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From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Sat, 20 Aug 2011 11:07:52 +0100
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Subject: LF: Re: Ferrite wideband antennas?
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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