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" <[email protected]>
To: <[email protected]>
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
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