LF: Transmitting with a small ferrite antenna

[Markus Vester <markusvester@aol.com>]

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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