My Crystal Radio experiments

MrChips

Joined Oct 2, 2009
30,712
@Will - You cannot use an LED as a detector diode. The turn on voltage is too high. Your best bet is to stay with the 1N34 germanium diode which is readily available.

You really need a variable capacitor which will allow you to tune into the desired station.
Look for Ham Radio flea markets or clubs in your area. What is your location?

You can try building your own with two concentric metal tubes that slide one into the other with a separating dielectric. I have tried using aluminum foil but was not successful.
 

alfacliff

Joined Dec 13, 2013
2,458
led's have been used, as have silicon diodes, with a variable bias to set them just below conduction to make them more sensative and compensate for the voltage needed to get them to conduct. some metalic crystals need a bit of bias too. check out "makeradio " website for crystal radio info.
 
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Thread Starter

WillS_70

Joined Apr 26, 2014
26
Thank you all for the comments.

...

Tuning coils as an antenna have some directionality - try rotating it, both horizontally and vertically, and see how your signal changes. You may find a sweet spot where the grounded tuning coil gives the greatest reception for a particular station. Or by rotating the tuning coil you may be able to reduce the strength of an interfering station, another way to improve the sound of a selected station.
Something along those lines had occurred to me, in my ignorance. I was contemplating the notion of creating a circular hoop antenna, inspired by the old RDF aerials that used to be mounted on World War II aircraft, and also to improve portability of the set. Otherwise, an extension of that thought had me wondering if my long aerial might still work well if wound, loose coil fashion, around a larger diameter section of PVC pipe, mounted on a three degree of freedom gimbal of a sorts. Precisely for tuning / reception purposes. However, I see there is already a thread for AM antennas out there on this forum, so I will go and read that, save myself from the risk of any possible ridicule :D, and reserve any further comments on that subject for now. Thank you for an excellent comment.

Something I've learned about radio reception is that almost nothing is as simple as it seems like it should be. That wire that you connect to ground and call ground is itself a length of RF receiving wire, and can supply some amount of RF energy to your crystal set. Even a wire connected to Earth itself doesn't automatically show zero RF energy at the surface of the earth, depending on the salt content and moisture in the soil, some length of wire below ground can still have RF energy on it.
Yes, this is what is happening. I cannot recall exactly what I did with the circuit sometime last week during experimentation, but it ended up to the effect of using the ground as the antenna, and it continued to receive. I cottoned on that my ground wire; A). is long, about fifteen feet, from the garden up to a second floor balcony, B). the buried 18" long motorcycle fork spring I have serving as a ground, 2 ft deep, is probably only just adequate as a good ground, and C). the gauge of the wire leading up to the balcony is the same as the antenna's.

A). I cannot do anything about this, unless I take the set down into the garden.

B). I have already "solved" this issue, but post in case I have made a glaring error. I used a four foot long piece of 5/8 inch diameter steel girder, drilled a hole in the end of it, attached a piece of 10 gauge copper wire to that with a nut and bolt, and drove the whole thing into the ground with a mallet, the bolt end first so that it would take the copper wire down with it. I avoided wrapping the copper wire around the girder to prevent the possibility of creating any accidental inductance around the ground, in case that is a factor.

C). I attached (soldered) a length of insulated, solid 22 gauge solid wire to the ground copper wire and ran that up to the balcony, the idea being that if it is a smaller gauge than the antenna, it will be less prone to interfering with the intended signal by receiving its own. It seems to work quite well, but I have questions. The wire run up from the ground is still solid (single), so may still be receiving signals. Would the use of multi-strand wire to run the earth up be a better idea? Is a smaller wire really the best way to go, or would a larger one be a better solution?

You really need a variable capacitor which will allow you to tune into the desired station. Look for Ham Radio flea markets or clubs in your area. What is your location?

You can try building your own with two concentric metal tubes that slide one into the other with a separating dielectric. I have tried using aluminum foil but was not successful.
Indeed, yes. Thank you for the comment. Around where I am (the term might be archaic, even offensive now for all I know, so my apologies beforehand, but I live in the "Third World") there is little possibility of picking up an air variable capacitor. I know it may be expected to be on the contrary, but take my word for it. :D

In any case, making my own will be an opportunity for learning something, perhaps, as well as reviving some old, long disused engineering skills! I started last evening, and the picture attached shows progress so far, on a test fitting.

@Will - You cannot use an LED as a detector diode. The turn on voltage is too high. Your best bet is to stay with the 1N34 germanium diode which is readily available.
led's have been used, as have silicon diodes, with a variable bias to set them just below conduction to make them more sensative and compensate for the voltage needed to get them to conduct. some metalic crystals need a bit of bias too. check out "makeradio " website for crystal radio info.
I would have been the first to believe that LEDs cannot be used as detector diodes had I not tried it with success before reading that. :) You actually do get a signal without the bias, though very, very weak. alfacliff's description is analogous with what I experienced / experimented with. I would be interested to read what results other people have obtained with LEDs and silicone diodes.

Thank you, again!
 

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Thread Starter

WillS_70

Joined Apr 26, 2014
26
Hello.

The title of this post is in fact a comment that my "contraption" drew in the household. The homemade variable air capacitor does work quite well, against expectations. Made from 22/1,000" galvanized mild steel sheet, and painted clear acrylic to insulate the rotor from the stator. 10 rotors to 11 stators. The range is from 31 pf to 243 pf, which is useful, and is quite linear (see graph), thankfully. Still some finishing to do.

I cannot say that I have learned too much, yet, as a result of the exercise. I will be dismantling it again and testing it from 1 rotor / 2 stators and up, to see the effect.

It was a fun project!
 

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MrChips

Joined Oct 2, 2009
30,712
I've never seen someone make a homemade variable capacitor just like the real thing.
You need to put a tuning knob on the shaft.

How did the cut the plates to be the same shape?

Would be nice of you to post it as a DIY instructable.

Amazing. Well done!
 

atferrari

Joined Jan 6, 2004
4,764
Hello.

The title of this post is in fact a comment that my "contraption" drew in the household. The homemade variable air capacitor does work quite well, against expectations. Made from 22/1,000" galvanized mild steel sheet, and painted clear acrylic to insulate the rotor from the stator. 10 rotors to 11 stators. The range is from 31 pf to 243 pf, which is useful, and is quite linear (see graph), thankfully. Still some finishing to do.

I cannot say that I have learned too much, yet, as a result of the exercise. I will be dismantling it again and testing it from 1 rotor / 2 stators and up, to see the effect.

It was a fun project!
Congratulations! It is hard to face a question like that!! :(

I recall that there was a technique to linearize them in the lower end of the range with small trimmers but cannot recall how they were connected.

Otherwise there is also the chance to alter the shape of few of them to compensate for the lack of linearity or even to change the alignment of the plates in the stator. Just by shifting one a little and recording the resultant values, will tell how many and in what direction.

Pity is that I forgot all I knew about that!!

Around a week ago I found several variable caps like yours, coming from RF equipment. I was sure that they were binned years ago. So many tests I could have done with them!

Keep doing and learning in spite of...questions :p
 

Art

Joined Sep 10, 2007
806
That tuning gang deserves some kind of award :)

They manufacture them so the lobes aren't properly circular
Only for tuning gangs that are going to be interacted with through the radio's interface.
That's how they make them linear.
 

Art

Joined Sep 10, 2007
806
I'm gobsmacked that you have the material and tooling to make that
in a place where you can't buy one.
We just can't go to a shop and buy a proper one either (Australia),
but there's plenty of old equipment around to salvage them from.
 

Thread Starter

WillS_70

Joined Apr 26, 2014
26
@MrChips: Well, actually there are two, and only two that I found on the internet (here and here), DIY tutorials on making variable capacitors. However, I already had the idea of what I intended to produce when I found them, so I pressed on with plan A. I do have some design plans I drew up, but as they are rough and in pencil (my engineering design instructor from years ago would have thrown them straight into the bin without comment and without changing his expression) I am hesitant to scan and post them. I will "clean them up" for that purpose.

Indeed, an insulated knob is an unavoidable requirement. I noticed while testing the unit that every time I touched the chassis the capacitance reading would go up by about 120 pf. Is the difference a reading of my personal capacitance?

@Art: The tooling is relatively simple, and building the unit requires only basic fabrication skills. Tin shears, files (2nd cut square, rat-tail for inner radius and a square smooth for final draw filing of edges), fine emery paper, a vice, drill, and anything that can stand in as a small anvil (my vice has a small one integrated). Also a guaranteed flat surface to mount the emery paper on (I used a square of thick glass), as the final, light surface polishing of the plates is done on this. Finally, a micrometer, though if washer thickness is a guaranteed standard in your location (unlike here), this is probably optional.

I had read that the elongated lobe design was used for attaining linearity, hence my employment of the concept. It seems to work. There is, no doubt, some rule which has to do with area of metal in proximity versus area not in proximity, in relation to total surface area of metal, I should not wonder. I am thinking about it; that is all I can say regarding. Has anyone heard or know about this? I notice neither of the tutorial links use elongated lobes. Odd.

Additionally, my stator plates are shaped to match the curvature of the rotor as much as possible. On one of those tutorials, they are just rectangular. It has me wondering what effect that might in itself have, in relation to the above ruminated, potential conjecture.

@MrChips: I notice your variable capacitor has slots cut into some of the rotors at certain angles. I wonder if this is an example of the sort of fine tuning for linearity that atferrari refers to.

Both tutorials also use aluminium, most probably because of the ease of cutting the material. I am not at all sure if better conducting material will in fact produce a more efficient capacitor, though it is plainly obvious that a capacitor of this kind - going extreme - would not work at all if made of wood!

Also, I am also considering proximity between plates. The gap between the plates of each component (on mine) is 118 thou, which gives an air gap of 48 thou between the rotor and stator plates when assembled (in practice, there are inevitably slight variations to this, of up to 6 thou, when measured with a feeler gauge on my unit). With this in mind, I am contemplating making another one (or modifying this one) in which the rotor surfaces are insulated with some thin plastic sheet and the gap is closed so that the insulation is brought down just into contact with the stators, in order to have the minimum possible gap, (ie; the thickness of the insulation) between plates. However, all this will not be before testing this one with less plates to, at least, "get a feel" for the thing.

I am reasonably optimistic it will work as is for my purposes (plus the knob, of course) in a radio circuit.

@THE_RB: A cat's whisker diode would make the whole crystal radio experience more or less essentially complete. I did not have much success with an attempt with the rusty blade / graphite version early on.

Finally, thank you for your comments and suggestions. I hardly think it is award winning stuff (my capacitor, I mean, not the comments! :)), but again, my sincere thanks for that positive note!

PS EDIT: I have just tested the device with my "mystery radio" coil. The results do exceed expectations. The first effect was anticipated, to an extent; selectivity is much easier and covers a good range of the AM band; a frequency card could easily be made for it. There is a bit of a gap off the scale at the highest capacitance, but this is an issue with the coil(s) inductance value not being properly matched. The completely unexpected result was a massive reduction in other station interference, applicable to using any of the coil taps, both on the antenna and detector coils. And the actual quality of the selectivity is improved across the range. In other words, it zeroes in on the stations and effectively "filters" out the background of a powerful nearby station that previously "intruded" on a good part of the range with the small "hobby" capacitor. Well worth the effort. My thanks for the advice to go ahead and use a variable air capacitor, thereby convincing me to proceed with what was initially only an idea I was "toying" with of making one.
 
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alfacliff

Joined Dec 13, 2013
2,458
in case you dont have any luck making variable caps, you could do it he other way, a fixed cap and variable inductor. wind your coil, and use some sandpaper to get the enamel off a small strip down one side. make a wiper out of brass or copper to silide along the strip to vary the inductance.
 

rjjenkins

Joined Apr 16, 2011
214
I seem to remember using trimmer caps on their own for tuning. Also, supposedly you can make a cats whisker from a lump of coke (not the drug). An old POW dodge, I was told.
 

Thread Starter

WillS_70

Joined Apr 26, 2014
26
Just a question;

Is self resonance of a coil an undesirable effect in crystal radios?

I have been pondering this a bit, and I imagine that it is likely not necessarily a good thing(?). Apparently, from what I am reading, tightly wound coils like those I have been making are prone to creating self resonance. I am sure I will be corrected if I am extracting the wrong conclusions.

Does it cause any sort of interference? Offset of the desired frequency? Does this mean that winding the coils, perhaps with a wire's width of the same gauge between them, and the length compensated to keep the desired inductance would be a good thing to do?

If reducing self resonance is advisable; in setups that have two coils in proximity (ie; an antenna coil and a tuner coil), would it be beneficial to reduce self inductance on both, or just on the tuner or antenna coils?

Thank you in advance.

EDIT:
Right. Scratch that, I have looked into this in much more detail this afternoon, understand the concept a good deal more than I did a while ago, and can determine that for receiving AM band there is very little risk of accidentally reaching the self resonant frequency of the coil/capacitor combination.

Plus an extremely good link for those interested in this subject and who may not yet have found it.
 
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Art

Joined Sep 10, 2007
806
A commercial radio from the 60's-70's I broke down for parts used fixed capacitors
and a dual gang mechanical inductor as the tuner. I still haven't seen it again in a commercial radio.



 

vk6zgo

Joined Jul 21, 2012
677
A commercial radio from the 60's-70's I broke down for parts used fixed capacitors
and a dual gang mechanical inductor as the tuner. I still haven't seen it again in a commercial radio.



Most car radios used this system,but few Mantel sets did.
 

Thread Starter

WillS_70

Joined Apr 26, 2014
26
@Art: Wow, that is a novel idea. Correct me if I am wrong, but that would appear to vary the capacitance by insertion of those two small cylinders or rods into sleeves in that block. That they are matt black is intriguing me; is that the insulation or are the rods made of carbon or graphite, or something? The pulley, no doubt, is for the band that drives the frequency "needle" over the face of the tuner card. Not surprised it is from a car radio, somehow. I bet my old, long since sold Ford Maverick's radio had one of them.

As I am considering trying out SW listening - maybe venturing into DX - soon, I did some preliminary looking into and calculating of the sort of coils and capacitor range I would need. It looks like I am going to have to make another variable capacitor with a very fine tunable, narrow range (0 - 50 pf, I would say), so any ideas like the above will be probable subjects of the next mini project.

My present "ham-fisted monster" creation will not do it for SW, that is, even though it is great for AM. Incidentally, I did try out that contraption with fewer rotor / stators, and plotted the changes. The results were pretty much as I expected with that. I also closed down the gap between stators to see the results. At a 60 thou inch gap between stators (two 18 thou inch gaps, approximately, either side of each rotor) , the maximum capacitance was "boosted" to 820 pf, with a minimum of 155 pf. Interesting experiment, but not a very usable range, so I put it back the way that it was.

@bertus: That is truly a great site. Thank you. In the picture, the third from left capacitor plates look like they are made out of bronze. Apart from it being quite aesthetic to look at, I wonder how that performs in comparison to other materials.
 

vk6zgo

Joined Jul 21, 2012
677
@Art: Wow, that is a novel idea. Correct me if I am wrong, but that would appear to vary the capacitance by insertion of those two small cylinders or rods into sleeves in that block. That they are matt black is intriguing me; is that the insulation or are the rods made of carbon or graphite, or something? The pulley, no doubt, is for the band that drives the frequency "needle" over the face of the tuner card. Not surprised it is from a car radio, somehow. I bet my old, long since sold Ford Maverick's radio had one of them.

As I am considering trying out SW listening - maybe venturing into DX - soon, I did some preliminary looking into and calculating of the sort of coils and capacitor range I would need. It looks like I am going to have to make another variable capacitor with a very fine tunable, narrow range (0 - 50 pf, I would say), so any ideas like the above will be probable subjects of the next mini project.

My present "ham-fisted monster" creation will not do it for SW, that is, even though it is great for AM. Incidentally, I did try out that contraption with fewer rotor / stators, and plotted the changes. The results were pretty much as I expected with that. I also closed down the gap between stators to see the results. At a 60 thou inch gap between stators (two 18 thou inch gaps, approximately, either side of each rotor) , the maximum capacitance was "boosted" to 820 pf, with a minimum of 155 pf. Interesting experiment, but not a very usable range, so I put it back the way that it was.

@bertus: That is truly a great site. Thank you. In the picture, the third from left capacitor plates look like they are made out of bronze. Apart from it being quite aesthetic to look at, I wonder how that performs in comparison to other materials.
You can have a fixed inductance & vary the capacitance,which is what is normally done,or have a fixed capacitance,& vary the inductance,which is what was done in the pictured equipment.

The matt black things are cores which adjust the coil's inductance.
They are probably iron dust,although they may have been ferrite.

In order to get a linear frequency scale,you have to either mess with the winding pitch across the coil,or as was done with some WW2 Australian Military radios,have a variable pitch on the adjusting threaded mechanism.
 

Art

Joined Sep 10, 2007
806
Yes they are some ferrite I think. It's definitely a pair of inductors though.
There are two coils with two connections for each.
 
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