Message-ID: <24E8E1045C204E9F810F0CF78CD6632E@gnat>
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <ccfd05230bf040948d51e71bc87ea859@kabelmail.de> <56A1372A.7090504@posteo.de>
Date: Thu, 21 Jan 2016 23:31:30 -0000
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Subject: LF: Re: How is this possible?
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Status: O

That is how you put out such a good signal Stefan :-))

Any wire entering the foil is a way into the inside. Even the folds are not 
proof! currents which circulate round the edges of the foil can be induced 
and these can radiate into the package.  A double skin with the two not 
connected to gether may be needed. Even the makers of expensive screened 
rooms have trouble at these levels. Ideally I suppose it should be copper 
soldered continuously round the seams .....then think of the skin depth at 
your tx frequency as well......-180dB is quite difficult to achieve :-((

Alan
G3NYK

----- Original Message ----- 
From: "DK7FC" <selberdenken@posteo.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Thursday, January 21, 2016 7:53 PM
Subject: LF: How is this possible?


> Hi all,
>
> While TXing WSPR i'm trying to shield my MF RX in about 30m distance to
> the antenna. See attachment. Inside this aluminium foil there is the RX,
> a USB stereo soundcard and a 9V-battery plus a DC/DC converter which
> produced 5VDC to supply the RX. The cable you can see is the USB cable
> going to the PC where i'm watching the input in SpecLab. When TXing, the
> signal is still detectable, 25 dB in 1 Hz.
> How is this possible?
> The foil is connected to the USB ground = the ground of the metallic box
> of the RX. The RX stereo input is made out of 2 RG174 cables which are
> terminated with 50 Ohm BNC resistors. The complete power supply is below
> the foil.
> The construction is inside a large building with massive walls!
>
> Does the foil construction somehow act as a probe, catching a common
> mode current which passes an impedance between ground and "earth" so
> there is a common mode - differential mode conversion????
>
> What is the solution? Bridging ground and "earth" or building a low pass
> filter for the USB supply cable?
>
> 73, Stefan
>