I'm not a ham, but the idea has always interested me, and so I've decided to test my interest by building a radio receiver. I'm not an electronics noob. It's been my hobby since I was a kid, and I've done a lot of things in digital electronics, including building a Z80 computer, and playing around with some microcontrollers. I've also built an EEG machine, which is basically the extent of my analog experience. I haven't done much with radio in the past (other than generally figuring out that it's hard) but it certainly shouldn't be something I can't do. I also once read a Radio Shack book about radio transceivers, which explained the whole super-heterodyne process (which is, interestingly, information that I've found useful for a lot of things despite not actually using it for radio yet). I decided to build a 40 meter receiver because the frequencies are within range of my 20 MHz oscilloscope (thus avoiding questions like "is it oscillating or not?"), and because a few 40 meter receiver schematics were the first ones I happened to find on the internet. At first I tried following a schematic (a simple regenerative receiver) as closely as possible, but it didn't work (and I'm tempted to include "obviously"), and at the time I figured it was because I didn't have the exact toroids listed. So then I decided that if I could simply make an LC tank oscillate, then I'd be able to construct a suitable inductor and capacitor pair, and move on from there. That started my week-long mission to build an oscillator. I have an air variable capacitor to use a tuner. I first spent a day or two trying to build an oscillator using it and an inductor as a tank circuit, with the intent of measuring the frequency in order to determine the value of the capacitor, but with no luck since I couldn't get it to oscillate.. Then I read on this forum about a "bridge circuit" and so I simply connected the output of a 4 MHz crystal oscillator to the variable capacitor, and subsequently decided it is variable between 15 and 450 pF, which is what I thought it was, but I never wrote it down and the web site where I bought it no longer exists. I then decided to connect another capacitor in parallel to the variable capacitor, and used Wolfram Alpha to calculate what size that capacitor should be, and what inductor should be used, to make it tune between 7.0 and 7.3 MHz. I also solved for some larger ranges with 100 kHz and 300 kHz buffer spaces on each side of the band. I then tried again to build an oscillator, but again with no luck. Then I decided the problem might be the solderless breadboard I was using. Too lazy to solder the whole circuit together, I instead simply soldered the tank circuit together. This made it work, and my oscilloscope confirmed I was near 7 MHz. About this time I decided I should also play with one of my 74HC4046 phase-locked loop chips which I bought probably ten years ago and never even tried to use. I used an 82C54 programmable interval timer for the two frequency dividers, along with an AT89S52 microcontroller connected to a PC via an FT245RL to program the clock divisors. Getting that to work was actually rather easy, which made me wonder if I shouldn't skip the idea of building a simple receiver and just build a super-heterodyne receiver. So then I decided to play with the LM1496 balanced modulator chips I'd bought at the same time and similarly ignored for the last ten years. I simply wired the thing up following one of the examples in the datasheet (or so I thought) and connected my oscillator (the first one, not the PLL) to it, along with a piece of wire for an antenna (being too lazy to build a pre-amp and just playing around) and connected the output to an LM386 audio amplifier. In order to have a signal strong enough to receive, I used my MP3 player to modulate the PLL signal, figuring that since the two circuits were right next to each other, it should be strong enough to be received, and without an antenna, it shouldn't radiate enough to bother anyone. To my complete surprise, this actually seemed to work. Even more to my surprise, I could receive other signals as well. Tuning the capacitor to the correct place, I could hear several streams of morse code at once, and tuning it elsewhere, I heard some Spanish radio, and somewhere else was my MP3 transmitter, though it was the worst signal of them all due to my modulation strategy being terrible. Strangely, the signals were receivable even after disconnecting the piece of wire I was calling an antenna. This was particularly strange with the Spanish radio station, as I can't imagine it being anywhere nearby. The next day, however, it didn't seem to work at first, until eventually I powered on my PLL circuit as well, which is on a separate solderless breadboard, at which point it worked again. So I tried tuning the PLL to different frequencies, but it seemed to make no difference what frequency it was tuned to -- only turning the variable capacitor changed the tuning of the received signal. Then I noticed that I forgot two very important resistors from the LM1496 balanced oscillator IC, such that the chip technically wasn't connected to a power source, which just deepened the mystery of how this radio worked. So I connected the resistors, but then it no longer worked. ...but, for the moment, I decided to forget about it. Instead I decided I needed to change the capacitor and inductor attached to the tuning capacitor. It was difficult to adjust the oscillator to receive a particular signal, and it would drift a lot. I considered attaching a large knob to it, something to make my movements more precise and remove the need for my hand to be anywhere near it when adjusting it, but I also decided I needed to remove the excess 300 kHz I'd designed into it both above and below the band. So I removed the capacitor and inductor, and attached a new larger capacitor and a new smaller inductor. Once again, I could no longer get it to oscillate. I spent the last two or three days trying to solve this situation, by replacing the inductor and capacitor again (although with the same values), and twice attempting to build the entire oscillator as a soldered-together mess of components that isn't attached to the solderless breadboard. I also spent some time filtering even more 60 Hz from my power supply, and I also tried powering the circuit with batteries. ...but, no matter what, it just doesn't want to oscillate anymore. I'm beginning to suspect that the problem is that some combinations of capacitor and inductor work better than others. Obviously, for any given target frequency, there's going to be a range of capacitance values and corresponding inductor values. Do they all oscillate equally well? I've been unable to turn up any information on this topic on the internet, despite searching on several occasions, as I've suspected some values work better than others since the beginning of this adventure. So, trying to figure this out for myself... I can't imagine why, with ideal components, the particular values of inductor and capacitor should matter. With nothing to dissipate the energy, it should oscillate forever. So the problem must be in non-ideal aspects, and all of the stray inductance and capacitance just add to the tank circuit, and so the problem must be entirely with energy lost due to unwanted resistance. However, I've already been making the inductors with very thick wire, and the variable capacitor is rather thick itself. (This might, however, explain why soldering the two together originally made it work, since doing so got rid of numerous resistances due to connections on the solderless breadboard.) I have thought of one other thing... Since P = I^2 * R, it would seem that reducing the current flowing in the tank would be far more important than reducing the resistance of the inductor windings. ...and, I think, that means that a tank circuit using a larger inductor and a smaller capacitor will work better, because a larger inductor's current will increase more slowly while a smaller capacitor will discharge more quickly, and the two together mean that the current is lower even though the frequency is the same. ...and, given that the I is squared while the R is not, using a larger value inductor should always work better, even if doing so requires using thinner wire. So I decided I'd try larger inductors and smaller capacitors and see if that helps. So I calculated some new capacitor and inductor values, but this time assuming I'd put the capacitor in series with the variable capacitor, but the results (1 pF in series with the 450 pF variable capacitor) made me suspect the tuning range would be incredibly lopsided, and so I spent some hours trying to find some application / web site to graph it for me before giving up and writing a Perl script to output the graph in postscript. It was far worse than I imagined, even after picking less extreme values (22 pF in series) which actually result in a far larger tuning range (thus making my original problem even worse). So it seems there's no point in trying a capacitor in series with the tuning capacitor. So I looked online, but I don't see anyone selling the things. Not even the cheap little plastic ones that come in those Radio Shack kits. I'm really not sure what exactly the things would be called. Been searching for "variable capacitor" and the like. The last ones I bought were from Ocean State Electronics, but they appear to have been out of business for years. So, I have questions, obviously: 1. Any advice on building an RF oscillator? Obviously I've read web sites and books, but for the most part, all I find are schematics that don't work, schematics without labeled component values, and a lot of "amplifiers oscillate and oscillators amplify" almost as if to say "yeah, no one really knows why these things work," which doesn't really make sense since how they work is rather simple, at least as I understand it. ...and that does make me wonder if there just isn't something to it that isn't well documented, like this idea I have about the current flow being more detrimental than the circuit resistance. 2. What about this idea I have about the current flow? Do you think it matters? Is it an idea you've heard before? ...or am I way off and you can actually point me in the right direction? 3. Any ideas on how to use a 450 pF variable capacitor as if it were much smaller, like 20 pF or 100 pF, but in a way that doesn't result in a painfully exponential tuner? 4. ...or should I just forget about that, and instead try to build one? I do have quite a lot of double-sided copper clad board that I'll probably never use for anything. However, the required tolerances seem annoying (even if I make it huge) and that stuff is a PITA to cut, so I don't think I like this idea. I may see what I can build from cardboard and aluminum foil. 5 ...or should I just rip some plates out of my air variable capacitor to reduce the capacitance? I really hate to do this, as I've loved the things ever since I first saw one when I was like 10 years old. Something about the things just seems so awesome. However, if I'm right with my inductor/capacitor trade-off idea, then to have the things be of such a large capacitance would actually be quite pointless and so I might as well turn them into something useful. ...but I really hate to do this without being certain that they're useless in their current form. 6. ...or should I forget about tunable capacitors and build a tunable inductor instead? It's certainly fewer moving parts, but I imagine difficulty with respect to getting linear performance out of it, e.g. I might glue an inductor core to a screw, thus allowing me a lot of turns to move the core a small distance, but if, as it turns, it wobbles closer and further from one side of the coil, the results may suck a lot. I'll have to search the internet for ideas on this one. Also, I'm curious just how easy I should expect reception of some 7 MHz activity to be. I live in New Paris, Ohio, if checking a map will help to figure this out. So far I've just been using a ten foot wire as an antenna. I know from my experience with CB radio that using such an antenna wouldn't pick up any transmitter that was further than a mile away, but I've also heard that the ham bands aren't nearly as noisy, and I once had a CB that could tune a little bit of 28 MHz, and IIRC there is a huge difference in the noise floor between the two bands. There's also that morse code and spanish radio I heard, though honestly I have no clue what frequencies those actually were since I have no clue why that circuit worked at all, and so I don't know if I should expect 7 MHz to be so easy to receive that I don't even have to know what I'm doing, or if that was some other band and 7 MHz will actually be very difficult to receive without building a 1/2 wavelength dipole outside (which I don't believe my yard is even large enough for, nevermind that I just don't want to buy the materials).