Dear Stefan, LF Group,
I spent some time reading various texts and data books, and doing some
calculations. The complete argument is too long and boring to type into an
e-mail, but very approximately it seems to be the case that for a ferrite
rod antenna and an air-cored loop antenna to have similar signal/noise
performance, the rod length needs to be similar to the loop diameter, which
seems intuitively reasonable. This means that the air-cored loop is better
for larger antennas (a 1m long ferrite rod is very heavy and expensive), but
the ferrite rod is better for smaller sizes (a 30cm long ferrite rod is
quite reasonable weight/cost, and less bulky than a 30cm diameter loop).
In that case, "is it possible to make a 30cm long ferrite rod antenna and
preamp with a noise level below the 136k band noise floor?" is the question
to decide if the ferrite rod is worthwhile for /P reception from a low-noise
location. Obviously there are many variables, but one can attempt at least a
rough calculation.
Assume Stefan assembles his rods into 2 bundles of 7; this would be roughly
equivalent to a single solid rod 28cm long x 21mm diameter. Assuming a high
permeability ferrite, this l/d ratio will multiply the flux through the
winding by a factor of about 70 compared to the same winding without the
core (called "mu_core" in Watt's "VLF Engineering"), so the rod antenna will
be equivalent to an air cored loop with an area of 0.025m^2. Assuming a
noise floor of 0.06uV/m per sqrt(Hz), a single-turn winding with this area
would have an induced EMF of 4.2pV/sqrt(Hz). With a low-noise preamp, assume
the internal noise level is all due to the resistive losses of the antenna,
which depends on the Q. Q of about 250 should be achievable; the inductance
of a single turn winding depends on another permeability parameter, mu_rod,
which depends on the rod geometry and the permeability of the ferrite; for
this rod about 100 from Philips' ferrite data book. L works out to about
0.16uH, and the loss resistance 540 micro-ohms. The noise voltage density is
sqrt(4kTR), 3pV/sqrt(Hz). So the internal noise is below the band noise by
3pV/4.2pV = 3dB. Hooray!
So it could actually be feasible. In order to make it work, it will be
important to achieve a high Q. Obviously, a single turn winding with
picovolt output levels is not very practical. I would aim for a parallel
tuning capacitance of e.g. 5nF, so it can be tuned across the 136k band
using a 500p variable. This would require L of 270uH, requiring about 41
turns of thick wire, preferably Litz or multiple strands of thin wire. The
parallel impedance at resonance with Q = 250 would then be 58kohms.
Connecting the tuned winding directly to a "miniwhip" type FET input preamp
should work well and add negligible amounts of preamp noise. The increased
number of turns and the high Q resonant winding winding will increase the
4.2pV/sqrt(Hz)noise floor at the preamp input by a factor of (41 x 250), so
43nV/sqrt Hz. In a 300Hz CW bandwidth, this would be about 0.75uV of noise,
so with a reasonably sensitive RX, no further gain would be needed
(worthwhile checking if it IS reasonably sensitive though...).
I stress that these are all very rough calculations - you will have to
actually try it out to find what the real values are. But they should be a
reasonable "first guess", and it seems to show that the expeiment is worth
trying.
Cheers, Jim Moritz
73 de M0BMU
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