Dear Daniele,
At my locaton in eastern Bavaria I am
using
such a vertical broadband loop pointing north from 10 kHz to 500 kHz
since several years and have also made experiments with a
horizontal loop
covering 1,8 to 30 MHz.
Their principle is as
follows:
A broadband transformer is needed between the loop
and the cable going into the shack.
Without such a transformer
the loop
inductivity has to be so high when feeding usual cable impedances
that a
ferrite core will be needed to realize the inductivity needed for
broadband
performance. I have used wire loops only so far.
The inductive
reactance
of the secondary of this transformer should be 4 times the cable
impedance (3.2
mH for 10 kHz). The loop circumference should be low in length
compared
to a quarter of the highest frequency (for VLF to MF usually no
problem,
for HF the circumference had been 2
meters).
The inductivity of
the primary of the broadband transformer should be equal to the
inductivity of
the loop (in my case 30uH and 17 meters circumference for 10 to 500
kHz).
When meeting these conditions, the inductive reactance of
the loop
itself will always be higher by a factor of 4 at least
compared
to the cable impedance transformed down by the turns ratio.
This is
the condition for the loop to operate broadband.
But the problem
for the
receiver may come in the shack: It must be capable of taking all
the power
delivered by the broadband loop, including the power received by
broadcast
transmitters! Otherwise the receiver may be overloaded and may need a
more
linear frontend or has to be equipped with a good front end
selectivity.
AT VLF to MF a long cable
into the
shack usually will be no problem because of its low attenuation at low
frequencies, therefore all additional selectivity I can provide in the
shack.
For HF and higher the situation is different of course and may need at
least
part of selectivity and preamplification already at the loop
output.
For
the time being however, I have problems using my loop for the reception
of SAQ,
since a solar panel had been installed on a roof two houses from
here, in
autumn 2010. Obviously the chopper of this installation is generating a
strong 300 Hz spectrum up to 20 kHz in my loop. As I could
clear interference into my active antenna completely the
interfering
field should be inductive. It seems to come from several
directions
(including the wiring of three houses because the installation
is feeding three phase current into the
mains)!
OK?
73 Ha-Jo,
DJ1ZB
"Daniele Tincani"
<[email protected]> schrieb:
Hello LF,
a very interesting discussion is in progress on the reflector on
ferrite
rx antennas. My understanding however is that until now we have been
talking
about narrow-band antennas. May I ask the experts if a wide-band
ferrite
antenna could be feasible, able to (partially) match the
"panoramic"
approach of SDR receivers, at least in the LF/NDB and/or MW bands?
Such an
"ideal" antenna would combine some advantages of air-cored loops with
a reduced size (uhm... too "ideal" to be real :-))
Regards
Daniele
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