PSN-L Email List Message

Subject: Re: it happened again
From: "Tom Schmitt" tschmitt@..............
Date: Wed, 26 Jul 2000 08:24:30 -0400

I am going to foreward this to an old friend who is a Ham and used to run
the Ga Tech seismic network.  He may have some favorite tricks that will

However, a 55 gal oil drum is a reasonably good Farady cage.   Stuffed with
fiberglass it is also has nice thermal properties.  or the cage to work
well, the electronics would have to be fairly deep inside the drum but it
might be wrth a try even if  you could not get it in far.    If the soil is
conductive that should help  with the E field.

Tom Schmitt


----- Original Message -----
From: Bob Smith 
Sent: Wednesday, July 26, 2000 7:51 AM
Subject: Re: it happened again

> John Tacinelli wrote:
> >
> > Well it jumped again tonight, right on schedule.  Fortunately I was
right there listening to KOLM radio, 1570 AM.  It jumped exactly as they
went off the air.  It must be their carrier signal during the day that keeps
it in one place and then when it goes off the signal changes.
Unfortunately, my homemade aluminum foil box shield did not work.  At least
not enough.
> John --
> My antenna theory is 40+ years old so I hope that some
> young, up to date engineer will feel free to jump in here
> and help.
> As I recall, a transmitting antenna emits an electrostatic
> field as well well as an electromagnetic field.  The
> electrostatic field falls off very rapidly with distance (a
> few miles??) and the electromagnetic field will soon
> predominate beyond some number (?) of wavelengths from the
> antenna.  Your aluminum foil shield may offer some
> protection against the electrostatic field, but will be a
> useless defense against the electromagnetic field.  Such
> protection will require a magnetic material - soft iron,
> alloys used to make transformer cores, mu-metal, etc.  There
> was a recent discussion of mu-metal here, a search of your
> archives may be useful.
> A better approach may be to try to filter off the
> interfering RF.  No - no, C4 on the tower legs is not
> allowed! ;-)
> The best defense against interference depends on the
> mechanism of that interference.  Here is one possible
> scenario:
> 1. Your pickup coil may be making a good antenna at 1.57mHz
> and dumping out copious amounts of RF.
> 2. If I understand seismometers correctly, the output of the
> coil goes to a high gain amplifier with response from DC to
> several hertz.  These are usually operational amplifiers
> with very high gain.
> 3. One likely mode of failure is RF entering the input
> stages, getting amplified and eventually rectified in an
> overloaded stage and saturating one of the gain stages in
> the amplifier.
> 4. To protect against this you should filter off the RF
> before it reaches the amplifier.
> 5. 1.57mHz is somewhat awkward to work with.  It is a bit
> too low for simple ferrite beads to be effective, so a
> lumped element filter will probably be necessary.  If I were
> in your shoes, I would start with a simple L section low
> pass filter in each coil lead.  Take two 1000 uH
> (micro-henry) inductors and insert them in the coil leads
> right where they enter the amplifier.  Then take two 0.1 uF
> capacitors and bypass the input leads to ground right at the
> amplifier pins.
> At 1.57mHz Xl for 1000uH is about 10,000 ohms, and Xc for
> 0.1uF is about one ohm.  This will give your prototype
> filter about 10,000 to 1 attenuation at 1.57mHz but very
> little attenuation at 10Hz.
> You must be alert for the possibility that the capacitors at
> the input will make the amplifier mad at you and cause it to
> oscillate.  Often capacitance at the input terminals of the
> amplifier will introduce an unwanted pole in the transfer
> function and cause oscillation.
> Be sure to use good low leakage capacitors such as
> low-leakage polyester film types.  Avoid electrolytics and
> tantalums.  Don't settle for the first thing you see on the
> Radio-Shack racks.  If you need help, I can look up DigiKey
> part numbers for you.
> I am hoping that someone on the list has solved this problem
> and can provide more specific advice.
> Happy witch hunting, Bob Smith
>   The folks at KOLM say they are going to get their 24 hour
> license soon so perhaps I will wait till then and depend on
> their steady signal to keep it centered.  I would like to
> shield it however.  I could add more tinfoil but it seems
> unlikely to help much.  Perhaps a wire cage of some sort?
> >
> > John Tacinelli
> > Earth Science Instructor
> > Rochester Community and Technical College
> >
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> --
> ---------  Avoid computer viruses  --  Practice safe hex
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>  * * Specializing in small, cost effective embedded control
> systems * *
> Robert L. (Bob) Smith Smith Machine Works, Inc.
> internet   bobsmith5@........ 9900 Lumlay Road
> landline   804/745-1065                 Richmond, Virginia
> 23236+1004
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Larry Cochrane <cochrane@..............>