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)
