Rap 'n Tap
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From: Lawrence Pizzella
Date: 9/3/99
Time: 9:48:45 PM
Remote Name: 165.236.189.200
Hi Jim,
You did not state how close either of these stations are. If the 50kW staion is close (<10mi) and the 1kW station far (>20mi) you might have to throw in the towel. At a spacing of only 20 kHz things are pretty tight.
I live in the city with lots of strong stations nearby, and have done quite a bit of experimenting with selectivity with rather good results. While I wouldn't say I have achieved superhet (ie table radio) selectivity performance, I have had good luck with multituned and trapped designs as Owen described. Be sure to check out his site for lots more info. In a sentance: "Big high Q coils, Loose Coupling, multiple tuned circuits and traps, and a light load on the detector coil, are what you need."
If you are willing to stretch the definition of a XTAL set a bit, you can also experiment with a Synchronous switch instead of a diode for the detector, but this is usually more complex than most people (other than rockhead nuts) wish to get. A synchronous switch that doesn't provide any power amplification is considered an "acceptable" detector for a Xtal set by most purists, even though it deviates from the notion of simplicity that is the great attraction of Xtal sets.
As to why a table radio (Superhet) has more selectivity, it is fundamentally because of the Passband charecteristics in the Intermediate Frequency (IF) amplifiers. High quality Superhets use quartz crystal, mechanical, or ceramic filters in the IF stage (or stages) to limit the received bandwidth. Todays consumer radios usually use a ceramic resonator or resonators to limit the IF passband. It has essentially nothing to do with loading of loopstick antenna coils, or any other such thing. Regenerative radios are another matter.
If you are successfull at seperating these stations with a Xtal set, we would all be interested in how you did it. --Larry
From: Nick Leggett N3NL
Date: 3/18/99
Time: 10:27:15 PM
Remote Name: 168.191.217.48
It is reported that some nations use their embassy locations as radio listening posts for intelligence purposes. If they use the conventional superheterodyne radio sets, other agents can receive their local oscillator radio frequency emissions and determine the frequencies that the embassy is listening to. However, if the embassy uses a crystal set there is no local oscillator. So the counter-intelligence people outside cannot determine the frequency observed by the crystal set. The sensitivity of the crystal set can be greatly increased by using an audio amplifier. Who said the crystal set was obsolete? Similarly, a hidden crystal set could be used by agents listening to shortwave numbers station broadcasts. This same approach was used in World War II to keep the Nazis from finding hidden radio receivers. Nick Leggett, N3NL