profile | register | search | main site
|Forums | Rap N Tap Clubhouse - Basic Crystal Radio Fun! | Crystal set aerial||Post Reply||Send Topic To a Friend|
|raypat||Posted - 31 January 2014 23:22 |
Hi Garry. I have the tuggle tuning in the antenna coil which is loose coupled to the mystery coil. I am struggling to understand the tuggle tuning schematic. I have the top of the coil going to one of the connectors on a dual gang variable cap. The way I am reading the schematic suggests that the two gangs are connected via the common chassis.I connected the bottom of the coil to the chassis and the other connector on the second gang to earth. The aerial is connected to the top of the coil. Does this sound right?
Edited by - raypat on 2/1/2014 12:43:28 AM
Edited by - raypat on 2/1/2014 12:44:15 AM
|Garry Nichols||Posted - 1 February 2014 8:11 |
Sounds correct to me, raypat!
Just to make sure, let me re-describe the connections.
First, the frame is common to both rotors on the dual variable. (a stator is the fixed, insulated, non-moving plates -- the rotating ones are called "rotors", ha-ha!)
So, the top of the coil goes to one stator. The bottom of the coil goes to the frame. The ground goes to the other stator. The antenna goes to the top of the coil (or the stator that the top is hooked to, as a matter of mechanical convenience).
This stage alone makes quite a good crystal set and is what I used in the recent "nontest" listening contest. (BTW, it is called a "nontest" just for fun, because we used to have actual "contests" where points were collected for stations heard and there were first place, second place, etc, announcements.)
With the exact setup I described above, one could tap onto the coil with a diode (I usually use 20% to 35% up from the bottom), which then runs to phones which then run to the frame. That's it. A complete radio.
The drawback to this single tuned set in my location (near Syracuse, NY) is frequent and often strong short wave interference. A single tuned stage does not have enough selectivity to reject SW sigs and some combinations of components actually resonate (unintentionally) in the SW bands.
The way the Tuggle circuit works (either way you use it) is that one section of the cap tunes stations along with the coil and the other section varies the coupling to ground.
At the bottom of the MW band the signal wavelength is becoming longer which makes the antenna shorter relative to the signal wavelength at the top. So the antenna is not as effective, and more ground coupling is required to get a stronger signal.
At the top of the band the opposite is true. The antenna is longer (relative to the signal's wavelength) and so is more effective (efficient) at delivering signals to the radio. In fact, the antenna and ground can load down the circuit so that it is hard to separate stations (this is called lower Q, "Q" meaning circuit tuning quality).
So we uncouple the set somewhat from ground to raise the Q (get more selectivity at the expense of signal strength -- the usual game played with simple radios).
It is equally effective to make a hard connection to ground and run the antenna through a fixed or variable cap to accomplish the same thing. Perhaps this is a type of Tuggle variation that you have run across.
Does this hang together for you?
|homebrew||Posted - 1 February 2014 9:13 |
Using a close coupled antenna tuner (coil sliding into the other), playing around with the mystery circuit grounding gives a good set of choices for selectivity vs sensitivity.
Here are some of the grounding choices on a loose coupled circuit:
|Garry Nichols||Posted - 1 February 2014 10:17 |
A close coupled setup would have to have a non-resonant antenna coil, wouldn't it?
If it was tuned to resonance it would be overcoupled, with very poor selectivity.
|homebrew||Posted - 1 February 2014 15:23 |
Generally a close coupled circuit would have only taps on the primary. And maybe a series antenna tuning variable.
A occasional problem is double humping where you tune a station to max, then it detunes a bit, then tunes to max again. Overcoupled I think.
That can be corrected by moving the inside coil out a bit and retuning. Generally the sweet spot is with the inside coil around half way out.
|raypat||Posted - 1 February 2014 20:19 |
Thanks for the advice guys. I have experienced most of the symptoms described when sliding the antenna coil in and out of the mystery coil.Especially the overcoupling when I get many stations together but its easy to just slide the antenna coil out a bit until I get back to reasonable selectivity. I have used this set to improve my aerial and ground setups and learn a bit along the way. I get all the local stations with pretty good clarity and reasonable volume. I think it's time to try and build a bit better set now and see if I can get some dx signals. I am thinking a couple of spider type coils face to face that can be angled (pivoted) closer together or apart. The other thing I would like to try is pulling a variable cap apart and seeing if I can improve the insulation along the lines discussed in this forum (I think).
|Garry Nichols||Posted - 2 February 2014 7:53 |
It's possible that you have not tried loose enough coupling. My double tuned set, with medium grade coils, uses about 8 inches spacing between the coils. They are arranged end to end.
I could suggest putting the coils end to end about 10 inches apart and then moving them closer by about an inch at a time.
Are you familiar with the concept of critical coupling? I used to set my coils so that tuning one circuit of the set did not "pull" the tuning of the other section at all (or VERY minimally, for the best sensitivity).
Tuning a double tuned arranged in this manner is a bit tricky at first.
At night you should be receiving stations from at least 200-500 miles away via skip. (Unless your geographic location is highly unfavorable.)
I've done this with just my Tuggle stage hooked up as I mentioned in a previous post.
What diode and phones are you using???
Have to run for now . . . .
|raypat||Posted - 2 February 2014 21:16 |
No Garry I have not tried moving the coil any more than about an inch away from the mystery coil. Mainly because of the way I have it set up. The mystery coil is fixed vertically and the antenna coil slides on a dowel set inside the mystery coil. I will try orienting them horizontally and increasing the distance between them as you suggest.
|Garry Nichols||Posted - 3 February 2014 8:38 |
OK, Ray. You are using heavy coupling. It is not a "loose coupler".
The coils do not have to be horizontal, but that might be easier mechanically. The spacing is what counts. Some folks build each unit on a separate board and adjust the distance between the units to set the coupling.
I seldom moved my coupling amount once set. I used two parallel wood dowels spaced about 1" apart and running between two vertical thin wood pieces. The coils were slipped over the dowels before fastening the dowels to the vertical supports. This allowed sliding the coils toward or away from each other.
When you get this going, try to find a strong station with the antenna stage tuning. Try a spacing of maybe 4 inches and try different settings of the detector stage tuning until you hear the station when tuning the antenna stage.
Then start to separate the coils a little at a time and carefully retune the detector stage. Do this repeatedly until the signal starts to get weak, then check to see if either stage interferes with the other.
Close the distance until you just start to get a bit of interaction, then open it a bit.
You will need to tune very slowly and carefully. Once you get a station tuned, you tune the band by moving one stage's tuning a little, then the other's in the same direction up or down the band. Tuning is done by leap frogging in this manner. Otherwise the two stages will be tuned to different frequencies and little or nothing will get through to the phones most of the time unless you have a very strong station around.
If this works out for you, consider calibrating one or both dials and logging your separation distance (it effects tuning). With at least one dial calibrated to stations you can more easily find your way around on the MW band.
It may take quite a bit of practice to get the hang of this once you are at critical coupling.
|gzimmer||Posted - 3 February 2014 19:27 |
And of course the term "loose coupler" had a different meaning back then...
Today, Double-Tuned Critical-Coupling gives optimum Selectivity and Sensitivity for narrow band signals (eg AM).
With Spark there is no carrier. The detector produces audio by mixing all the "sidebands" together.
Plus the old style "Loose Coupler" usually did not tune the secondary winding. It was purely a transformer winding to give best impedance match to the detector.
I think these two points explain why the old style Loose Coupler had such tight coupling.
Edited by - gzimmer on 2/3/2014 7:29:16 PM
|raypat||Posted - 3 February 2014 19:57 |
Thanks guys. I think I get the gist Garry. I will give it a try and let you know the results.
|raypat||Posted - 5 February 2014 18:24 |
Well I tried loose coupling as suggested by Garry. It worked pretty much as described.It took a bit of fiddling but after I got the hang of it I was able to tune stations with reasonable volume and much better separation. The weird thing was the volume was OK when my hand was near the variable antenna tuning cap, but would halve when my hand was removed. The coil for antenna tuning was about 70 turns of 24gauge magnet wire close wound on a 2.25 inch pvc former. I tried a couple more coils I have and found that the one I had wound with insulated hookup wire worked much better with the hand proximity not affecting the volume at all. I have no idea why this was but decided to wind another coil using heavier gauge magnet wire that I recovered from a microwave oven transformer ( This is the same wire I wound the mystery coil from). I used a 4.5 inch pvc former and machined a groove in it at 12 turns per inch which gave the wire one diameter spacing between each winding. I read that this is good for a coil so I tried it. The wire was 18 gauge which was 40 thousands thick and I wound 25 turns. The coil was easy to wind. Most of the work was setting up the pvc tube on the lathe. Anyway I connected the new coil in the antenna circuit as before and WOW.What a difference. Great volume, great selectivity. Thanks to garry I have made the single biggest improvement so far. Still no Dxing yet. I have messed around a bit with an in line wavetrap, but I think I am getting that part wrong. This post is already way too long so I will give the wavetrap details another time if anyone is interested.
Edited by - raypat on 2/5/2014 6:28:26 PM
|Garry Nichols||Posted - 5 February 2014 19:14 |
Congratulations, Ray! However, I cannot explain your hand capacity effect change when swapping coils. The only thing I can suggest is that the tuning knobs be fairly big and of good insulating material, so your fingers stay well away from the shaft and frame of the variable. A Tuggle circuit puts the shaft and frame above ground potential and so more likely to be effected by body capacity.
By any chance did you mount the second and third coils tried farther from where your hands are when you operate the set?
Professor Coyle is telling me that your new space wound coil is about 79 uH. The first coil you mention was about 241 uH (if I remember my previous calc). So you likely cannot tune as far down the band with the new 25 turn coil.
It is likely that it is performing better because the wire gauge is bigger, it is space wound and it has less wire on it, so there is less copper loss. These are all pluses.
How far down the band can you tune with it?
I don't know why you are not hearing DX. What's the farthest away station that you've heard anytime after sunset?
|raypat||Posted - 6 February 2014 0:54 |
First off I need to answer a couple of questions asked earlier that I didn't notice until re reading the thread.The phones I am using area a pair of salvaged earpiece and mouthpiece from an old Bakelite phone. They have 5620 DYN stamped on them and the makers name Fernsig. I mounted them in a cheap ear protector as advised on the forum elsewhere.I have no idea if they are high impedance or not.They work better than the little earpiece although I have to admit I am cheating a bit now having made a little kit amplifier bought from Jaycar for about $8, hooked up to a tiny 8ohm speaker. The volume using my current coil setup with proper loose coupling is amazing. I need to wire in a volume control. I have checked every station with the headphones though and I can hear all of them with pretty good volume. The diode is one from Jaycar as well .I think it is a standard 1N34.If anyone knows anything about my headphones I would be grateful for feedback. As for the new coil. I purposely wound less turns as I was having trouble tuning the top of the dial. Now I get the locals all the way up to 1665kHz and still can tune the bottom station at 576kHz. I will wind a 50 turn version next. Haven't really tried to get any DX stations yet but that is next on the list. Or maybe I will try and get the wavetrap right first.I have disconnected it for the time being.With the loose coupling I found I needed to move the antenna coil a full 11.5 inches away from the main coil in order for it to have no effect on the tuning of my strongest station. From there I moved it back a bit until tuning the variable on the antenna circuit made the station fade. If I read garry right this is the critical coupling point. It's around 10 inches separation. The other variable I have neglected to mention is the caps. I only own three and all from ebay. The only thing written on them is in pencil someone has written 480 on one gang and 500 on the other. I assume they are all the same as I bought them all together. Maybe that explains why I can tune the bottom of the dial with the 25 turn coil?That's as far as I've got so far.
Edited by - raypat on 2/6/2014 12:56:01 AM
Edited by - raypat on 2/6/2014 1:02:49 AM
|Garry Nichols||Posted - 6 February 2014 7:48 |
I Googled Fernsig 5620 DYN and it appears that that is a type of sound powered telephone. However no spec info is available nor does Fernsig seem to have a web site.
I suggest contacting Scott Balderston who repairs and sells old phones for info:
For simplicity and comfort, I like simple crystal/ceramic earplugs and have a pair from Mouser that I use in series with a 1N34 diode:
I think that they are about as good as my old Brandes Superior antique 2k DC phones.
But these can be cantankerous. There are connections inside which oxidize and loose contact. Occasionally I have to tap them on the desk to get them to work again. Sometimes with some force.
I can now see how you are able to tune down to 576 with those high C variables and a lower uH coil.
With the double-tuned set, I always liked to adjust my coil spacing so there was no interaction between the two stages. I think that if you space for some interaction you may be more than critically coupled, but I am no expert. I thought it was easier to tune the rig when there was no interaction.
Have to run. Did I hit all the questons?
Click Here To Close Thread, Administrators & Moderators Only.
Show All Forums | Post Reply