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I was trying to build a passive cell phone detector this evening, and finally got one to work.

Along the way, I tried two different antennas: a 1 wavelength loop, and a dipole.

I had the hardest time getting a signal out of the dipole, but the loop worked like a charm - hooked it up, and got a blink from my phone.

Here are the circuits:

schematic

simulate this circuit – Schematic created using CircuitLab

In both circuits, I get pulses of voltage across the 1N34. In both circuits, the pulses across the 1N34 are several volts (varies depending on distance and phone activity.)

In the loop circuit, I also get pulses across the LED - it lights up.

In the dipole circuit, I don't get voltage pulses across the LED. It acts like an open circuit - my oscilloscope shows 50Hz hum as though I had connected a length of wire to the probe.

The dipole will not light the LED in this configuration.

I don't understand what is different between the two that causes this.

I did find, though, that I can place a voltage multiplier across the 1N34 in the dipole circuit and get a blink.

That looks like this:

schematic

simulate this circuit

The dipole with voltage doubler works, and has about the range of the loop antenna.

Since the voltage doubler worked so well for the dipole circuit, I thought I'd try it on the loop.

That was a disappointment. Putting a voltage doubler on the loop antenna actually makes it less sensitive.

Now it gets interesting.

I put a second stage on the voltage doubler on the dipole antenna, and drastically improved the range.

Now, down the the questions:

  1. Why doesn't the simple dipole circuit work?
  2. Why does the voltage from the loop circuit actually drop when using a voltage doubler?

I've decided to go with a dipole and a couple (or three) of stages of voltage doublers to finish my gadget, but I'd like to understand what is going on here.


Possible explanations:

For (1):

a. There isn't a DC path through the dipole in the simple circuit. The RF passes through the 1N34 and on out the opposite antenna half, passing through the LED due to capacitance. The rectified DC, however, has nowhere to go. The dipole is only a closed circuit for RF.

b. Suggested by Glen Willen: The loop is acting more like an inductor than an antenna. It reacts differently to the voltage doubler because the load changes the resonance of the inductive circuit.


"Range" is something of a misnomer. The dipole with a two stage voltage doubler will blink a couple of centimeters from the phone. The loop blinks when it is almost in contact with the phone.

This is fine for what I'm doing. This isn't a functional circuit. It's more of a decorative thing.

The band on my hat broke, and I'm going to make a new one out of leather. The cell phone detector goes on the new hat band in place of the original escutcheon.

A dipole is mechanically easier to use than a loop - the dipole can hide behind the band, whereas the loop can't. And, a loop long enough to go around my head won't receive cell phone signals.

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  • \$\begingroup\$ With capacitors that big, I think you are responding to packets, not the individual sinusoids. \$\endgroup\$ – analogsystemsrf Dec 23 '18 at 3:51
  • \$\begingroup\$ @analogsystemsrf: Yes. I get pulses the width of the packets. The voltage doubler was cobbled together out of the junk box. Those are the values used, but they are probably not anything close to optimal. \$\endgroup\$ – JRE Dec 23 '18 at 3:53
  • \$\begingroup\$ A Quad antenna has directional gain relative to a dipole. \$\endgroup\$ – Chu Dec 23 '18 at 8:35
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I don't have a full answer (and it will be hard to get one without a lot more measurements with instruments you maybe don't have -- antenna design is a black art even with good tools.) But at the distances you're considering, near field effects are likely present. So the loop may be picking something up inductively, which the dipole cannot. Inductive coupling will be much less sensitive to the exact length of wire involved -- tuning an antenna to resonance without tools, by just cutting to length, is unlikely to work very well except by luck, so your antennas are probably not that close to being resonant.

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  • 1
    \$\begingroup\$ I used 2NEC4 to simulate the antennas. They seem to be fairly broadband, and the actual circuit bears this out. It takes a change in length of centimeters to make a large difference in sensitivity. It doesn't matter much if the dipole arms are 7 or 9 cm instead of 8. If you go too far, of course, it gets worse. Same with the loop. Give or take a couple of centimeters, no problem. Change by 4 or so, and it gives up. \$\endgroup\$ – JRE Dec 23 '18 at 13:47
  • \$\begingroup\$ I will check and see what 2NEC4 says about nearfield, though. \$\endgroup\$ – JRE Dec 23 '18 at 13:47
  • \$\begingroup\$ And, you guess right. My little work room does not include equipment for analysing GHz stuff. My old scope has a maximum bandwidth some where far south of 50MHz. When it was built, GHz circuits were the stuff of military projects and advanced research projects. \$\endgroup\$ – JRE Dec 23 '18 at 13:51
  • \$\begingroup\$ The inner near field is chaotic and non polarised, so inductive coupling is perfectly feasible. \$\endgroup\$ – Peter Smith Dec 23 '18 at 15:25
  • \$\begingroup\$ Oh, interesting that you simulated the antennas and they're not sensitive to length. It sounds like your guess is probably better than mine given your level of tooling. \$\endgroup\$ – Glenn Willen Dec 23 '18 at 23:04

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