PSN-L Email List Message

Subject: Re: it happened again
From: Bob Smith bobsmith5@........
Date: Wed, 26 Jul 2000 07:51:06 -0400

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 g
oes 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

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
> __________________________________________________________
> Public Seismic Network Mailing List (PSN-L)
> To leave this list email PSN-L-REQUEST@.............. with
> the body of the message (first line only): unsubscribe
> See for more information.


---------  Avoid computer viruses  --  Practice safe hex 
 * * Specializing in small, cost effective embedded control
systems * *
Robert L. (Bob) Smith			Smith Machine Works, Inc.
internet   bobsmith5@.............. Lumlay Road
landline   804/745-1065	                Richmond, Virginia

Public Seismic Network Mailing List (PSN-L)

[ Top ] [ Back ] [ Home Page ]

Larry Cochrane <cochrane@..............>