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This is an old revision of this page, as edited by 108.244.204.22 (talk) at 01:12, 10 January 2015 (→Modulation fix wanted: new section). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Naive?

I object to the use of the perjorative "Naive" in regards to the simple circuit presented. It implies a preference or judgement that has no place in an encyclopedic article.

For example, getting 8-year-olds to build a working radio in less than an hour requires some simplification, and the the circuit presented does that. Perfect for the application.

Thbusch (talk) 16:53, 27 August 2009 (UTC)[reply]

I agree, it does imply judgment, but there is nothing wrong with sound engineering judgment, it is something we need more of.
... and perfect? Actually, that circuit does come pretty close to naive by adding a part (variable capacitor) that does more harm than good, it reduces the tuning range over the simpler slide tuner circuit. Many people add the variable capacitor because they are unaware that most inverted L antennas already have the needed capacitance. I am very sympathetic to your motive of making this article assessable to young people, but eight-year-olds are not going to understand how a variable capacitor works anyway, a simpler, slider tuner radio would be easier to build and understand.
That circuit is just plain hard to justify from any angle. John (talk) 03:04, 31 August 2009 (UTC)[reply]

2 of the first 3 links are broken, 1 to the 'enthusiasts', the other to using modern components for 'hi' performance.
Just after I added some text to the cites too! ADVICE always check a link works before trying to 'improve'it!
Dug out a few links that may be relevant. Some are obviously commercial sites, for those who are interested.

--220.101.28.25 (talk) 09:28, 6 December 2009 (UTC)[reply]

Expanded 'How it works' section

Added subsections under 'How it works' describing the different parts of a crystal set, addressing some of the topics discussed on this page, such as impedance matching and selectivity. --ChetvornoTALK 06:40, 12 July 2010 (UTC)[reply]

This revision has some problems. It adds a capacitor across the coil, which will not work to tune the BCB, and is not the simple circuit, it does not address the antenna as the capacitor that tunes the inductor. We need to restore it to the previous version. John (talk) 06:22, 18 August 2010 (UTC)[reply]
Which part are you referring to specifically? --ChetvornoTALK 07:36, 18 August 2010 (UTC)[reply]
Much of the added subsections under "How it Works". This material is assuming a variable capacitor can tune the BCB with a coil, and it is not practical. We don't want to mislead people with this. See "The Naive Circuit" section. Electrically short antennas are capacitive and coil tunes the antenna in a single pole circuit, a variable capacitor cannot be used in this circuit. John (talk) 18:51, 18 August 2010 (UTC)[reply]
All of the diagrams in this section were sourced from actual circuits. Keep in mind that this is a "How it works" section, for nontechnical people. It's important to emphasize for them that a capacitor is always necessary in a tuned circuit, whether provided by the capacitance of the antenna or a descrete component. So the text mentioned that some circuits use the capacitance of the antenna alone, but most of the circuits I chose used tuning capacitors. And you're certainly mistaken if you think that practical BCB crystal sets didn't use tuning capacitors. There are literally hundreds of circuits of crystal sets that were tuned capacitively. See here I didn't have time to check how they solved the tuning range problem, but it looks like many use tapped coils and cover the band in separate sections. I think it's important to have a historical POV and show a variety of the circuits that were actually used. Your point about tuning range is important, and deserves a section, but I think removing all circuits with tuning capacitors is going too far. --ChetvornoTALK 02:38, 19 August 2010 (UTC)[reply]
Sets do use tuning capacitors, but it is not physically possible for practical sets to use them the way this diagram shows. Does what I said above suggest removing all sets with tuning capacitors? I do not. That diagram shows a variable capacitor and an antenna, and is not correct. I am very familiar with that site, and I am extremely familiar with how this problem is resolved: by not using this circuit. It is a common mistake and this article needs to avoid it. Please become familiar with this issue before editing this. I will be happy to discuss it with you. BTW, I am not sure hundreds of circuits means something works. Please do not put this figure back. I would like to make the "how it works" section correct. John (talk) 02:56, 20 August 2010 (UTC)[reply]

This edit war on the article needs to stop. Discuss the matter here with regard to Wikipedia's policies and guidelines, then change the article in accordance with compromise and consensus. Discuss the reliability of the sources and the representation of the cited sources in the article text and diagrams, not each other's intelligence, abilities or personal views. Having said that, here is my input: First, the scan of the 1950s article by L.B. Robbins looks authentic to me. Does anyone have any evidence that it is a modern con with technical errors introduced by someone else? If not then it seems to be a WP:RS reliable source and may be used to support the article. Second, I have made that circuit and so I know it works. This is WP:OR and so cannot be used in the article, but should be enough to stop the bickering. (I was trying to re-create a crystal radio made by my grandfather during WW2 and so wound a coil like his on a wooden former, but used a schottky diode as a detector and an audio amplifier instead of Hi-Z headphones. My grandfather's design was capacitor-less with a sliding earth-tap on the coil, connected by sandpapering away some insulation lacquer after winding it. Adding a small trimmer capacitor increased the selectivity and 'peaked' the reception slightly, but still only the most local medium-wave station was really receivable). So, the circuit is authentic, and it works. What's the problem with mentioning it in this article? --Nigelj (talk) 08:29, 20 August 2010 (UTC)[reply]

P.S. A completely different inductance is needed with the capacitor compared to without it, for the same long wire antenna and same reception band. I can dig out my old experiments and count the turns if anyone's interested, but it's largely irrelevant as it's WP:OR, however it shows the principle is sound. --Nigelj (talk) 08:37, 20 August 2010 (UTC)[reply]

Right you are, Nigelj, no more EWing. The argument is not over whether the circuit works; I think John would agree it does. The problem is that the circuit is impractical for an actual radio because it can't tune the entire broadcast band. John thinks that for this reason it shouldn't be included in the article, I think it should. John, I understand the issue. The AM broadcast band requires a tuning range of 3.09:1. So the tuning capacitance requires a range of the square of this: 9.5:1. Since the minimum capacitance is the antenna capacitance, the tuning capacitor must be at least 9.5 times the antenna capacitance. For a 15 meter antenna, which has a capacitance of about 135pF that's about 1300pF. It's difficult to find air variables that large, so with a standard capacitor the radio won't tune the whole band. So what? These are technical details. The circuit does in fact work. Many people build crystal radios just for the educational value and don't care whether it covers the entire band.
More importantly, the purpose of the schematic John deleted is not to present a practical radio, but to illustrate for novices the operation of a crystal radio with the simplest, most easy-to-understand circuit. Since the tuning capacitor is mentioned in the text, it has to include a tuning capacitor. The circuit I used is the one most commonly employed in the literature to introduce crystal radios, and is very widely seen: 1, 2, 3, 4, 5, 6. It says in WP:DUE that "due weight" must be given to different aspects of a subject "in proportion to their representation in reliable sources". Considering the wide appearance of this circuit in crystal radio literature, I think it probably should be included.
I have no objection to mentioning the drawbacks of this circuit in the article, in fact I insist on it. But I don't think there is a better circuit for illustrating how crystal radios work. John, I have given 6 references above, plus the 2 in the article, on the wide use of this circuit. Where are your references backing up your claims that it is unusable? --ChetvornoTALK 10:08, 20 August 2010 (UTC)[reply]
Tuning across the whole MW band is irrelevant. The poor overall Q-factor due to the inefficiency of a (short) long wire antenna, the almost complete lack of impedance matching at every point (antenna -> tuned circuit; tuned circuit -> headphones), and probably a poor earth, means that the selectivity of the receiver is woeful. If you have a relatively local AM station, that is all you will copy, with all the others appearing as noise, wherever you tune in the band. All you really change is the s/n of the local station compared to all the others heard together. I found better selectivity in some of the low shortwave bands (4 MHz, 7 MHz?) using a smaller inductor, if I remember correctly. I found a variable inductor (sliding tap) essential in the absence of being given a precise 'recipe' including number of turns, length of antenna, short connection to ground spike or buried pipes, etc. --Nigelj (talk) 15:10, 20 August 2010 (UTC)[reply]
Shouldn't we keep this article from becoming an article about tuned circuits, it is how simple crystal radios work. It is easy to get carried away. The way most simple crystal radios work is to tune the BCB by tuning a short (capacitive) antenna with a coil only. The diagram does not reflect that. Adding a capacitor across that coil simply aggravates the tuning problem, doing nothing to help it, and is going in the wrong direction. Also, we have articles on resonance and Q that we can reference for details. John (talk) 19:51, 20 August 2010 (UTC)[reply]
Adding a capacitor improves the Q and therefore the sensitivity and selectivity of any xtal set (I assume by masking the combined R & C of the antenna with a purer C term). Capacitors may have been expensive and rare in the 1920s and 30s, unavailable under Nazi occupation in the 1940s, but in fairly standard usage by the 1950s if that reference is anything to go by. --Nigelj (talk) 21:29, 20 August 2010 (UTC)[reply]
I agree. John, there are hundreds of crystal circuits and most of them do use a tuning capacitor of some kind. I have researched xtal sets back to the 20s and have added more than 80 references to this article. I can give you dozens of examples. This article is not just about "simple" crystal radios, but many "simple" circuits do use a capacitor, and solve the tuning range problem easily by taps on the coil or a loading capacitor in the antenna circuit. Besides the lower Q, the main problem with omitting a tuning capacitor is that the tuning range varies with the capacitance of the antenna. If a longer antenna is used, the coil may not tune to the top of the band.
And you're missing the point of this particular image. It needs to be a simple introductory circuit people can refer to while reading the "How it works" section. The majority of readers will be nontechnical people who don't know what a "tuned circuit" is. If the example circuit lacks an explicit tuning capacitor, it will be confusing. The section already has a pictorial schematic of a coil-only circuit, right at the top. --ChetvornoTALK 21:57, 20 August 2010 (UTC)[reply]

Removal of mention of capacitance in 'How it works' section

I notice that, in addition to deleting the image, all mention of capacitance has been removed from the "Tuned circuit" bullet point at the top of the 'How it works' section. This is grossly false, and misleading to nontechnical readers, since it defines a tuned circuit as an inductor alone: "[A tuned circuit] . . . consists of a coil of wire called an inductor or tuning coil used to tune in different stations." Helllowwwww???? As the text made clear before it was eviscerated, a tuned circuit in a crystal radio always consists of an inductor and a capacitance, but some circuits use the capacitance of the antenna. The original description was thoroughly sourced. I don't want to be accused of continuing the above edit war, but unless somebody comes up with a compelling reason this dreck should stay, I'm changing it back. --ChetvornoTALK 22:24, 20 August 2010 (UTC)[reply]

I agree, it was clearer before (e.g. [1]) and I don't see any reason why the capacitor version should be expunged from the article. Maybe it can be better still with the compromise of introducing the capacitance inherent in a 'short' (detuned) antenna, and the possible use of this as part of the tuning 'tank', with disadvantages, when a capacitor is unavailable or impractical. I'm not sure where this new material should go, but somewhere in the 'How it works' section. --Nigelj (talk) 22:53, 20 August 2010 (UTC)[reply]
Changed it back --ChetvornoTALK 23:05, 20 August 2010 (UTC)[reply]
What I read now is better in my opinion. I did not intend to remove 'all mention' of C, if I did (?) I got confused. However, there are still substantial issues with this discussion of resonance, and how the antenna's L and C get tuned with reactances in crystal sets. There is still wrong stuff being stated as fact; this topic is not trivial and kinda beyond the scope of most readers of this article. I wish y'all would fix it or leave it to the antenna folks in the antenna article. I would suggest consider omitting stuff rather than misleading people. I hate to criticize without offering the solution, but I simply don't have time now. John (talk) 05:46, 21 August 2010 (UTC)[reply]
Sorry, I should have realized the capacitance thing was just an oversight. I hope you'll explain about the errors you mention when you have time. BTW, I appreciate you raising the issue of the tuning range of the "naive circuit", even if I disagree with your solution. This article is kind of a specialized subject and I'm glad there are knowlegeable editors working on it. --ChetvornoTALK 07:22, 21 August 2010 (UTC)[reply]
No problem, I will try but it may take me a few weeks at least. If someone else wants to take it on, that'd be nice; my hope was to leave it alone in this article but it keeps coming up. This is involved because an antenna acts like both coils and capacitors and they depend on the frequency. An electrically short antenna has too much C and needs L in the radio to tune the whole system of radio & antenna. It also affects Z matching since X is part of Z = X + R. The previous version of Fig 1 would have had to add too much L to tune with C, and the C would have needed way more range than any physically realizable variable cap could muster. There are references for this issue in some of the old 1920s textbooks but thats another research project.John (talk) 16:39, 23 August 2010 (UTC)[reply]
Yes, they used loading coils and capacitors to tune out the reactance of the antenna. I didn't go into that in the "How it works" section, in the interest of keeping it short and accessible for general readers, but maybe something should be said about it. Do you think the existing text states things that are false or misleading? Where? --ChetvornoTALK 23:25, 23 August 2010 (UTC)[reply]
To mention a couple, I think I see problems with the treatment of Capacitor in the Tuned Circuit section and the the Crystal Detector section is completely wrong in stating that detection efficiency is related to low forward threshold. John (talk) 14:16, 26 August 2010 (UTC)[reply]
Yes; technically there is no sharp diode "threshold" but only the exponential IV curve. But I didn't want to go into the Shockley equation and the resulting reciprocal relation between a diode's forward current and its small-signal AC resistance. The idea of a "threshold" for conduction is an approximation commonly used for diode's IV curves. It's an easy way for novices to visualize the detector's sensitivity problem, and especially why a bias voltage can improve sensitivity, by getting rid of the threshold. Crystal radio detectors' lack of sensitivity is often explained that way, as shown in the references. If you can come up with a better way, I'm all ears. --ChetvornoTALK 17:48, 25 August 2010 (UTC)[reply]
I think the best way to illustrate the problem, found in early books, is to give a graph of a diode's IV curve with the applied sinusoidal voltage against the horizontal axis, and show graphically how it creates a very poorly rectified output current waveform. Maybe such an image could be gotten from an early book and put in the article. --ChetvornoTALK 17:35, 25 August 2010 (UTC)[reply]
You could make a real curve for a 1N34 with pSpice along with the real detected signal curves. But more importantly, there is no sharp diode threshold, but further, there is no diode threshold at all, this notion is not even approximately right. This comes as a shock to most engineers, but the diode curve has no knee (NONE) at all, it is a complete optical illusion. Mathematically the diode equation (and real diodes conduction) is an e-to-the-x form curve (with slight variation, corrupted slighted by "n" the so-called efficiency parameter), and that has no point of max curvature (knee), it just does not exist. Don't take my word for it, check the second derivative of a real 1N34 plot, or plot this and look at it on different scales, the "knee" changes. I realize the notion of a knee is very popular (to the point it is emotional to many), but utterly fictitious. It misleads people to think germanium diodes will always detect better than silicon. Not true, it completely depends on the impedance of the radio tuner and transformed antenna. A lot of crystal set design is substantially suboptimal because of this misunderstanding; being popular doesn't make it technically right. It doesn't even make sense, though college professors actually teach this nonsense in top universities. Reality is that, for signals weak enough for the curve to be approximated by a linear segment, the inverse slope of the IV curve at the operating point (AKA detection Z) needs to match the Z of the radio, again not a trivial issue. It is a tough point to communicate, and few will believe it, I would suggest just deleting the wrong stuff in the article and reference some Agilent papers or something. John (talk) 14:16, 26 August 2010 (UTC)[reply]

The illustration incorrectly describes the slider

The slider in the illustration is on the right, not the left. —Preceding unsigned comment added by 68.42.44.186 (talk) 13:22, 18 July 2010 (UTC)[reply]

Fixed it. Thank you very much for catching my screwup. I must have had a touch of dislexia :) --ChetvornoTALK 16:20, 18 July 2010 (UTC)[reply]

The detector

The explanation of how the crystal detector is wrong. I have made several attempts to correct (perhaps not very well) but my edit is simply removed. There does not seem to be any point in correcting errors if the corrections are simply removed and the incorrect explanation retained.

david.tucker2@virgin.net Davidpetertucker (talk) 17:09, 30 January 2011 (UTC)[reply]

Crystal detector

This explanation would only work is the signal measured several volts peak to peak.

Since the signal in only micro volts p-p how can a diode possibly rectify such a signal?

The real explanation relies on the fact that the diode is a nonlinier conductor and this mices the sidebands and carrier frequencies resulting in the recovery of the modulating frequency (among others). The purpose of the bias is to move the opperating point to the part of the diode's curve which maximises the amptitude of the desired signal. 

— Preceding unsigned comment added by Davidpetertucker (talk • contribs) 09:55, 2 February 2011 (UTC)[reply]

The latest revision is better because it avoids the false myth of "a knee in the diode curve," and is more small signal oriented. A technical explanation of this can take the frequency domain approach of collapsing sidebands, but that is very abstract for many. There is also a very clear time domain explanation that requires a very clear understanding of detection. 128.49.9.18 (talk) 17:25, 9 November 2011 (UTC)[reply]

I wrote the original explanation, that used the "knee of the IV curve" language. I was aware that this was an inaccurate explanation, but I thought that "the signal voltage has to overcome the diode's forward voltage drop" was a more concrete, visual way of explaining to a general reader why biasing the junction improves sensitivity (the same wording is used on many crystal radio websites, see refs). I couldn't think of a way to explain the actual nonlinear curve understandably. Davidpetertucker and others rightly pointed out that this was a bogus explanation. I reverted your edits; I felt at the time it was more important to keep it simple. 24.152.166.100 recently did a good job of correcting the explanation. I understand the desire to correct the "myth of the knee" and present an accurate picture of small-signal rectification in a xtal set, but its inclusion makes the description of the how the diode works awfully "fuzzy". --ChetvornoTALK 05:47, 10 November 2011 (UTC)[reply]

Wrong Units

"energy of only 10−16 W/cm2"

W/cm2 is a power density NOT energy (which should be Joules). Someone put this right, please. 194.72.120.131 (talk) 10:23, 11 May 2011 (UTC)[reply]

Fixed. But you can fix things, too. Wikipedia:Be Bold used to be a guiding principle around here. --Wtshymanski (talk) 13:52, 11 May 2011 (UTC)[reply]

Need to delete Construction section

Do we need a section about how to make a radio that is ineffective at tuning? John (talk) 04:43, 28 June 2011 (UTC)[reply]

It wasn't really about construction and operation, was it? I"ve moved it to the discussion of tuned circuits; it's an interesting point and this was exactly the explanation I needed as a kid to explain why I could only get CFRW (1470) and never CBW (990) on any of the crystal sets I made or had given to me. --Wtshymanski (talk) 13:46, 28 June 2011 (UTC)[reply]

Buzzer?

Was this actually used in the day, or some contributor's original research? I've never seen a book or Web page describing this technique of setting a cat's whisker, and One thing that troubles me is how one is expected to hear anything in the headphones with a buzzer running within arm's reach. --Wtshymanski (talk) 22:32, 10 November 2011 (UTC)[reply]

nevermind. [2]] says the WWI BC14A artillery spotter receiver (no tubes!) had a tuning buzzer. --Wtshymanski (talk) 22:44, 10 November 2011 (UTC)[reply]
Yes, they were actually used. Check the reference at the end of the sentence, that's what its for. --ChetvornoTALK 23:49, 10 November 2011 (UTC)[reply]
Just because most Wikipedia references are junk doesn't mean I should always ignore them. A bad habit of mine. --Wtshymanski (talk) 15:32, 11 November 2011 (UTC)[reply]

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Crystal earpiece impedance

The article describes the impedance of the crystal (piezo) earpiece as being ~ 1 Mohm. This is the DC value. IIRC, the AC impedance is still quite low (~5kOhm), so a coupling transformer is desirable. Have I got this right? I haven't changed the article; someone better informed may wish to. — Preceding unsigned comment added by 87.194.171.29 (talk) 02:29, 10 August 2012 (UTC)[reply]

Ground connection

In order for a radio to work it is necessary to have a ground connection or the radio needs a very large capacitance to act in lieu of the earth.

This requirement is similar to that for a power delivery system. The current from a power plant arrives at your home, it passes through various devices in your home and then travels to the ground wire in the circuit box and from there to a ground wire buried in the earth under your home. In a sense the earth itself is the conductor that returns the current to the power plant where there is a massive grid of conductors buried in the ground. The earth is not a great conductor but there is a lot of it and so it works. There is no wire that returns the current to the power plant. The earth could be replaced with a gigantic capacitor but that would be prohibitively expensive. Similarly, a radio needs either a ground connection or to be connected to a large capacitance that acts like the earth. The use of early crystal radio sets in aircraft and such was problematic due to the vibrations to which the radio was subject not due to grounding problems. I am not certain about the grounding of aircraft but will speculate that the tremendous amount of air through which a moving craft is passing will act like a sufficient ground/capacitance. Zedshort (talk) 16:16, 12 September 2014 (UTC)[reply]

I suspect Wtshymanski understood that; he may have been thinking of counterpoises. As you alluded to, a lot of early crystal sets used capacitive grounding via a counterpoise instead of an earth ground; that is what early aircraft and zeppelin wireless sets used. 1, p.312 From the early 1900s there have been portable crystal sets carried on the body which could not have earth grounds. For example, in the photo at the top of the page, the crystal radio the boy is listening to does not have a ground wire, but is grounded sufficiently by the capacitance of his body through his hand holding the set. --ChetvornoTALK 16:56, 12 September 2014 (UTC)[reply]
Cell phones. Airplanes. Spacecraft. How much capacitance is there between a satellite in synchronous orbit and the Earth? A Mars rover and the Earth? (For that matter, light is just really really high frequency radio waves and propagates just fine over intergalactic distances.) The Tesla model is inappropriate - radio receivers work with the free space field and any use of earth is for convenience. At the frequencies used for AM broadcast, it's inconvenient to build an antenna that collects enough power to operate an earpiece, so you use the earth as part of the system - but it's not really correct to think of Tesla-style currents running through the Earth from transmitter to receiver. --Wtshymanski (talk) 23:59, 12 September 2014 (UTC)[reply]
What are you basing the above upon? Is it just opinion? A ground is shown in most all diagrams of radios. Why are they shown? What purpose does a ground have? In the case of a rover or spacecraft, they too have enough capacitance to function as a ground. Your injection of a counterpoise into the definition makes no sense. A counterpoise is used in the case that a proper ground cannot be made at the transmitter site. Zedshort (talk) 04:42, 13 September 2014 (UTC)[reply]
Zedshort, were you addressing Wtshymanski or me? A crystal radio does not have to have a ground, it could use a dipole antenna; it's just that with the long wire antennas needed, a dipole would require a second long wire, doubling the length of the antenna, to give the same reception. A monopole, due to the ground reflection, has 3 dB more gain than an equivalent dipole. Another issue is that AM stations broadcast in vertical polarization, so a horizontal dipole isn't as efficient as a vertical monopole. A crystal radio with a monopole antenna without a ground can also work, because the radio's case and the hand holding it can function as the other half of a dipole antenna. It will be much less sensitive but is probably adequate for the strong signals in an urban area. --ChetvornoTALK 10:14, 13 September 2014 (UTC)[reply]
A dipole antenna does not use a ground, but a monopole antenna, which is what was used with crystal radios, requires the other side of the transmitter or receiver to be connected to some type of conductive ground plane, which functions as a reflector, not a radiator. 1, p.142 It doesn't have to be connected to the Earth. The size of ground plane required depends on the frequency, it should ideally extend at least a quarter-wavelength from the antenna. The MF and LF broadcast freqs received by crystal sets require a large ground plane, at least 1000 ft in dia., so the Earth is the most convenient conducting plane available. If an Earth ground connection cannot be made, a counterpoise, a wire screen capacitance to Earth, can be used.1, p.142, 2, p.523 "Counterpoises" were widely used during the wireless era for receivers as well as transmitters. 3, p.157, 4, p.217, 5, p.278, 6, p.311-312, 7, p.45 However early radio researchers didn't really understand how they worked. If a counterpoise is too small or too far away from the Earth, its voltage will not remain constant and it will radiate (or in a receiver pick up radio waves), simply functioning as the other half of a dipole antenna. That is actually how the whip antennas in modern walkie-talkies, CB, and portable FM radios work. 8, 9, p.53, 10, p.48-49 The whip is nominally a "monopole" antenna, but the other side of the transmitter or receiver is merely "grounded" to the radio's chassis or circuit board. So the chassis and whip function as the two halves of an asymmetrical dipole antenna. This is also how small portable crystal radios without grounds work. --ChetvornoTALK 09:04, 13 September 2014 (UTC)[reply]


This photo shows a woman in 1922 with a crystal radio in a parasol. It has a wire antenna in the parasol, but is not grounded. The radio itself and the woman's body function either as the "ground" or the other half of the antenna. If they have enough capacitance to ground they function as a counterpoise. If not, they function with the parasol as the two halves of a dipole antenna; the incoming radio wave induces opposite phase oscillating voltages in the antenna and the radio's case including the attached earphone wires, etc. which constitutes the input signal applied to the tuned circuit. --ChetvornoTALK 09:04, 13 September 2014 (UTC)[reply]

Well, this has been a truly fascinating exchange and collection of factoids. Now, would someone please explain the presence of a ground connection in diagrams of radios without using the circular reference of a counterpoise, and do so in twenty words or less. Thank you. Zedshort (talk) 14:03, 13 September 2014 (UTC)[reply]

Sorry, I may have had too much coffee last night. What it boils down to is: The oscillating current from the wire antenna, after passing through the radio, needs some conductor with a high capacity to flow into. The Earth is ideal, but it doesn't have to be the Earth. --ChetvornoTALK 18:47, 13 September 2014 (UTC)[reply]
So, in order for a current to slosh back and forth within a circuit, the charges needs something to slosh into and out from that has lots of space to allow the sloshing, that thing being a capacitor. Whether the antenna is connected to the ground directly via a conductor into the ground or coupled to the ground by a counterpoise makes no difference, provided the capacitance is sufficiently large relative to the need. But of course, in writing the article, that idea needs to be expressed. Should the article describe the "ground" as a very large capacitance? If it is described as I did, as a large reservoir, that might work. The ground wire analogy with a physical connection back to the transmitter obviously fails in the case of spacecraft communications. Zedshort (talk) 20:05, 13 September 2014 (UTC)[reply]
The simple statement is that crystal radios need good antennas because the power that drives the headphones comes from the antenna. The more power the antenna delivers, the better. A half-wave dipole is a good antenna, and it does not need a ground (but for practical reasons might need a balun). Over-the-air powered RFIDs can use dipole antennas. AM transistor radios also used ferrite loopsticks; they capture the magnetic field and do not need a ground. Consequently, a good ground is not a requirement. Back when crystal radios were popular, the operating frequency was low, the transmitted power levels were high (that pesky atmospheric noise required it), and people often used a crappy, untuned, single-ended, long-wire antenna. Grounding those antennas improved them. If the ground were perfect (which it isn't), it would make an image antenna and cause current to flow in the antenna wire. Simplistically, if there were no connection to ground (and no capacitance to ground), then no current (and no power) would flow from the single-ended antenna. Glrx (talk) 01:17, 15 September 2014 (UTC)[reply]

Impedance Matching

I have made a quite serious edit to this section. Given the interest in crystal sets evident in this talk page, it is likely that what I have done will prove to be controversial, so I shall try to explain. Firstly I have corrected the statement of the max power transfer theorem, so that it reads correctly. Impedances should not be equal for max power transfer, but one should be the conjugate of the other. I do not expect any disagreement over this. However, it seems to me that the only respect in which this theorem can be applied to a crystal set is that the impedance of the power source (i.e. the aerial) should be matched to that of the the load (the earphones). Ideally, the equivalent resistance of the tuned circuit at resonance should be severely mismatched to the source impedance of the aerial in order to minimise resistive losses in the tuned circuit, which can only be wasteful. If the tuned circuit were power matched to the aerial, maximum power would be transferred to the resistance of the tuned circuit, which is not what is required. It is for this reason that I have deleted all references to the idea of matching the aerial to the tuned circuit. Transforming the equivalent resistance of the aerial has significance in relation to selectivity, and this concept is best treated separately. Both topics (impedance matching for max power transfer to the headphones, and aerial resistance transformation for improved selectivity) are complicated by the fact that so many different crystal set circuits are used. In the circuit which accompanies the "Impedance Matching" section, it is clear that the left-hand slider can be used for tuning, and the right-hand one for impedance matching, and that the right-hand slider will also affect the Q of the inductor, and therefore the selectivity. In more complicated circuits where a variable capacitor is used for tuning, and the aerial is connected to a variable tap on the inductor, the useful effect of the different tap positions will be to improve selectivity when the tap is low, since then the Q of the parallel tuned circuit will be at its greatest. In this type of circuit, the resistance of the headphones will lower the Q of the parallel tuned circuit, so for greater selectivity, the resistance of the phones should be high. In this circuit, power transfer to the phones will also be affected by the tap position, so that the tap control can be used either to improve selectivity, or to achieve best sensitivity, depending on requirements. g4oep — Preceding unsigned comment added by 77.96.60.31 (talk) 11:43, 4 January 2015 (UTC) G4oep (talk)[reply]

Modulation fix wanted

In the section about "use as a power supply" it states that AM stations modulate only 30 percent. This is only partially true. Aircraft navaid stations modulate their IDs 30% to leave room for voice modulation over the ID. But broadcasting stations routinely modulate 100% and even 125% in the positive direction. Can someone fix this please?

108.244.204.22 (talk) 01:12, 10 January 2015 (UTC) Chip Veres 1-08-15[reply]

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