TheRadioBoard

[Dan McGillis]

Hi all.

Here’s yet another “Mystery” set variation.

Paraphrasing one of Dave Schmarder’s comments --- I’ve shamelessly stolen ideas from everyone. If you’ve ever posted advice, an idea, or a result on Rap’nTap or Dave’s RadioBoard Forums, you might recognize it in this set.

This mystery set uses a ferrite rod with the 2 coupled coils wound on top of each other - not bifilar wound. Simple and easy. About 21’ of 165/46 litz is used for the 2 coils.

The litz, plus a fairly low-loss ferrite rod and a good tuning capacitor results in a one-tuning-knob crystal set that’s versatile and a lot of fun to play with. It has enough “oomph” for some daytime stations to be able to drive a speaker and be heard across the room. Or, enough selectivity can be switched-in to make it a pretty good nighttime DX set. It tunes from about 520 khz to 1700+ khz, regardless of antenna length. Take the ferrite rod out of the coil, and it tunes from about 1.5 - 7 Mhz for some band spread and short-wave listening.

With 60’ of wire up about 20’, I heard:
-- with a daytime, single 10 minute band scan,- -15 stations.
-- two of these would drive a speaker.

With 200’ of wire thrown over a tree near the house, I heard:
-- with a nighttime, single 20 minute band scan,- - 24 DX stations.
-- plus 6 SW stations and a time signal.
-- (with no short-wave “ghosts” in the medium-wave band.)

As Mike Tuggle and others have said about this kind of set, it’s not as good as a decent double-tuned arrangement. But it’s versatile, simple and fun to experiment with.

The schematic and breadboard of the set are shown below. (The breadboard isn’t pretty, but it works well.)

73, Dan

[Dave-N2DS]

I like that one, Dan.
I can almost hear "Winchester Cathedral" coming out the speaker from here.

I've built a few "mystery type" sets. They are a lot of fun to operate because they
don't tune like the usual crystal sets do. While not the most selective, they are
probably one of the loudest sets that one can build.

I've never compared the bifilar winding to the "Telefunken winding" of one coil on
top of the other.

Dave
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[peeblesoriginals]

Dan,

That's nice. . .

The "Telefunken-type" secondary winding is far better, in my opinion.

I don't use any insulation material between the windings. Use the same gauge for both windings, and wind the secondary in the middle of the primary. The secondary will fall into the "threads" of the primary, utilizing the wire's own insulation material. This gives maximum coupling, for maximum sensitivity.

Mystery/Telefunken sets are fun, and good performers for single-tuning sets.

Thanks for sharing that, Dan.

BTW: You can use a 39" whip on top of the primary, this gives you a great set for locals. You need to use very good construction practices with plastics, quality variable, hi-"Q" coil, etc, to obtain max. performance here. I think you move the ground to the bottom of the primary, but you can experiment this for maximum performance. The whip will add some capacitance to the "tank", so removing a turn or two on the primary may be needed to obtain the top-end of the band. Once had a set like this and was amazed at its performance.

Mike
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[Dan McGillis]

Thanks guys -- nice comments indeed from 2 icons.

Dave, yes - strains of "Winchester Cathedral" from oldies radio AM740 out of Toronto on a crystal set. Doesn't get any better than that!

Mike. Thanks for the whip idea. I have one around here somewhere so I'll try that. Would make for a nice radio.

BTW, if you put a variable capacitor between points 1 & 3, ie. add some more capacitive coupling, you can squeeze out more volume - at the expense of selectivity. You can really drive that little funnel speaker.

The selectivity of this little set is pretty impressive though when you switch-in the antenna coupling capacitors. For example, it easily separates 3 fairly loud stations here at 1050,1060 and 1100 khz.

Fun stuff.

73, Dan

[homebrew]

You have the first half of a antenna booster circuit. Hook a diode to 5 and a .0001 cap in series to ground to complete the circuit. It works extremely well in some xtal circuits and has minimal effect in others. Originally designed for Radiola and AK sets with no antenna tuning circuits.

[exray]

I like the clever band changing technique

-Bill

[Dan McGillis]

Thanks fellas.

Homebrew, I tried your suggestion, but heard no effect on this set-up.

73, Dan

[Dan McGillis]

Hi all.

To complete the show-&-tell, here are pictures of the finished Mystery Set. I made the case large so the coil had room to breathe -- trying to preserve Q. The left-to-right controls across the top are for: antenna coupling, tuning, dc load (Benny), ac load (Bogen tap).

The boxed-up set works very well for both daytime listening and nighttime DX. I have it on all the time and use it as a comparison baseline. Fun stuff.

73, Dan

[golfguru]

Nice work Dan.

[Bagna]

Dan, I am curious about the thinking behind your use of disk caps/switch in preference to a variable in this setup?

Bagna
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[exray]

I've found, with my particular antenna of ~115 feet, that around 20-40uH of loading helps it a lot on the lower end of the band. Never had SW ghosting problems but suffice it to say that the inductance may serve a dual purpose.

As for the series switchable caps...Again, in the context of my own antenna length, I tried that once and it didn't really have much effect. Certainly not enough to merit a variable cap. The idea is to unload the ant ckt from the tank as a tradeoff for volume vs selectivity and the numbers change across the band.

This is one of those jellowy-ish things thats hard to nail down because of different antenna lengths, different tank Q, different freqs. In my experience that cap is quite broad so that a couple/few selections does the job adequately.

I'll let some one else add up the 4 series caps and tell us what the total result is. The coupling is quite small.

[Dan McGillis]

golfguru, thanks for the nice comment.

exray -- your description of the situation is exacly what I ran into. FWIW, the total capacitances are in the lower right-hand-corner of the schematic.

Bagna, there is no deep thought on my part. I used other people’s ideas -- I’m a novice. Here is the evolution - sort of.

a.) My goal was to have a single tuned radio that could be easily changed from loud (sensitive) to fairly selective - and preferrably with only one tuning knob. I have 1 daytime station I like to listen to on a speaker (funnel + piezo), but I also like to casually listen to DX at night (with piezo headphones) and more selectivity is needed then.

a.) Dan Petersen’s and Mike Tuggle’s Mystery Set work pointed me in the direction to originally use 2 variable caps. One across the coil (Cp) and one in series with the antenna. That set-up workrd very well. A dual gang capacitor would have worked ok - but I didn’t have one. And, with my antenna-ground, the variable cap in the antenna line seemed to be non-critiacal as to it’s exact value. Lots of capacitance gave me more sensitivity, and less capacitance gave more selectivity. Furthermore, the most sensitivity occurred when the antenna bypassed the in-line variable capacitor completely and was connected directly to point 2 on the schematic.

b.)So, I was looking for a way to switch the antenna variable capacitor in-&-out so I could connect directly to point 2 and get maximum sensitivity when I wanted it.

b.) Steve B had suggested in one his posts that ceramic disk caps in a string worked well as antenna coupling caps. So I “borrowed” his idea as a way to switch-in capacitance - or - switch directly to point 2. I used a signal generator and plotted the audio output voltage versus in-line capacitance at various frequencies (600 khz, 1100 khz, and 1600 khz) to get an idea of what capacitance values were needed. This arrangement gave me one tuning knob, plus a way to switch-in degrees of sensitivity/selectivity regardless of the tuned frequency (something I couldn’t do if I used a dual gang variable cap). Being able to “switch-in” only as much selectivity as is needed at the moment is proving to be useful.

So, that’s the story. It’s all other peoples’ ideas that they’ve posted here or on Rap’nTap, or they have published on the web.

73, Dan

[Bagna]

Thanks for the info guys. I tried a cheap (poly ?) cap on the Peterson rig and noticed benefits as you pointed out. Think I'll try the disks to see differences in performance, and then build another rig based on some form of the "Mystery" concept. Reception now is not all that great, so will work on this after things improve.

Bagna
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Activity manifests the essence.

[krystallo]

Hey Dan Great stuff.

What would be the (approximate) maximum amount of coupling you'd try between points 1 and 3?

de K

[Dan McGillis]

Hi K:

Thanks for the nice comment.

Here's what I've found so far re increased volume by coupling points 1 & 3. I suspect this effect is very dependent on your particular antenna-ground.

a.) the increased volume effect seems to require the 40 uH SW trap in the antenna line. (My antenna is ~ 200'). This is in line with exray's comments about his experience.

b.) A capacitor of about 200 pf (air or polystyrene) between points 1 & 3 almost doubles the volume. AND a direct hard-wired connection also works on this particular set-up. Again, caveat emptor.

c.) Connecting points 1 & 3 GREATLY decreases selectivity and shifts the tuning capacitance required to higher values.

Except for this "point 1 & 3" connection business, the radio is very well behaved and seems to tune about the same REGARDS of the antenna or SW trap I hook to it.

I'll also mention that I've tried various turn ratios of L2 to L1 (which is 56 turns). As the # of turns on L2 went from 27 to 40, the audio output increased a small amount (~ 1.5 dB), the rf bandwidth decreased a bit, and a lower Bogen tap was required for maximum audio. This suggests a better impedance match to the FO-215 diode with more turns on L2 -- for this set-up.

Fun stuff!

73, Dan