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THE MYSTERY CRYSTAL SET by Joe Patrick. A few months ago, I was asked by Chris Wells, PARS President, if I would be interested in “coming up” with a club.
Table of contents
- Upcoming Events
- Dave's Homemade Radios Crystal Set #49 Australian mystery radio with extra bells and whistles.
- An Explanation of how the "Mystery Crystal Radio" Works
- The “Mystery” Crystal Set?
- Mercator: The Man who Mapped the Planet.
- Mystery Crystal Radio circuit analysis.
- Missing Aaron.
- Atomic and Molecular Manipulation (Frontiers of Nanoscience);
- Party Favor.
- mystery crystal radio set - Google zoeken | crystal radio 2 | Pinterest | Crystals and Mystery!
I did not bother with the 0. It serves no useful purpose, other than to increase component count. The cathode of the detector is wired to an output jack on the front panel. I use an audio matching transformer between the detector output and the headphone.
The audio matching unit and headphones are shown below: The matching unit consists of a Bogen T audio transformer, and two 6-position rotary switches connected to taps on the transformer. One switch selects the input tap, and the second switch selects the output tap. The headphones are Western Electric Model D sound powered phones. The elements have an impedance of ohms each, and are wired in series, for a total impedance of ohms.
Even with the low selectivity, the reduction drive still makes tuning much easier. The antenna and ground connection screws are shown at the back on the base board. The underside of the base board has been hollowed out using a router, and the wiring and detector diodes are located there. The vertical wood dowel with the knob on top is used as a support for the leads coming off of the coil.
This is shown in more detail in the following front view: Now, I must confess that these are rather bad construction techniques for a crystal set, if the intention is and it should be to get the maximum possible Q. The results are shown in the following graph: With a maximum Q of just over , it's certainly not a big league set, and not surprisingly, the selectivity is not particularly good. However, the sensitivity is better than I expected, and I have been able to pick up several stations more than km distant.http://pps.vn/components/vabuh-march-16.php
Dave's Homemade Radios Crystal Set #49 Australian mystery radio with extra bells and whistles.
I entered this set in the Radioboard DX Contest , and following is my reception log: The following map shows the locations of the stations received. This in combination with another circuit from Australia, rounds out this design. This is not my first mystery set. My 3 set was also a mystery set. It is wound on a 6 inch styrene form. The coil is supported with four styrene legs that I machined. The two coil windings are bifilar wound in the middle.
This means that the two coil wires are wound in tandem. Other coil details will be made available at a later time. Overall, double tuning this Mystery set increased the signal strength by about 2. As is expected of double tuning, the selectivity is greatly improved over that of a single tuned set. I was very pleased to be able to easily hear the daytime ground wave signal from 5 kW stations at the low end of the band - , , , and kHz - that are 45, 84, 56, and 63 miles away.
Hearing low band stations on a crystal set has always been a problem for me. It picks up and clearly separates virtually all of the daytime stations that I can hear on an inexpensive table radio. This would give a bandwidth of 5 kHz at 1 MHz, which seems reasonable from what I hear listening to the set. To look at this coil issue some more, I did the following experiment using signals from daytime stations.
This makes the set an ordinary double tuned crystal set. And, I listened to close stations at: The output voltage is reduced using the Mystery coil - stepped down - as expected if the Mystery coil is acting as a transformer. Less loading on the tank, with subsequent higher loaded Q and smaller bandwidth. The usual crystal set trade-off is operating - more selectivity at the expense of sensitivity. I tried 8 different diodes in the double tuned Mystery set to see which gave the most audio output.
A 26 mVp-p, kHz signal, modulated at Hz, from an Eico signal generator was fed through a 0. This gave a just barely audible Hz tone in the piezo headset when using the least sensitive diode in the Mystery set. I estimate that the impedance of these elements at Hz is about 6k, about 12k for the headset. The headset was connected to the 20k tap of a Bogen T, so I assume the audio impedance presented by the Bogen was about 24 k. The average bandwidth was found by varying the frequency both above and below the resonant frequency. I did it this way because there was a lot less capacitive loading and detuning by the scope probe.
The Hz audio voltage across the piezo element headset was measured by putting the 10x scope probe across the headset and measuring the Hz peak-to-peak voltage.
This is the maximum sensitivity separation. Audio volume is loudest here.
An Explanation of how the "Mystery Crystal Radio" Works
As the separation is increased further, the signal level starts to decrease - but the selectivity starts to increase, ie. Interesting effects can take place there -- double peaked frequency response, asymmetric frequency response, etc. I stayed out of that area.