Rap 'n Tap
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From: Owen Pool
Date: 8/30/99
Time: 9:16:15 PM
Remote Name: 207.124.26.204
Jim, it depends on how close they are together, but you can often get the small guy by (1) having a reasonably selective set (2) using an antenna tuner - another stage of filtering if you will and (3) a wave trap to cut the big guy down to size. Try tuning the trap onto the big guy from the side opposite the smaller station, and you may be able to cut the signal enough to separate them. A wave trap inductively coupled to the sets tank circuit does nicely for this, as you can vary the coupling as well as the sharpness of the null. If you cut the big station too much, you may supress the small guy as well. Good luck. You can also try triple tuning, but that's a whole level of difficulty, unless!, first tune the trap to the frequency of the weak station (may need a signal generator to do this), and then insert the trap between the antenna tuning coil and the set's tuning coil. You may still need a trap to null the big guy some; note that you are now tuning 3 and more circuits at the same time. Good Luck, Owen http://thebest.net/wuggy
From: Steven Coles
Date: 3/16/99
Time: 8:21:12 PM
Remote Name: 207.149.222.158
Tim,
1) What will work as an antenna depends on a lot of things including the distance to your local radio stations and their power. See Owen Pool's web site for a link to this information.
2A) A vertical wire 1/4 wavelength long (75 meters at 1.0 MHz) should work extremely well. Shorter will work proportionally less well.
B) The standard for flat-roofed schools around here appears to be bribing the wood shop instructor to arrange 2 2x2 poles about 8 feet tall each with crossed 2x2 feet. These are placed on opposite roof corners with sandbags on the feet. Hang the wire antenna between the poles. There shouldn't be any problem placing 3 or 4 antennas on a large flat roof.
C) Yep. When you hook 2 xtal sets to the same antenna, the tuned circuits are in parallel. It acts like one tuned circuit with twice the capacitance and half the inductance. A distribution amplifier would solve this problem and give potent signals to test the sets on. However, designing and building a wide-band amplifier is a rather extreme solution. I'm sure others have better ideas.
3A) If your building's that well shielded you could try building 0.1-watt transmitter and sending the signals from one end of your classroom to the other.
B) The Xtal Set Society carries a very good collection of books. If you want something a little sketchier, but free, try some of Owen Pool's links. Most of the links assume the students either have a little electronics background or have substantial mentering.
C) You should be able to receive local short-wave stations 24 hours per day. Daytime reception of more distant stations depends on the ionosphere, which in turn depends on the sunspot cycle, seasons, time of day and chance.
Steven