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Gordon McCallPosted - 17 January 2014 9:18  Show Profile  Email Poster  Edit Message
Hi Ray. The arrow on the schematic symbolizes that the capacitor is variable. It doesn't matter which way you connect it.

Gordon

Gordon McCallPosted - 17 January 2014 9:20  Show Profile  Email Poster  Edit Message
Hi Ray. The arrow on the schematic symbolizes that the capacitor is variable. It doesn't matter which way you connect it.

Gordon

Gordon McCallPosted - 17 January 2014 9:25  Show Profile  Email Poster  Edit Message
My apologies for the multiple posts. I was getting error messages when I posted so I kept trying.

Gordon

raypatPosted - 17 January 2014 19:27  Show Profile  Email Poster  Edit Message  
Thanks Gordon.How about tuggle tuning?Is it simply a dual gang variable cap connected in the antenna circuit with one gang in parallel with the aerial and the other in the earth leg?

Ray

Gordon McCallPosted - 18 January 2014 2:14  Show Profile  Email Poster  Edit Message
It's been awhile, so someone correct me if I'm wrong. The Tuggle tuner consists of a dual gang variable cap with one gang in parallel with a coil, in series with the other gang to ground. The antenna is connected to the other end of the parallel cap/coil. Here is a link to a schematic. http://theradioboard.com/best-of-the-best/mcgillis-double-mystery-radio.htm
homebrewPosted - 20 January 2014 9:10  Show Profile  Email Poster  Edit Message  
There are several versions of the Tuggle tuner.
That is one of the best I've found for the mystery set and should fit into your designs good.


Edited by - homebrew on 1/20/2014 9:34:38 AM

raypatPosted - 20 January 2014 21:56  Show Profile  Email Poster  Edit Message  
I will give the tuggle tuner a try. Meantime I wound a smaller diameter coil. 75 turns on a 2.25 inch pvc tube 24 gauge wire. My mystery coil is wound with 18 gauge.I put a dowel in the centre of the mystery coil and the new antenna coil slides right down inside. The results were excellent. I get one new station at the top of the dial (2MM 1665)with the antenna coil almost all the way inside and with some slide adjustments volume and selectivity is great on all the other stations. I can't get any stations outside of the Sydney area yet but I am gradually improving my set. I need to learn more about coil inductance and capacitance as I have no idea how many turns to wind my coils.So far its all been luck.

Ray

SWseniorPosted - 21 January 2014 8:43  Show Profile  Email Poster  Edit Message  
[ I need to learn more about coil inductance and capacitance as I have no idea how many turns to wind my coils.So far its all been luck.]

That is a problem I too have, and not knowing how to solve all the equations for making coils makes it very hard to find the right match. It takes me a lot of messing around winding a coil and finally get one that works and gets what I want. The professor coyle is good, but not always accurate when you don't know the exact amount of the capacitor. I guess it's part of the "fun" of building the sets.

SW

Edited by - swsenior on 1/21/2014 8:45:00 AM

_J_Posted - 21 January 2014 10:30  Show Profile  Email Poster  Edit Message
The Mystery Set is a novelty design that used capacitance between the wires of the interlaced coils to load (couple) the antenna and it affects tuning in a very complex way. It is very hard (impossible?) to approach with analysis or calculations. Any change in the coil changes the design to something else, very much luck driven.

What we usually call Tuggle tuning is a completely different setup to load (couple) to the antenna. Not sure how one would apply that to the Mystery Set?

BTW, I have found that #18 wire when wound with a little space between the wires makes very good performing coils.

John Davidson

Garry NicholsPosted - 23 January 2014 7:18  Show Profile  Email Poster  Edit Message
Hi John and all;

On your #18 solid wire comment, would simply using PVC insulated hookup wire degrade the benefit much? Coils are so much easier to wind with close wound hookup wire than with air spacing!

A trick that I sometimes use is to put a powered radio with ferrite bar antenna next to a coil and variable cap under test. Carefully swinging the cap through its range should peak up the signal on the transistor at some point, because the setup acts as an external antenna tuned to resonance. But be careful, the peak can be so sharp that you miss it! Be sure to use an insulated knob on the cap or your body will confound the result.

However, when you attach an antenna and ground the tuning range of the coil/cap combo will change (lower). So you can then repeat the experiment with an even more profound peak in the signal on the transistor (or any other powered radio with a built in ferrite bar antenna).

Playing around with these setups will allow you to get a feel for coil and cap sizes that cover the MW band with and without your antenna/ground.

Try to orient the test coil on the same axis as the radio's ferrite bar antenna, but start with about a 6 to 8 inch spacing, end to end. (The bar antenna is usually along the top edge of most radios.) Experiment with the spacing. Farther will give a weaker but sharper peak.

At some point, you could carefully measure your coil dimensions and see what frequency the test combo (with no antenna and ground attached) is tuned to as heard on the powered radio (with the test variable at or near max C -- work you way down the band leap-frogging the tuning of the two).

Enter the coil dimensions to get a ballpark inductance value. Then go to the resonance calculator and enter the inductance. Then juggle the capacitance value in Coyle until the frequency fits your test frequency. That would likely present a ballpark estimate of your variable capacitor value (with the plates nearly or fully meshed).

I've left out some fine details because this is getting rather long!

Questions?

Garry
near Syracuse, NY

homebrewPosted - 23 January 2014 9:39  Show Profile  Email Poster  Edit Message  
Coil calculators:
40 pf and 365 pf are good values to use for the capacitor. If you get short changed tuning wise you can play with the variable capacitor values until you get calculated frequencies close to the actual and use those values for future coils.
Other typical values are 260 pf and 430 pf.
You may also run into a low around 22 pf.
Since I reuse my coils on various circuits and with different variables I usually target
210 uh which will cover the band with a 22 to 430 variable. And come close to where I do not lose much with other value variables.

Edited by - homebrew on 1/23/2014 9:41:42 AM

Edited by - homebrew on 1/23/2014 1:07:43 PM

SWseniorPosted - 23 January 2014 18:16  Show Profile  Email Poster  Edit Message  

What I have found out recently is that looking at a schematic and seeing that the coil is X diameter and X length will make the coil X uH. One then can change the coil size/dimensions using Professor Coyle and come up with the same uH. This has made it easier for me to wind a coil for a certain crystal set.

I'm learning!

SW

_J_Posted - 26 January 2014 0:4  Show Profile  Email Poster  Edit Message
Yes, Garry, PVC insulation might take a big bite out of the gains of optimal wire / coil size. 100 lb test nylon fishing line laid between coil turns is better.

On wire / coil size: I am skeptical that there can be an optimal wire size / coil size for solid wire Q, yet that is what I seem to find. In theory, coil loss goes down as all dimensions increase. I can't make that happen. Likely something's wrong, surely I am overlooking something???

On capacitance: The amount of capacitance needed depends on what other capacitance is across it. Thats where reading schematics comes in handy. If the antenna-ground is across the tuning capacitor, you need much less capacitance, and as a practical matter, you can't get a capacitor that tune the whole BCB wired like that.

John Davidson

homebrewPosted - 26 January 2014 9:58  Show Profile  Email Poster  Edit Message  
Quote:
On wire / coil size: I am skeptical that there can be an optimal wire size / coil size for solid wire Q, yet that is what I seem to find. In theory, coil loss goes down as all dimensions increase. I can't make that happen. Likely something's wrong, surely I am overlooking something???
J:
What are the parameters of the coils that do not fit the 1:1 rule of thumb?
Keep in mind that the "ideal squared coil" is a 1920's rule for typical magnet wire close wound coils.
1 to 4 inch formers
20 to 30 ga wire
240 UH and a 365 PF variable.
Terman did the math behind it down to two decimals. But his simplification of the equation by eliminating parameters that were too small to matter limits it to the typical early commercial radio coil sizes.
Larger coils do better using loop antenna design parameters calculating optimum wire spacing which will give you a shorter coil.
Optimum spacing on a 5 gallon paint bucket with 16 guage wire is around 0.1 inches Roughly two wire diameters using loop design.
The loop formula works best for 2 ft to 8 ft loops.
Both formulas use wire AC resistance for a fixed value inductance so Litz would be spaced closer and stranded wire farther apart.
*
PVC wire - usually has a thick insulation layer. The flexible grade PVC used on wire has higher losses than the pipe and the thickness adds to the losses.
The PVC coated Chinese automotive wire I bought actually spaced the wire two diameters
and had a lot higher DC resistance per 100 ft.



Garry NicholsPosted - 26 January 2014 10:39  Show Profile  Email Poster  Edit Message
Hi John;

I guess you would remove the nylon line after the coil is wound? Or are you saying that leaving it in place would likely give a better performing coil than using the same gauge of PVC insulated wire?

I'm surprised how well my #22 solid PVC insulated wire on a small outbox performs. With a 1n34a, crystal earplugs in series and an ordinary dual variable out of an old radio I can copy around 30 stations evenings on the BCB from here near Syracuse, NY. (The NE US is a signal rich area, however.)

This is a Tuggle circuit with the diode tapped up 20% from the bottom of the coil. Antenna is about 60 ft at 18 feet and ground wire runs down the side of the house from a second floor window to an outdoor water tap.

Nothing exotic in use, here. However, I've always felt that two crystal earplugs in series were notably superior to one, not to mention doubling the impedance seen by the diode.

Must be the cold, dry weather up here. I've only had to wack my earplugs once to make them work again during the recent RadioBoard contest.

Garry

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