I came across the following circuit on net which is a simple RF/cellphone signal detector:

enter image description here

Before I build and try I decided to simulate it in LTspice as follows:

enter image description here

I'm stuck at point about modelling the loop antenna for the simulation. You can see I put ??? question mark there in above schematics.

What type of source should I place at ??? to mimic the antenna? Should I use a 1GHz sinusoidal voltage signal source or a current signal source at 1GHz for instance? And what would be a realistic amplitude or frequency of the signal for a cell phone nearby like 5 10cm apart from the antenna?


Regarding an answer:

enter image description here

Now what can we say about the model for the antenna coupling?

I used a current source with 1uA amplitude and 1 GHz freq. Coupling done by a transformer above with 1uH inductances and 0.01 coupling factor.

Would this be realistic for a loop antenna for 15 cm diameter 10 cm far away from a typical cell phone?


Ideally, you'd put an inductor there that represents the inductance of the actual antenna. You would weakly couple that to a second inductor that is driven by a current source.

If you have inductors L1 and L2, you might couple them with a statement like

K1 L1 L2 0.01

somewhere on your schematic.

See Using Transformers in LTspice/SwitcherCAD III

After that, I'd measure the RMS power that the coil is delivering to the detector circuit (multiply its voltage and current, take the average). Turn down the current generator amplitude until the circuit stops working, and figure out what threshold level of power this represents.

Whether or not an actual cell phone can induce that amount of power in an actual coil can only be determined by trying it.

  • \$\begingroup\$ Thanks I updated following your answer. Please see my edit. What do you think now about the quantities? This works in simulation turns on the LED. \$\endgroup\$ – user16307 Feb 23 '18 at 21:45
  • \$\begingroup\$ See my edit above. \$\endgroup\$ – Dave Tweed Feb 23 '18 at 22:37
  • \$\begingroup\$ A tuning capacitor has to be added somewhere to tune to the frequency (1GHz), correct? Physically, it goes across loop antenna terminals, correct? The question I have is where to put this tuning cap in the LTSpice model to model that correctly? Current source side of the transformer or the receiver side? \$\endgroup\$ – acker9 Dec 2 '19 at 21:13
  • \$\begingroup\$ @acker9: Not necessarily. It might be desirable for the receiver to be as broadband as possible. \$\endgroup\$ – Dave Tweed Dec 2 '19 at 21:19

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