James,
so I have some clue to try the formulas in Electronics World article 8/2002....,
and tnx fer your thoughts about screening and grounding.
You don't need a complete screen, a wire screen should work. Believe a french
named 'Rouse' made some work on this.
But, with 80Hz bandwidth (Siemens D2006 Level meter) or using my FT-902
(500kHz BW) with 74HC4066 converter
the background noise peaks considerably on the S-meter when antenna is tuned to
resonnance. The antenna-tuning is pretty sharp and you'll note when it
is on the side of resonnance. So I really doubt that a pre-amplifier could
improve as long as the background noise level is so high, and I live in a
suburban area, don't believe it is so much man made noise there.
Discovered that when Siemens D2006 was modified, I would have been able to use
it
for CW reception on 17.2kHz for SAQ using 80Hz BW filter.
The noise level would have been improved by 30dB instead of using 3.1kHz bandwidth,
but it is a lot of noise from different local sources down there, particularly
around 18kHz
I used an active antenna with 1m rod and the signal to noise was 6dB, inspite
that
the signal was 1mV! hi
73
Jan-Martin
LA8AK
-----Opprinnelig melding-----
Fra: James Moritz [mailto:[email protected]]
Sendt: 12. november 2002 15:24
Til: [email protected]
Emne: LF: Re: RX Frame Aerial for 136kc/s - still experimenting.....
Dear Jan-Martin, LF Group,
At 12:03 12/11/2002 +0000, you wrote:
Signal strength last night for DCF39 was 80-150µV (-75...-80dBU)
At your distance, the field strength of DCF39 should be roughly +60 dBuV/m
- so if under quiet band conditions the band noise is around 0dBuV/m in a
bandwidth of a few hundred Hz, the receiver will have to have a noise level
below about -135 or -140dBu to achieve optimum sensitivity. So you might
need a preamp!
Not sure if it is still a point to ground the center of the tuned loop.
I have used several loops here for LF reception, and have never found any
advantage in grounding the loop centre tap, allthough in theory it ought to
improve rejection of E-field signals. That might just be because of the
nature of the QRM at my location, though. I have to say I am skeptical
about the idea of E-field rejection being an important benefit in reducing
noise pick up at LF. In fact, I find a small, screened, LF loop to be quite
a good mains cable detector. The big advantage of a small loop RX antenna
over TX type antennas is that is easy to move the loop around until you
find a position where the noise level is low - but you can do the same
thing with a whip antenna. The other benefit of the loop is the directional
nulls, of course.
Cheers, Jim Moritz
73 de M0BMU
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