Dear Jim,
Thanks for the most interesting details, always a pleasure to read your
mails :-)
Yes, really looking forward to what i actually will get (L at 50 turns
or so.) Ebay already told me that the rods are sent so probably i'll
give some data within this week.
What do you think about the 50 Ohm output winding and a separate
resonated winding? Then using your preamp AND my RX converter.
Would you try either 2x7 rods or 3x5 rods? Both would fit in my
backpack. If i read your explanation, a longer rod would be an advantage...
73, Stefan/DK7FC
Am 15.08.2011 23:14, schrieb James Moritz:
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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