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From: Bill Steinhour
Date: 11/11/00
Time: 3:14:58 PM
Remote Name: 152.163.204.196
Ben,
Ken's advice to be sure impedances are matched between stages in the receiver is right on target. One of the best articles I've seen to date on practical crystal radio circuits is the "Circuit Circus" Column in the Nov. 1996 issue of Popular Electronics magazine. Charles Rakes, the author, gives diagrams, instructions and explanations for six progressive crystal radio circuits, a couple of which I hadn't seen published elsewhere. He starts by giving winding information for the two coils used for all the projects along with pictorial diagrams with measurements.
Then, the first radio circuit using one of the coils is a simple untuned broadband receiver for checking to see if your antenna system is adequate to receive strong signals.
Each succeding circuit is an example of another improvement for better performance, including tapping of the coils to better match impedances, use of parallel and series resonant circuits, an antenna tuner configuration frequently used to match the impedance of your antenna to a receiver. Then figure 7 shows a double tuned series resonant combination that, in addition to matching to the antenna, matches the tuning stages to the relatively low input impedance of the crystal detector. His circuits also use choke coils to offer a DC return path for the rectifier circuit which helps sensitivity.
The last configuration he presents is more of a novelty circuit than an improvement for sensitivity or selectivity. It simply center taps the coil of a parallel resonant tuner which allows using two rectifier diodes to form a voltage doubler. While selectivity will suffer as a result of loading both polarities of the received sine wave, it does provide audio drive that can power loudspeakers if strong local stations are available.
Other improvements for honing your circuits for the last degree in sensitivity and selectivity involve winding coils for maximum Q (Quality factor) and minimizing detector and audio transducer losses. Ben Tongue's site addresses these last two issues elegantly and in in depth.
High Q winding techniques include (1)using either large-guage wire or many-stranded litz wire to reduce resistance and impedance losses and (2)separating each turn by an air gap to reduce distributed capacitance losses (as in basket-weave or the more effective space-wound coil configurations).
Ferrite or pereferably, iron powder cores in coils help Q by requiring shorter lengths of wire for a given inductance value (thus trimming wire resistance/impedance loss). However, the typically close spacing of turns and their close proximity to the core material introduce distributed capacitance, offsetting to a degree the increase in Q gained by lowered resistance.
I recently noted this demonstrated on a japanese website where the author indicated that a 32 turn coil space-wound on a 10" dia. wastebasket with what appeared to be about #12 AWG wire beat a ferrite-core tuning coil in performance.
Silver plating the coil wire to the "critical depth" thickness for the frequency involved (thinner as frequency increases) helps since silver offers ten to twelve percent less resistance to current flow than does pure copper. Gold plating would offer even less resistance if someone out there can afford to try it.
So - that's my two cents worth on gaining sensitivity and selectivity. The best treatise I've found that explains all of the above (and much, much more)is ELECTRONIC COMMUNICATIONS, by Robert L. Schrader.
His section on impedance matching radio-frequency amplifier stages shows a number of ways variable inductors and capacitors can be adapted to design tuned and untuned coupling circuits for extremely high performance crystal sets.
By the way, on a recent trek to the east coast, I stopped by Ocean States Electronics in Pawcatuck, Rhode Island to check out their selection of variable capacitors - Wow! what a selection.
The best deal I saw was a triple-gang TRF unit with each section variable from 15 to 500 pF. Prior to that visit I would have given at least one of my family jewels to find one. Now I know I only need part with sixteen bucks.
Ocean state also stocks just about everything else, routine or esoteric, that a radio experimenter/ham operator could want. They're on the web at www.oselectronics.com. Be sure and tell them I recommended Ocean States to you and that I send my best regards.
Good building and better listening, Bill S.