Quote:
Originally Posted by Mike(Mont)
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Quote:
Originally Posted by Qiaozhi
The video either shows some interaction between the calculator and the AM radio caused by EMI or, as joecoin says, the guy is "... manipulating the volume with his thumb ...".
Does anyone here have a small AM radio to hand? This would be a very simple experiment to try.
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This little experiment does work, and has been around at least since the 1960's. In my "vintage" detector collection, I have a "Ross" from the 70's, which is literally an AM radio, slightly modified with a search coil. There have been several BFO magazine projects which have a simple Colpitts oscillator for the TX, and an AM radio for the RX.
This experiment works best if either the oscillator is easy to shift (like a Colpitts), or the receiver (like a poor AM radio design). A calculator uses a crystal oscillator so it's not going to shift, therefore you need a cheesy AM radio to get a decent BFO operation. But sure, it will work. Distances are on the order of an inch or less.
I own a Radio Shack AM radio ("Flavoradio"), which happens to be identical to the AM radio stuffed inside the Treasure Scope Raven LRL.
This is one of the last AM-only radio designs and uses a pretty stable AM radio-on-a-chip, so it does not make for a good BFO when used with a calculator. But it will still "beat" with the calculator and produce a buzz. When you move a metal target nearby, the volume (not frequency) of the buzz decreases, because the metal is stealing induction energy from the radio. So it works more like an off-resonance design.
Now for the funny part...
"What calculator did you use, Carl?"
Well, since you asked, I used the Karce calculator from my Ranger-Tell Examiner. It has an all-plastic case, so the RFI leaks out unimpeded and the experiment works as I described.
But wait... I have a second Examiner, which uses the HP-6S calculator. This calculator has an all-metal case: solid on the back, and with lots of holes on the front for the buttons and display. With the
front of the HP facing the AM radio, the holes for the buttons leak enough RFI for the experiment to work.
"But Carl, what happens when the HP's solid metal back is facing the AM radio, the way it faces the "induction circuitry" inside the Examiner?"
Good question!
Nothing happens. The metal back blocks the RFI, and the experiment fails.
So, if you're still inclined to believe the Examiner's calculator couples a signal into the internal circuitry, it is probably worth pointing out the stupidity of using a calculator with a metal case that blocks that very "signal".
- Carl