LF Group,
I think Jim has it pretty much right. I would add the point that the
ground and shielding will not improve the signal strength much if at
all. What it can do is improve the depth of the rejection off the nulls
off of each end. The ability to turn and tilt the loop receiving antenna
to reject a noise can be useful and may be the most important property of a
receiving loop over an e-field probe. The depth of that null can become
important.
This aspect can be tested by observing the amount of rejection of a strong
station comparing the strongest level and weakest level. The null point
will become sharp and require fine adjustments in both azimuth and tilt to
achieve the deepest null.
A poorly shielded and balanced loop will have shallow nulls. I expect at
least 60 dB nulls.
Frank K0BRA
At 02:24 PM 11/12/02 +0000, you wrote:
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
Frank Gentges KØBRA
<[email protected]>
LF web site at <http://www.amrad.org/projects/lf>
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