I've just read the review of the welbrook Loop in the latest RadCom,
and wondering if anyone here knows the preamp configuration.
There are two ways of doing it...
Hopefully, and almost certainly the best solution has a broadband
low input impedance preamp, ideally with balanced input giving a flat
field strength response. I did one such loop+amp many years ago for
work, using a pair of 2N5109 devices (or may have been 2N3866, can't
recall which) in common base with a 0.3m diameter loop between the
emitters. It worked well enough . but all we wanted then was for
broadband horizontal polarisation skywave HF, omni coverage, so
the loop was used horizontally.
This approach, if the amp has zero input impedance (short circuit)
gives a flat OP vs. field strength characteristic.
The amp design is made quite a bit easier if a step up transformer
were used to couple the loop into the preamp; that would also solve
isolation / balance issues, but the transformer would have to cover
30kHz to HF - not that difficult now, but still needs the right choice
of ferrite.
These days with high frequency op-amps with good signal handling, I
suspect it may be possible to synthesize a virtual-earth input design,
giving the low impedance without having to resort to common base
push-pull.
Other way of amplifying the loop ouptut is, simpler but with a big
disadvantage: terminate the loop with a high input inpedance
amplifier. Now, the output voltage is proportional to frequency which
means massive output signals at HF, especially high HF, and very
little at LF. Which isn't necessarily as bad as it sounds as LF has a
lot of background noise anyway, but I suspect signal handling due to
the high HF sensitivity could be more complicated. The amplifier
response could then be flattenned, and if this were done on the input
stage would solve the strong HF signal issues - but it all seems a bit
tacky and unprofessional - a bit of a bodge.
So, does anyone have an inkling which way Wellbrook do it?
I'm guessing low-Z flat response, and possibly with an op-amp or some
virtual earth solution
Andy
www.g4jnt.com
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