Antenna engineers use to say about design goal limits of antennas:
Small
Efficient
Wideband
Pick any two (meaning you can't have all three...).
73
Clemens
DL4RAJ
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Saturday, August 20, 2011 12:07 PM
Subject: LF: Re: Ferrite wideband antennas?
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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