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I'm just getting into radio (inspired by YouTube videos), and like many, I plan to start by building a crystal set (or multiples) before moving up to vacuum tubes (which I prefer over transistors).

All the crystal set schematics I've seen, however, are aimed at AM Broadcast band (540-1610 kHz). I'd like to build one that covers a broader band selection -- ideally, up to 80 m or even 40 m.

Am I correct in understanding that simply tapping the main tuning coil isn't sufficient to extend the working frequency up into the 7 MHz range? Is it even possible to do this without physically switching coils? Or are signals in 40 m and 80 m likely to be too weak to receive with a crystal set, or impossible to separate without making a receiver as complex as a tube regen set?

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A crystal radio generally consists of a simple LC network and a diode detector. So in theory, altering the LC network would allow you to tune higher frequencies. The real issue is that signals you will find in the higher bands, simply do not have enough power to be detected without additional amplification.

You have an even larger obstacle, however. A CW signal will not be properly demodulated by a simple diode detector even if the signal is sufficiently strong. CW is simply the turning on and off of a carrier. If you were to listen to this with a simple diode detector you would hear the carrier when a dit or dah is being sent - this would be denoted by the absence of background noise. The space in between a dit or dah would be denoted by the sudden rise of background noise. So instead of a nice CW tone of 800 hertz or so, an "S" sent via CW would sound like Silence-Pffft-Silence-Pfft-Silence and then prolonged Pfft. To overcome this, a CW receiver creates a side tone through various techniques. This is beyond the scope of a simple crystal radio.

The same problem exists for the SSB voice modes on 80 meters and 40 meters. Your best hope is to catch the occasional AM QSO and then only if the signals are strong enough - you will probably lose patience quickly.

If you are interested in a simple CW receiver kit, for example, there are many on the market that are inexpensive and simple to build. Here is one example from Four State QRP Group:

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An Internet search will reveal many kits for SSB/CW receivers, transceivers, and multi-band versions with a wide variety of features and price ranges.

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  • $\begingroup$ There are voice transmissions in 80m band, right? I've seen CW crystal set designs; they include an oscillator and, of course, require a power source to run the oscillator. I've got most of the parts already in hand to build a regen receiver, which I understand receive CW just fine when set at the edge of oscillation. I'll eventually upgrade that to a regen/reflex to get speaker output. I just wanted to start with a crystal set to get practice winding coils and building RF. $\endgroup$ – Zeiss Ikon Jan 8 at 14:24
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    $\begingroup$ @ZeissIkon Yes, there are certainly CW signals on 80 meters and a regen (not crystal) set would do the job! Have fun with the project. $\endgroup$ – Glenn W9IQ Jan 8 at 14:48
  • $\begingroup$ Oh, I'm looking forward to building with tubes. I plan to build a QRP tube transmitter or transceiver once I learn code have my license (I ran through the entire question pool on the ARRL site and scored above 80% with no prior study -- regulations and antenna questions were the only problem children). $\endgroup$ – Zeiss Ikon Jan 8 at 15:23
  • $\begingroup$ @ZeissIkon Excellent! Good luck with the exam. $\endgroup$ – Glenn W9IQ Jan 8 at 18:03
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    $\begingroup$ I've edited the question to remove mention of CW -- but as noted, unless there's high power AM voice in the SW bands, it may not be worth trying to go much above 1610 kHz. $\endgroup$ – Zeiss Ikon Jan 8 at 20:30

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