You have this quite wrong. A Q of 500 is a loss of 0.2%, and
1500 is a loss of only 0.067%. Increase your coil area by 0.13% (a fraction of a millimeter of diameter) and you recover the power you lost by "only" having a Q of 500, by gathering more power to start with.
Furthermore, because the signal of interest spreads +-5Khz or so, when you use a super high Q, you *reject* the parts of the signal that are outside the passband. So yeah, you get more voltage for for the low audio frequencies, but *less power overall*.
Duh. Doesn't sound like such a good deal now, does it?
The carrier, and the sidebands, which spread either side by the bandwidth of the audio being broadcast - perhaps 5 or 8KHz. That means as soon as your Q exceeds 200 (@1MHz), you start to reject some of the transmitted power.
You really know so little that you can't even conceive of how little you know.
You start to *reject* some of the received power as soon as the Q passes 200ish, and you destroy the audio at the same time.
You can quadruple your received power by doubling your coil diameter, or fitting a longer wire. That's *far* FAR more effective than saving 0.2% by using higher Q.
With a crystal ear-piece, you still may have an impedance matching problem. For that, you should use an audio transformer *after* the detector.
in a crystal radio the tank is fairly heavily loaded that reduces the effective Q, every picowatt that isn't lost in the tank parts is a picowatt you can send to the earpiece.
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You don't have any idea of what he wrote. Read it again and try to understand what he is talking about before you dismiss it so (actually not so) glibly.
That is the _unloaded_ Qu. Still 500 sounds excessive high for ordinary LC circuits or loop antennas. At higher frequencies a silver coated cavity resonator would easily ac hive that.
When you connect the load you get the _loaded_ Ql. This Ql determines the actual receiver bandwidth. For AM reception a 10 kHz bandwidth would be desirable, so at 1 MHz the loaded Ql=100 would be suitable.
The Qu/Ql ratio determines the signal losses. a Qu/Ql=10:1 gives less than 1 dB losses, a 2:1 ratio and the loss is a few decibels. A
500/100 ratio gives about 2 dB losses. In receivers with amplification, this loss is directly added to the noise figure. For this reason, it is undesirable to have too high loaded Ql in front of the first amplifier stage. On the other hand, a wide front end can easily overload the first amplifier or mixer. A high Ql filter between the first amplifier and mixer helps a lot. This is an issue on VHF and above.
It should be noted that BW = f / Q only gives the -3 dB point of the response curve. There might be a 40-60 dB signal on the adjacent channel, which will effectively kill the reception of the wanted signal.
There are cases where the Q of the tuner and antenna combination restricted the way the transmission can be modulated.
In aviation, there are non-directional beacons (NDB) working in the MF range. Due to obvious mechanical restrictions, the vertical antennas are very short compared to the wavelength (70 ft compared to 4000 ft). We had to limit the frequency of the AM modulated identifier Morse code from the standard
1050 Hz to 400 Hz to pass the bandwidth of the antenna system.
And yes, there was some power, typically 500W into the antenna and a couple of W into the air.
I'm sorry you're so confused by facts, but I'm not responsible for fixing your innumeracy. I'll just get the popcorn and watch while you chase unicorns.
Exactly, not 500 or 1000. Still, even with a somewhat matching loaded Q the audio quality is mediocre because a simple tuned circuit has a poor shape factor. This results in higher audio frequencies being muffled, speech to be less easy to understand and as you said further below a strong station on the next AM channel will spoil the whole experience by swamping the signal.
Which is close to impossible to even hear.
There is probably some sort of sports ambition where the guy with the highest Q wins a trophy or at least a free beer :-)
Most of all it muffles higher audio frequencies in the sidebands which makes the listening experience less than pleasant. This is why good receivers like the ones in my lab have 6- or 8-pole crystal filters.
As the old Romans said, hic Rhodus, hic salta.
Give us an example of such an antenna with a Q of 500 or more. Just one. And yes, I have a ham radio license and decades of RF experience. My clients pay me for that.
I'm afraid you are missing some facts on crystal radio crystal optimization. Here's research by one of the real gurus of crystal radio optimization, Ben Tongue of Blonder Tongue fame.
There is a lot here so A quick look says article 22 and 24 may help, but others have relevant information. Mikek
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Bell labs reported a bandwidth of 300Hz to 3000Hz is range required for intelligible speech. When you slip your headphones on, all you need is to identify the call letters of a distant station. No one complains of excessive Q, it is so easy to spoil. I have a coil on my Q meter now, adding a 10M resistor across a coil with Q=1300 lowers it to Q=1100.
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It's funny to see so much emotion over somebody else's hobby circuit. Building the ultimate crystal set is like making an oscillator that will run on a 5-mV supply. It's not good for much, but it can be a lot of fun if you have time on your hands and nothing to prove.
I can see the fun of trying to make the highest-Q coil you can, just as I can see the beauty of shiny silver-plated air variable caps.
Think stamp collecting, not designing for 1e6 units.
Cheers
Phil Hobbs
(Who once built a completely impractical capacitive gauge that was linear to 0.2% right down to where the plates touched. Nice polished brass things they were, too.)
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Dr Philip C D Hobbs
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ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
Crystal radios do not use the coil to collect RF energy from the air, a long wire antenna of 25ft to 150 is attached tothe coil to pickup the RF signal. If you want to discuss loop antennas, start a new thread.
Take a breath! I'll start simple, Coil and tuning cap have a unloaded Q of
1000. To extract maximum power you use a load that matches the Q times Xl of the coil. Loading the coil with it's matched impedance lowers the Q to 500. But before you do that match, you need to insert a diode to demodulate the signal The diode adds a series resistance into the circuit, that is before the matching audio transformer. Mikek
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The highest Q's I've seen are just over 2000, using 2 parallel 660/46 litz wires solenoid wound. The site that it was on is defunct and the Wayback machine server for the link seems to be down.
However, I did find the link to a Ferrite coil with a Q over 2000.
And, isn't it great that we have evolved to a point that food, water and shelter are not concerns, and we can play at anything we want!
Mikek
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