A few years ago I made an active broadband loop antenna for HF for
'official' purposes. This consisted of a 300mm diameter loop of 3mm
copper wire into a common base amplifier made from a 2N5109 transistor
operated with 50mA collector current. The input impedance was about 0.5
ohm with a collector load of 50 ohms, and no negative feedback was used
for a reasonably flat response across the higher part of the HF
spectrum. Performing down to 2MHz at least, although sensitivity was
falling off there it still gave more than enough signal to hear
background noise.
This loop vastly outperformed the commercial Procomm active whips we
were using at that time as HF collection antennas and even performed
better than a homebrew active E Field plus high impedance amp design,
based on one in VHF communications some ten years ago (which itself
easily outperformed the Procomm). Furthermore, it was immune to the
huge locally generated interference levels on this hill overlooking the
industrial areas of north Portsmouth !
The ony reason we didn't go ahead with a whole array of these and
instead stayed with E Field whips was that the loops were directional
against vertically polarised signals. Which was something definitely
not wanted at the time. However its omnidirectional properties for
H-Pol signals when mounted horizontally made it a good antenna for low
angle skywave detection.
Such an amplifier with a 1m loop consisting of a few turns of wire would
make an excellent LF antenna and could cope with strong signals up to
nearly 1 Watt input before overloading. Purely for LF work, an op-amp
with feedback could be used and perhaps an even lower input impedance
can be achieved.
Andy G4JNT
This is compensated by the negative feedback .
The active antenna has a frequency response that is flat from 5kHz to
over
30MHz. It stays linear in field strengths up to 10V/m. Great attention
has
also been given to proper noise behaviour ("noisemanship"). The input
is
protected by diodes against high voltages. Because signal voltages at
the
input are very small due to the low input impedance little signal
current
flows into the diodes and they do not impair the favourable
characteristic
of the active antenna.
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