtblank: hfactor: 0.000e+00
vtblank: afactor: 20.000
initial reference phase -89.4 amplitude 7.496e-04
phase 0 0 0 0 0
vtblank: end of input
vtblank: dropsum 0 64750004, nfp 359999999
vtblank: dropfactor 0 1.799e-01
found rank 14143 ber 4.2548e-01 Eb/N0 -2.5 M -2.143712342e-02 ph 0 0,0,0,0 [DK7FC/P]
carrier phase: -10.6 deg
carrier Eb/N0: -0.4 dB
carrier Es/N0: -15.33 dB
carrier S/N: 15.63 dB in 133.5 uHz, -23.11 dB in 1Hz, -57.09 dB in 2.5kHz
elapsed 92

The message was: DK7FC/P

Link to Stefan’s email:


Record distance for a VLF transmission from an earth dipole antenna.

Recently we’ve done a successful  transmission from an earth dipole on 8270 Hz.

This is the first amateur transmission on VLF from Poland which has been received in another country. This is also the record distance for a transmission using a ground dipole antenna – over 1330 km.

Link to Paul Nicholson’s post about it:

Link to Paul’s VLF milestones list:

This is probably the first VLF signal which has been transmitted from Poland after World War II. We are not aware of any military or other transmissions. Before the war we had the Babice Transatlantic Radiostation (callsigns AXO/AXL/SPO/SPL), which was similar to Grimeton/SAQ.

VLF transmit tests from a ground dipole

Encouraged by our last experiments with ground dipoles we decided to do a transmit experiment with this antenna on VLF 8.270kHz, 36km (36000m) wavelength. Thanks to  Fab LAB Trójmiasto we had a great location on the see on a cliff near the Baltic Sea in Gdynia. Our goals for this experiment were:

  • Try transmitting from a ground dipole on VLF
  • Assess the field strength near the sea
  • See if it is possible to reach greater distances with relatively simple equipment and low power.
  • Test a broadband transformer for VLF-LF-MF

Continue reading “VLF transmit tests from a ground dipole”