Re: LF: Transmitting with a small ferrite antenna

[John Rabson <john.rabson07@gmail.com>]

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 <markusvester@aol.com> 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)

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