Dear Daniele, LF Group,
Regarding bandwidth, the first thing to note is that the same principles
essentially apply to both air-cored loop and ferrite rod cored loop
antennas - the main difference is that air-cored loops are wide and flat,
but ferrite rods are long and thin ;-).
Assuming you can make a preamp with a low enough noise level, the minimum
usable signal level "sensitivity" of a loop antenna depends on the ratio
between the induced signal level, and the level of thermal noise produced by
the resistance of the loop windings, core losses, etc. So this sensitivity
depends on the construction and size of the loop/rod, and in principle it
does not matter if it is tuned for narrow-band resonance or loaded to
produce wide bandwidth, provided the tuning or loading arrangements do not
introduce additional noise. But in practice, tuning/loading and
preamplifiers will introduce some additional noise.
The big advantage of a tuned loop is that the resonant circuit can provide a
high "passive gain". So Stefan's rod antenna probably produces an EMF in the
nanovolt range for usable received signal levels, but the high Q circuit it
forms with a parallel capacitor increases this voltage by more than 50dB The
actual signal power level is not increased by the resonant circuit, but the
much higher signal voltage is easily handled by a simple preamplifier with
insignificant additional noise introduced. The resonant circuit also has a
very narrow bandwidth - this might be an advantage for attenuating strong
out-of-band signals, but is a drawback if wideband reception is required, or
remote tuning of the loop is needed.
In many commercially available wideband loops, the loop is loaded by a
preamp with a very low input impedance. This provides a flat frequency
response, since the loop EMF rises in proportion to signal frequency, but
the signal current at the preamplifier input is maintained constant by the
reactance of the loop inductance, which also rises proportional to
frequency. This flat response is very popular for measuring applications and
wideband reception. But the preamp design is much more difficult, because
the input signal amplitude is effectively attenuated by the combination of
high loop reactance and low preamp input impedance. So careful preamp design
is needed, to provide a low input impedance, very low noise voltage, and a
low noise figure when fed from a highly mis-matched, relatively much higher
source impedance. The "noiseless feedback" techniques such as "Zwichenbasis"
amplifiers mentioned by DF6NM or "Norton" feedback amplifiers can be
usefully used. But even with careful preamp design, relatively large loops
(~1m) seem to be neccessary to achieve a reasonable sensitivity. Of course,
if loop size is not an issue, one can simply increase the loop area to
produce a greater signal amplitude, and all that is needed is a large wire
loop terminated by a low impedance receiver input.
In my view, for communications reception purposes, creating a flat output
voltage vs. field strength relationship for a wideband loop is not
particularly useful - the background noise field strength decreases with
frequency, so if you keep the "natural" signal EMF-proportional-to-frequency
response of a loop, the background noise at the receiver input remains
fairly constant with frequency. I have used 2x2m and 4 x 5m loop antennas
where the loop inductance forms the input inductor of a low-pass filter with
cut-off frequency of about 550kHz, in order to attenuate powerful broadcast
signals. These give reasonable results from VLF to 500kHz without any tuning
adjustments.
Cheers, Jim Moritz
73 de M0BMU
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