I just wanted to bring up two points.  Please re-read my suggestion that
you add some series impedance to the lines at the entrance to your
pre-amp and then do your bypassing.  This has a two-fold purpose =96 it
increases the series impedance from the antenna structure formed by the
seismo lead-in wires so that the bypass caps you use don=92t have large
circulating current in them.  This helps if the ground structure of the
preamp area is not real good.  Secondly it allows a higher attenuation
in the single section lo-pass you=92re attempting to create.  In many
instances, the attenuation is the ratio of the capacitance of the
lead-in wires to ground, some hundreds of pF probably in your case, to
the bypass cap value.  With the use of a resistor, this value becomes a
pF or two, making possible higher attenuation ratios.

The second point is where Chris Chapman did a brief field strength
calculation in part and wrote: =93This gives E =3D 0.23 V / m=94   This i=
s OK
as far as it went, the point I want to make is, that in practice, this
near (2 miles) to the station, the AC power lines play a big part in the
perceived signal levels.  Think about how AM radio fades in and out as
you approach power lines sometimes. So if you=92re on the same AC feeder
line, you can have considerably higher levels.

A simple test is to take just about any meter, put it on the AC scale,
and measure from your preamp ground to the seismo lead-in wires.  The AC
voltage measured is almost certainly due to the broadcast station.  Just
about any meter should work, as it takes effort to cut-off the frequency
response in the AM radio band.

Charles R. Patton


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