[sdiy] 100 MHz EMI, what can it be?

[Czech Martin]

If I connect the (long) ground clip of the oscilloscope
probe to the tip, I get a nice loop.
I can see 10 mVpp, around 100MHz. No matter how I orientate the loop,
it will not disappear. 

What can this be? Computer is of, cell phone is much higher,
wireless phone should also be higher.

Of course, connecting the probe tip to shield via 5mm wire
will make the noise disappear, so it is clearly something
from outside the scope.

I live near a railway track, is this railway communication radio?

m.c.