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I want to build a simple RF radiation detector that shows if there are any RF devices transmitting at a certain frequency. I want to use it to detect RF radiation in the 2.4 and 5 GHz range. I have a 1SS86 diode which works at high frequencies, a MMBR941 HF transistor (works up to 8Ghz) and a 2SC3356 transistor.

I also have a DC ampere meter (range 0-100uA) which I would want to use to measure the signal strength. Or I could use an LED instead. Or better I could use headphones to measure the radiation intensity as sound.

Basically I want to extend the range of this device (I think it's called lectenna.)

I tried to build it myself like it was shown in the video and the LED and ampere meter only detects the radiation when the 1SS86 diode is near the Wifi antenna (the meter shows a 10uA at a distance of 10 cm from the antenna and spikes up to 100uA when I move it right near the antenna). But if I move further away from the router, like more than 15 cm away, the meter does not record anything. So I thought that I need to somehow amplify the signal so thought to use a transistor but that did not work for me so maybe some of you could help me?

For the 2.4GHz I use a dipole antenna with a total length of about 5.9cm (2.95 cm in each side), like it is shown in the video. I do not have a variable capacitor to create a LC circuit so I am using fixed length of the antenna in a hope that this can bypass the use of variable capacitor.

If I absolutely need to create a LC circuit, I can build the coil myself and for the variable capacitor, I could try to make one using aluminium foil and paper like in some Youtube videos although I am not sure that it will work at such high frequencies.

This is what works

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  • \$\begingroup\$ Show what circuit you have tried, including your transistor circuit. \$\endgroup\$
    – JRE
    Oct 20, 2020 at 11:14
  • \$\begingroup\$ Hand made capacitors have little chance of working properly at such high frequencies. \$\endgroup\$
    – JRE
    Oct 20, 2020 at 11:15
  • \$\begingroup\$ I tried this circuit circuits-diy.com/am-signal-catcher-using-1n914-diode and this one circuits-diy.com/simple-rf-detector-circuit-using-transistors but they were not working for me. \$\endgroup\$
    – avg_cat
    Oct 20, 2020 at 11:17
  • \$\begingroup\$ Add the references to your question. Put the schematic diagrams in your question and indicate what you changed, or even better draw diagrams of your own circuits and indicate the parts you used in the diagram. \$\endgroup\$
    – JRE
    Oct 20, 2020 at 11:22
  • \$\begingroup\$ Simple lectennas don't have much range when used with low power signals. I built one to wear on my hat, just because. It can detect 2.5GHz WiFi at a "range" of a couple of centimeters. You're going to need a better diode than the SS86 to get up to 5GHz, though. \$\endgroup\$
    – JRE
    Oct 20, 2020 at 11:27

2 Answers 2

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The ISS86 diode has a capacitance of 0.85 pF and, at 5 GHz this is an impedance magnitude of 37.45 Ω making it pretty unsuitable as a diode detector because, in the forward direction, it will rectify (what you want) but, in the reverse direction it will still have high AC conduction due to the 37.45 Ω capacitive reactance.

The MMBR941 has an \$f_T\$ of 8 GHz and this means that at 8 GHz the current gain has dropped to unity. At 4 GHz the current gain will double to 2 and, at 2 GHz \$h_{FE}\$ will be 4. In other words a fairly poor device for your targeted operating area.

The 2SC3356 has an \$f_T\$ at 7 GHz. Enough said.

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  • \$\begingroup\$ I see. Thanks for the explanation Andy. If I am not able to make the deice work for 5Ghz, how can I at least expand the range of the 2.4Ghz Lectenna? I mean currently it detects the radiation only about 10 cm from the router but how can I make it detect radiation at 30-40 cm from the source? \$\endgroup\$
    – avg_cat
    Oct 20, 2020 at 11:46
  • \$\begingroup\$ I'd consider using a chip like this one, the MAX4000. But you will also need extensive band-pass filtering on the input side to the chip to ensure that what it receives is broadly in the 2.5 GHz part of the spectrum. \$\endgroup\$
    – Andy aka
    Oct 20, 2020 at 12:01
  • \$\begingroup\$ There's also the LTC5533 - it will work up to 11 GHz and has an input power range from -32 dBm to 12 dBm. It's a very specialist area. There is also this device from TI, the LMH2120. \$\endgroup\$
    – Andy aka
    Oct 20, 2020 at 12:05
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    \$\begingroup\$ Also, I have a (less engineer-targeted) answer over at the amateur radio SE site: How to make an RF probe for accurate power measurements \$\endgroup\$ Oct 20, 2020 at 12:37
  • \$\begingroup\$ @MarcusMüller looks good. \$\endgroup\$
    – Andy aka
    Oct 20, 2020 at 13:02
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One approach is to forward_bias the diode.

Create a series circuit: resistorA, diode, resistor+Capacitor, with top tied to 1.5volts and bottom tied to 1cm square of copper foil

(Aluminum foil instantly has an insulating oxide surface, so you cannot get a reliable contact with that foil).

If can get a HewlettPackard 5082-2935 diode (100 picosecond reverse recovery time) , then life may be simpler.

By forward biasing the diode, with a few microAmps, the needed voltage should drop by 10:1, giving you 10:1 more range.

In that series circuit, use 470 K Ohms from the +1.5 volts to anode of the diode.

Connect the antenna (your only frequency_selective component) to junction of diode and that 470 K ohm.

On other side of diode, (the cathode (bar) end), remember you have a parallel R+C network. Have 100pF capacitor to that 1cm square foil, along with Resistor of value 10Kohm (or could use 1kohm, but that may risk your 100uA meter needle being wrapped around the top-end mechanical stop pin) that connects to + lead of the meter.

You could use a 10Kohm linear_taper (to avoid pot selection confusion) with 1 K ohm in series, to replace that fixed resistor in parallel with the 100 pF. This provides sensitivity adjustment.

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