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 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 > > __________________________________________________________ > > 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 http://www.seismicnet.com/maillist.html 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 23236+1004 __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>