Hi John, LF!

The radioamateur experiment in France several years ago with 3.3 khz PSK31 
communication in caves.

http://f6kcz.free.fr/Actualites/2011/Carrieres%20Fleury/Carrieres%20Fleury.htm

Profesional cave phone:

http://bcra.org.uk/creg/heyphone/

Russian communication system FERRA from Omsk radio-factory:

http://www.radioscanner.ru/forum/topic46535-3.html


73!
Roman

> In the cave radio field, we have tried a number of such configurations 
> underground and also in out-of-service rail tunnels.
> 
> Discussion of the results can be found in several issues of the Cave Radio & 
> Electronics Group Journal. Go to http://bcra.org.uk/pub/cregj/covers.html and 
> search for ferrite transmitting antennas. Back copies of the Journal are 
> available for download – a subscription available for a modest fee.
> 
> The electrical properties required of such an antenna depend on frequency and 
> bandwidth. For cave radio speech systems, a common centre frequency is 88.5 
> kHz (87 kHz USB) with a bandwidth of about 3 kHz and a maximum transmitter 
> output power of the order of 4 W. These systems normally work in the in the 
> induction regime, the ERP is not significant and may be of the order of uW.
> 
> Data systems have also been investigated on this and other low frequencies. 
> In general these require significantly less bandwidth.
> 
> For a theoretical analysis of such antennas see David Gibson’s doctoral 
> thesis (Leeds 2003?).
> 
> Some years ago, I was sent a ferrite-rod based antenna for analysis. It was 
> intended for use with the Molefone. Unfortunately it arrived shortly before I 
> emigrated and the device disappeared somewhere among my household effects 
> (420 cardboard boxes to start with). It has recently come to light and I hope 
> to examine it in detail later in the year.
> 
> 73 John F5VLF G3PAI
> 
>> On 22 Jul 2018, at 15:32, Markus Vester <[email protected]> wrote:
>>
>> This morning I attempted to transmit from a small ferrite antenna. It 
>> consists of a number of 9 mm diameter ferrite rods, with 7 bundled in 
>> parallel. The bundles were stacked with overlap to ~ 35 cm total length, and 
>> n=47 turns of litz wire were wound around the middle. Total ferrite cross 
>> section is a=4.45 cm^2 and volume V=156 cm^3, weighing 0.77 kg (including 
>> the coil). The coil was resonated and matched by several 1 nF high-Q ceramic 
>> capacitors.
>>
>> Under small signal conditions (-17 dBm), the electric parameters at 137.5 
>> kHz were
>> L = 0.50 mH, R = 0.62 ohm, Q = 690.
>>
>> Applying about half a watt significantly increased inductance and losses, 
>> and the tuning became sharply hysteretic ("jumpy"). For fine tuning, a small 
>> rod was placed at a variable distance beside the antenna.
>> Then I connected my PA and drove about 25 Watts into the antenna. Losses and 
>> inductance increased further:
>> I = 1.3 A, U = 867 V (rms), L = 0.77 mH, R = 14.8 ohmn, Q = 45,
>> with the Q-factor now so low that tuning jumps disappeared again. The 
>> central part under the coil became quite hot, so a tiny fan was added which 
>> held the steady-state temperature at ~55 °C.
>>
>> From the induced voltage we can calculate the flux density in the middle of 
>> the rod as
>> B = U / n / a / omega = 48 mT (rms),
>> decreasing approximately linearly toward the ends (similar to a small 
>> electric dipole). The average Bav=24 mT then results in a dipole moment of
>> iA = V Bav / µ0 = 156 cm^3 * 24 mT / µ0 = 3.0 Am^2
>> which results in a radiated power
>> EMRP = 62.34 kohm * iA^2 / lambda^4 = 25 nW.
>> Thus the efficiency of this transmit antenna is only 1 ppb !
>>
>> Anyway I attempted to detect the tiny signal on the DL0AO LF grabber, 48 km 
>> from here:
>> https://lf.u01.de
>> Transmitting on 137.780 kHz (6:29 to 8:22 UT) indeed produced a detectable 
>> trace in the QRSS-60 window (below the Slonim Loran line on 137781.25 Hz). 
>> Then I attempted to send a 4-character EbNaut transmission on 137.510 KHz, 
>> which was successfully decoded with some margin.
>>
>> Now if that's not QRP... anyway fascinating, considering that the small 
>> ferrite antenna might be carried in a handbag, buried in the ground or taken 
>> to a cave.
>>
>> Best 73,
>> Markus (DF6NM)
>>
>> <Ferrite_TX_Antenna.jpg><dl0ao_qrss60_180722_0820_small.jpg><dl0ao_07220844_result.png>
-- 
73!
Roman, RW3ADB