Dear Daniele, LF Group,
You wrote:
So my rough summary is:
- small-size (e.g. ferrite cored) loops need tuning = high Q to provide a
reasonable signal level.
...Or a more sophisticated preamplifier design that reduces loaded loop Q
without also reducing SNR. But practically this becomes more difficult to
achieve as the bandwidth increases.
- intermediate-size (e.g. 1m diameter or so) loops can be wide-band but
require low-noise, very low-impedance pre->amps.
There are quite a few amateur and commercial designs around of this type,
and they can be quite good. The reason for having a preamp with a very low
input impedance is that the ratio between output signal voltage and input
field strength is then nearly constant over a wide frequency range. This is
useful, e.g. for making wide-band field strength measurements, but not so
helpful for reception purposes, since it just effectively means the noise
level at the output rises as the frequency is reduced.
- bigger loops (e.g. > 2m diameter) can be wide-band even when connected
directly to a low-impedance receiver input
(without a pre-amplifier), may be through a broadband transformer to
furtherly reduce the load impedance as seen by the >loop.
...But unless the loop is very large, good receiver sensitivity is still
required - be aware that many receivers have degraded sensitivity at low
frequency.
In general, loop size is more important for sensitivity than permeability
of core material and number of turns.
The number of turns essentially only varies the source impedance of the
loop, but does not change the available signal output power. High
permeability cores have slightly greater effective area, but usually have
higher losses.
For practical reasons, for rx activity at home I'm building a ~1m wide-band
loop. The reference design for the amplifier is >M0AYF's (despite it is not
a very low-impedance pre-amp, I think).
Nevertheless, the discussion about ferrite-cored antennas here on the
reflector is more and more pushing me to build my >own "loopstick" and
experiment with it.
Could it be possible to tune a ferrite loop over a frequency range as wide
as 100-500KHz? Or at least over the EU NDB >band (say 300-500KHz)?
Any practical suggestion (and/or references on the web) about how to build
it?
A narrow-band loop that can be tuned over a wide range of resonant
frequencies is quite easy - A winding inductance of a few hundred uH could
be tuned with a capacitance varying from a few hundred pF to several nF to
cover that range. This could be achieved with a combination of switched
fixed capacitors and a variable capacitor. Or using the same tuning
capacitor with windings of different inductance, like a long wave / medium
wave ferrite rod antenna. A high input impedance preamp like the one used
with my prototype loop, connected directly to the tuned winding, would give
adequate gain for reasonably sensitive RXs. If you want a wide-band loop
covering 100kHz - 500kHz with no tuning, that is more difficult...
About the ferrites, there are several sellers on eBay who offer a russian
MU400HH type. Would it be suitable for LF? >Are there some recommended
alternatives?
Probably the best choice is ferrite intended for medium wave antenna rods,
which combines low resistive losses with fairly high permeability. The
losses generally decrease at lower frequency, so if it works at MF, it
should also be good at LF.
Cheers, Jim Moritz
73 de M0BMU
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