Message-ID: <5.0.2.1.0.20021112173802.026003d0@pop.mindspring.com>
Date: Tue, 12 Nov 2002 17:50:30 -0500
To: rsgb_lf_group@blacksheep.org
From: =?iso-8859-1?Q?Frank_Gentges_K=D8BRA?= <fgentges@mindspring.com>
In-reply-to: <5.1.0.14.0.20021112133603.00a927d8@gemini.herts.ac.uk>
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Subject: Re: LF: Re: RX Frame Aerial for 136kc/s - still  experimenting.....
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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
<fgentges@mindspring.com>
LF web site at <http://www.amrad.org/projects/lf>