Crystal Radios and Followups

[Vanadium 50]

TL;DR

Discussion on some of the isues with crystal radios.

In another message, I discourage a crystal radio as a first project for a kid:

I would not start with a crystal radio. They are fussy.

• Using an iPod headphone will not work. In general, people want to substitute parts and use what they can find, This seldom works.
• Most of the kits today use a variable inductor and a fixed capacitor. These inductors and finicky and while they look beefy, they are fragile. If the kit has you build them, its another source of trouble.
• The antenna is critical and most likely too short. The radio may be OK, but there is note enough power. Remember it's not just the source of signal but the source of power.

As a first project, it might be discouraging.

Don't get me wrong - I think they are great. But I wouldn't start with one. Anyway, this brought up some issues.

(1) The ugly tan earphones that come with the kits. What are they? Piezoelectric?

(2) An iron-core inductor and a variable cap is smaller, and more likely to work. However, it seems hard/expensive to find them. You know the drill: 1000 for $1000, 500 for $999, 200 for $998, etc. Any decent sources?

(3) 10-year old me had very good luck using a rooftop TV antenna. In retrospect, I doubt it was the antenna itself - it was likely all the internal twinax in the huse going to the televisions. Today there are few rooftop antennas and good luck even finding the twinax runs.

Note that for most people an AM band dipole is unrealistically large. String wire and hope for the best is pretty much the option.

Where I think things get interesting and might grab a kids attention is improving on the basic circuit. Ten-year old me hooked the audio output to an amplifier and played it out the speaker. It worked but was not exactly audiophile quality. That could be improved.

As I have been thinking about it, I wonder if one could use a transistor to rectify the system and get a little gain.

One could further improve on the crystal circuit. I think building a superhet is unrealistic, but a superregen shouldn't be too hard. They are a little antisocial, I admit. I dimly remember a circuit where the same tube amplified both the RF and the audio. I wonder if a transistor would do the sane thing.

Anyway, I would say that for a motivated kid, this would not just be a project but a source of projects.

Comments?

 

[marcusl]

Here are some comments from my limited understanding. (I invite Baluncore and other experts to correct me if I go astray.)
1) Piezoelectric operation for the tan headphones seems right. See, e.g.
Amazon earphones
They have an impedance of a few thousand ohms. Why not 8 ohms, which is standard for HiFi/stereo speakers and headphones? Because of the need to match the load impedance to the crystal rectifier. The broadcast AM band is, for discussion purposes, pretty close to DC so we can look at the DC impedance of crystals, which is typically a few hundred ohms (see, for example, the center column in this table from Scaff and Ohl, Bell System Technical J., 1947).

These World War II microwave diodes literally used cat whiskers and semiconductor crystals inside of a cartridge.
Choosing the load impedance is a balance between best power transfer, which occurs with a matched load (500 ohms, say), and good selectivity (narrow bandwidth to capture just one station at a time) which requires high Q and therefore high impedance. I'm guessing that ~3000 ohms was empirically arrived at as a reasonable compromise.
EDIT: Another consideration is the need to forward bias the diode over at least part of the RF cycle in order to provide rectification. For a given power intercepted by the antenna, a higher circuit resistance will provide higher voltage across the diode. The forward drop sets the minimum voltage: 0.3V for germanium and 0.7V for silicon.

2) As a boy, I built a crystal set with enameled magnet wire wound on a toilet paper tube and held in place with airplane glue (toxic stuff, probably can't buy it anymore). The tube was nailed horizontally to a piece of wood and I used sandpaper to rub off the insulation along the top. A tin strip, nailed to the wood so it could pivot left to right and bent up to contact the coil, tuned the radio. I think this is easier than acquiring a ferrite antenna and variable capacitor, as you say.

3) I connected the RF input to a rain gutter downspout and the ground to a cold water pipe, and it worked! Gutters of old were steel with soldered connections, however, so this might not work so well with today's aluminum gutters and glued seams. Still, it's worth a try.

I think transistors are an obvious next step as the builder increases in ability to understand electronics (transistor action is not obvious...).

 

[DaveE]

Any decent sources?

digikey.com

 

[marcusl]

I just looked up air variable capacitors and they are quite expensive!
Variable Capacitor

 

[Baluncore]

(2) An iron-core inductor and a variable cap is smaller, and more likely to work.

An iron powder core will not work across the BC band. It needs to be ferrite. That is why a ferrite rod is used in portable radios as the antenna and tuned RF.

Note that for most people an AM band dipole is unrealistically large. String wire and hope for the best is pretty much the option.

Yes, string a wire between two trees as a capacity hat. Take a single wire down from the midpoint, to a tap adjustable inductor at a ground wire. It makes an H on its side, with inductance between the top and bottom wires. Vertically polarised, tune inductance to taste.

One could further improve on the crystal circuit. I think building a superhet is unrealistic, but a superregen shouldn't be too hard. They are a little antisocial, I admit.

A direct conversion receiver built using CMOS analogue gates for the only mixer, with a synchronous local oscillator, is the simplest receiver.
Any regenerative receiver is usable over 100 kHz to 30 MHz, given five or six RF coils. Super-regenerative should be avoided.

I dimly remember a circuit where the same tube amplified both the RF and the audio. I wonder if a transistor would do the sane thing.

You are referring to "reflex" receivers, which were used when the price of each vacuum tube was a weeks pay. The same tube could be an RF, an IF, and an audio amplifier, all at the same time. That time has thankfully passed.
https://en.wikipedia.org/wiki/Reflex_receiver

 

[Vanadium 50]

I think this is easier than acquiring a ferrite antenna and variable capacitor, as you say.

I had more trouble with that. It was relatively easy to short, I was not the best winder, but most troublesome of all was when my hand went near the tuning coil, the frequency would shift.

A direct conversion receiver built using CMOS analogue gates

Well sure. One could go to full SDR, or for that matter, listen on the internet. But where's the fun in that?

transistor action is not obvious.

Transistor Man! They all have tiny people insider operating them.

 

[Tom.G]

WOW! Those 365 pf variables are expensive.
270 pf variables als work and are cheaper and multi-sourced.

A Google search finds pricing ~$4, to $1000+ for a high-power transmitter.
https://www.google.com/search?&hl=en&q=270pf+variable+capacitor&oq=270pf+variable+capacitor

Cheers,
Tom

 

[Baluncore]

... I think this is easier than acquiring a ferrite antenna and variable capacitor, ...

eBay search: 'ferrite rod antenna'
Cost between $5 and $10 including postage.

Well sure. One could go to full SDR, or for that matter, listen on the internet. But where's the fun in that?

NO, not SDR. Just a local oscillator, (near synchronous with the RF carrier), and one CMOS gate chip to do the RF conversion to baseband, followed by a simple 10 kHz low-pass filter, and audio amplification.

 

[Swamp Thing]

A superregenerative receiver for the aircraft band could be the next project. The students will be like officially mind blown (as they say these days) hearing pilots and ATCs talking to each other.

CAUTION WARNING ACHTUNG: Could be illegal to operate in some jurisdictions and areas e.g. near an airport.

 

[Baluncore]

A superregenerative receiver for the aircraft band could be the next project.

Please don't use simple regenerative or super-regenerative receivers.

Regenerative receivers re-transmit on the listening frequency. Super-regenerative receivers also re-transmit on the channels above and below the listening frequency.

You do not need the attention you will get from an unattended 'Jenny' on the air-band.

 

[Swamp Thing]

Please don't use simple regenerative or super-regenerative receivers.

Regenerative receivers re-transmit on the listening frequency. Super-regenerative receivers also re-transmit on the channels above and below the listening frequency.

You do not need the attention you will get from an unattended 'Jenny' on the air-band.

Best use one of these black box LNA things to isolate the antenna from the superregerative circuit's input.
https://www.amazon.com/HiLetgo-0-1-2000MHz-WideBand-Amplifier-Noise/dp/B01N2NJSGV?tag=pfamazon01-20

You likely won't need to actually wire the output of the LNA to the receiver; you just need to glue a short piece of insulated wire with one end near the output of the LNA and the other near the input of the receiver, and put both into a metal box.

 

[Baluncore]

You likely won't need to actually wire the output of the LNA to the receiver; you just need to glue a short piece of insulated wire with one end near the output of the LNA and the other near the input of the receiver, and put both into a metal box.

Just like a feedback oscillator, cavity-tuned.

 

[Swamp Thing]

On second thoughts, better still to put the LNA in one metal box and the receiver in another. Mount a connector on each box and run a coax. But in the receiver box you just have a short piece of wire hanging from the connector terminal and near the receiver circuit.

 

[Rive]

WOW! Those 365 pf variables are expensive.
270 pf variables als work and are cheaper and multi-sourced.

I wonder if a suitable replacement could be produced DIY. These receivers are generally not expected to last too long and a bit of an inconvenience is generally accepted during usage. No quality product is needed.

 

[Swamp Thing]

I wonder if a suitable replacement could be produced DIY. These receivers are generally not expected to last too long and a bit of an inconvenience is generally accepted during usage. No quality product is needed.

Here they have used some 3D-printed parts and PCB laminate stock to make a variable cap.



----

And this next one would be easier to make, but you might need to stack 3 or 4 with a single shaft through them, in order to get up to 300+ pF.



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Nother one here:

 

[Vanadium 50]

but you might need to stack 3 or 4 with a single shaft

Or, if a switch is cheaper, add one of them in with fixed sized caps. No reason your knob can't have a range from 500-1200 and another from 1000-1700. No need to have the variable do all the work.

 

[Swamp Thing]

Looks like we are totally spoilt for awesome options....

 

[Baluncore]

No need to have the variable do all the work.

If there is only one tuned circuit, you fix the capacitor, and tune the inductor by sliding the ferrite core in or out. Some AM car radios in the 1960s used that technique with a mechanical press-button station selector.

The MW BC band, 540 kHz to 1620 kHz, has a frequency ratio of 3.0
The variable capacitor needs a capacitance range of 3²= 9.0

 

[Vanadium 50]

We were discussing tuning with a capacitor.

 

[Rive]

And this next one would be easier to make, but you might need to stack 3 or 4 with a single shaft through them, in order to get up to 300+ pF.

That one with the CD case is promising, I think. Maybe I should give it a try... Looks easy to make. With some tuning it could be perfect...

 

[Baluncore]

We were discussing tuning with a capacitor.

I am discussing stable tuning arrangements for DIY projects.

Fabricating a variable capacitor, while eliminating stray capacitance to the external variable "wet biological" environment, is particularly difficult. That problem is reduced with inductors because an external metal box reduces inductance only slightly, rather than increasing the minimum capacitance significantly, which limits the critical capacitive tuning ratio. Notice how variable capacitors are often constructed with chassis connected parts outside, surrounding and screening the insulated active plates on the inside.

DIY projects that make their own components benefit from a study of the early history of the field. Before variable tuning capacitors, and RF ferrite, there was the "variometer", an air-cored inductor with a variably coupled internal secondary winding. When connected in series, the axis of the secondary could be rotated, to add or subtract virtual turns from the primary.
https://wg2xka.wordpress.com/the-variometer/

 

[sophiecentaur]

Choosing the load impedance is a balance between best power transfer, which occurs with a matched load (500 ohms, say)

It's important to note that tuning for 'max power' is not the holy grail that some people seem to think it is. It's very useful in cases when reflected power is a problem ( long wires and short wavelength signals etc.) but where there's a source of high power (a transmitting amplifier or a generator) you actually want as near as possible 100% of the supplied power to go the load. Efficiency is very relevant in many cases.

 

[Vanadium 50]

We could mine our own germanium too.

 

[marcusl]

It's important to note that tuning for 'max power' is not the holy grail that some people seem to think it is. It's very useful in cases when reflected power is a problem ( long wires and short wavelength signals etc.) but where there's a source of high power (a transmitting amplifier or a generator) you actually want as near as possible 100% of the supplied power to go the load. Efficiency is very relevant in many cases.

The rest of my post points out that other considerations drive the optimal impedance away from a power match.

 

[Baluncore]

It's important to note that tuning for 'max power' is not the holy grail that some people seem to think it is.

That is very true with higher Q tuned circuits in radio receivers. You want all the power to resonate, then you can sense the higher signal voltage with the gate of a FET, extracting minimum power from the resonant circuit.

 

[sophiecentaur]

The rest of my post points out that other considerations drive the optimal impedance away from a power match.

I wasn't disagreeing; just emphasising your point. Those "other considerations" are often far more important than going for a power match, particularly in elementary experiments.

 

[Vanadium 50]

The rest of my post points out that other considerations drive the optimal impedance away from a power match.

I think that was an excellent post. I had not thought about crystal radios in decades, so I had not properly considered impedance. I am more impressed than ever with the designers of the Radio Shack 75-in-1 sets. The diode needs to be properly matched for the earphone or the radio won't work. But that's not the only project involving the diode. So they have to pick component values that work in a wide range of projects.

I'm also impressed with people who used actual crystals. They not only had to find the right spot on the crystal to rectify, they also needed to get the right impedance for the rest of the radio.

 

[Vanadium 50]

Let me also dial back the hate on superregens. Yes, I called them "antisocial" and yes, they do have a tendency to radiate. But they don't have to. My grandparents had one from maybe the 20s or 30s. No interference at all.

I still would in general avoid it, as better alternatives likely exist, but it's not the radio equivalent of toxic waste.

"It's not a bad circuit. It's just misunderstood."

 

[Baluncore]

.... and yes, they do have a tendency to radiate. But they don't have to. My grandparents had one from maybe the 20s or 30s. No interference at all.

A regenerative set must be adjusted to a stable point, below the point of oscillation. If you walk away, and it begins to oscillate, it will cause interference on the listening frequency, to other listeners on that frequency.

The "super" in super-regenerative means that it is a regenerative receiver, designed to jump in and out of RF oscillation, automatically and continuously, at a "supersonic frequency". The interference from a super-regenerative set is ultrasonic, so you cannot hear it on your listening frequency. A super-regenerative set is always radiating that ultrasonic interference, on the frequency channels, above and below your listening frequency.

"It's not a bad circuit. It's just misunderstood."

That your grandparents never received a complaint, does not mean it was not causing continuous interference, but, how do you know it really was a super-regenerative set that your grandparents had, and not the more common, or house, regenerative set?

 

[Vanadium 50]

how do you know it really was a super-regenerative set

Because it said so on the front. In what I am sure the manufacturer thought was a futuristic typeface.