TheRadioBoard

6f6 · Joined: 15 Jan 2009 Posts: 7 Location: Illinois, USA

I want to build a crystal set which is a little bit more selective and somewhat easier to tune than the usual variety (like Dave's #12). My idea is this: wind two coils on a form, one clockwise; the other of equal size counterclockwise. Have them butt up against each other on the coil form and join the two ends that meet in the center. Connect this center tap to the common point on a dual gang variable cap. Connect the other two ends to their respective ends on the variable cap. You connect one tank circuit to the antenna and the other tank in series with the diode and earphone. Do you think that this will work?

Norm_N_Tam · Joined: 19 Jul 2008 Posts: 252 Location: Connecticut

Your idea sounds kind of like a contra coil tuner to me. Is that correct?

http://www.makearadio.com/coils/contracoils.php

Norman
KA1GUK

6f6 · Joined: 15 Jan 2009 Posts: 7 Location: Illinois, USA

That is something like what I'm talking about. The coils would be wound around a cylinder like the one Dave wound around the PVC pipe on web page you linked, but the hookup is even simpler: basically two tuned tanks, like an Intermediate Frequency transformer in a superhet. Unlike the IF transformers, these will be continuously variable. Unlike a superhet, there will be no amplifying devices; therefore one end of each coil will have to be tied to ground, as will the rotor of a dual gang variable cap. One stator will be connnected to one coil; the other stator will be connected to the coil wound in the oposite direction. Again one tank will connect to the antenna and the other will connect to the diode and earpiece. Will this work? If so, would there be any adavantage or disadvantage giving minimal spacing (1/8 in or less) between the two coils?

exray · Joined: 30 Apr 2007 Posts: 1806 Location: Vieques, PR

I visualize this as two opposing fields. I think you would want them wound in the same direction although you'll get some degree of coupling regardless.

Another caveat is the ganged tuning capacitor. The antenna is going to introduce some capacitance to the 'ínput' side of the scheme so you cannot depend on that section of the cap to track with the other one...unless you're particularly lucky. I'd say try two individual caps.

How are you connecting the antenna to this configuration?

J.B. Weazle McCreath · Posted: Wed Mar 10, 2010 6:18 pm Post subject:

6f6,

What you describe with the two coils wound in opposite directions and tuned
by a ganged cap is similar in design to a 160 meter antennas which is in the
ARRL Antenna Compendium Vol. 6. It is called the Optima receive antenna.

When I first read the article I was puzzled by the two windings being wound
in opposite directions, but it apparently works quite well as designed, so you
may be onto something with your adaptation.

Good luck with it, and do be sure to let us know your results.

Cheers, J.B.
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6f6 · Joined: 15 Jan 2009 Posts: 7 Location: Illinois, USA

The fields would oppose if wound in opposite directions? I'll take it that you are correct here. The antenna would be connected through another variable cap- here I'll admit that I didn't figure on the capacitor and antenna detuning a crystal set as much as it would a Doerle-type regenerative set.

6f6 · Joined: 15 Jan 2009 Posts: 7 Location: Illinois, USA

I guess I'll have to wind two pairs of coils and see which works better.

exray · Joined: 30 Apr 2007 Posts: 1806 Location: Vieques, PR

In a 2-circuit set like you describe the coupling is a factor. What would make a good experiment is to wind them on 2 separate forms in a manner that you could experiment with. Twist it around, vary the coil end coupling, etc. With extremely high-q coils you might find the best selectivity with them 12 inches apart!

gusnaz · Joined: 07 Nov 2006 Posts: 783

exray · Joined: 30 Apr 2007 Posts: 1806 Location: Vieques, PR

I suppose you could. The effects wouldn't be as obvious with a lower Q coil. Then, if you rewound with litz, the ideal spacing would probably change.

6f6 · Joined: 15 Jan 2009 Posts: 7 Location: Illinois, USA

12 inches apart and it still trandfers signal?- I don't expect any miracles here. All I have for an antenna is 20 of wire run up the side of a tree. As far as coils are concerned, I plan to use magnet wire, I start with 22 gauge and go down to 14 or 12 after I'm sure that what I'm attempting to do will work. BTW, you guys don't happen to know of anyone who sells enameled 10 gauge (for shortwave above 8MHZ)?

gusnaz · Joined: 07 Nov 2006 Posts: 783

6f6,

10 ga enameled? Ready for this,,,,,,,,,,,,, just googled it for you..

http://www.magnet4less.com/product_info.php?products_id=175

Bottom of page gives you a price! Ouch! Might want to rethink about this! Wink

You might try an electrical outlet supply or local electrician for a small quantity.

Gus
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6f6 · Joined: 15 Jan 2009 Posts: 7 Location: Illinois, USA

I did some searching and found a place that would sell 16ft for $15.95- not too bad.

homebrew · Joined: 15 Jan 2008 Posts: 397 Location: Dallas, Tx.

Quote
I start with 22 gauge and go down to 14 or 12
As a general rule you lose selectivity as you increase the wire and former sizes on short antennas at broadcast frequencies.

Broesel · Joined: 07 Apr 2008 Posts: 287 Location: Villach, Austria

6f6

sounds as you want to make a kind of a bandpass radio since you want to have just one tuning cap. The latest one of Dave was such kind of radio, maybe some of your questions are already answered there.
http://makearadio.com/crystal/76.php
Otherwise I got something wrong...