The Simplest Way To Spot 2.4GHz RF

When the cool kids are showing off their SDRs it’s easy to forget that a radio receiver can be very simple indeed. The crystal set is one of the earliest forms of radio receiver, a tuned circuit and a diode that would pick up those AM broadcast stations no problem. But lest you imagine that these receivers can only pick up those low frequencies, here’s Hackaday alum [Ted Yapo] with a handy 2.4GHz receiver that picks up strong WiFi and microwave oven leakage.

It’s about as simple as it gets, an LED with a UHF diode in reverse across it. The clever part lies in the wire leads, which are cut to resonate as a dipole at 2.4 GHz. The resulting RF voltage is rectified by the UHF diode, leaving enough DC for the LED to flash. If you are wondering why the LED alone couldn’t do the job as a rectifier you would of course be on to something, however its much worse high frequency performance would make it not up to the job at this frequency.

The glory days of analogue broadcasting may now be in the past, but it’s still possible to have fun with a more conventional crystal radio. If you are adventurous, you can even make one that works for the FM, band too.

24 thoughts on “The Simplest Way To Spot 2.4GHz RF

  1. Something like this was in Electronics decades ago. The diode with the leads as an antenna, but a cheap “tuning meter” as the indicator. Presented as a “microwave leak detector”.

    1. I can’t say how good it works on 2,4GHz, but i did try that with a simple half wave antenna at 144MHz. I could light it throughout the room with around 3w of power.

      People who live somewhat close to a big/national medium wave transmitter can pick up enough voltage to make the led glow perceptibly in darkness.
      Those people can also use a germanium transistor to make an amplified crystal receiver with a few decibels of gain for the weaker stations, if you use a separate resonant circuit to supply the voltage to the Ge transistor.

      1. A friend of mine had a washing machine that picked up the Vatican radio broadcast. Not a digital washing machine, this was in Rome back in the 80’s. It was one of those old glorious contraptions with mechanical switches driven by a timer motor. No buzzers or speakers, but you could faintly hear the broadcast when the machine was off and you put your ear close to its control panel. My theory is that in some way the timer motor windings were tuned to the AM broadcast and either oscillated in sympathy or managed to vibrate the motor magnet just enough that it would be audible. Probably helped by the fact that Vatican radio used to broadcast at power levels that were outright illegal.

        1. “Probably helped by the fact that Vatican radio used to broadcast at power levels that were outright illegal.”

          Considering that Vatican City is an independent nation, the power levels might have been legal (to them).

          “It is good to be King!”
          -Mel Brooks

        2. In Italy, in the 1980’s, private TV stations would broadcast 24 hours a day to stop anyone else from squatting on “their” frequency. And Vatican Radio was not alone in broadcasting at higher than registered power levels.

  2. I tried so many leds and diode combinations and this trick never worked, not on 8watt transmitters nor microwave ovens. Unless all my diodes are fakes Ill call this a the rickroll of RF.

    1. I built about a dozen using RadioShack LEDs, and Schottky Barrier Diodes, and Popsicle sticks from an early 1980s Radio-Electronics article. They all worked, and the LED flashed when compared with a commercial microwave leakage detector “yellow zone”.
      I still have one in my toolbox, the others I gave away.

    2. From a tweet in that thread, it is a 61 mm long antenna.
      2457.3 MHz has a wavelength of 122 mm (~4.803″)

      So 61 mm (~2.4″) would suggest that it is a half wavelength antenna near the middle of the 2.4GHz WiFi range.

      …oooOOO 2.45GHz microwave ovens (maximum power is typically less than 0 dBm ; 1 mW ; 0.224V) OOOooo…
      Microwave ovens for legal compliance need to emit at most 1mW/cm^2 at 5 cm from the oven (0dBm) during operation (the same power level as a Bluetooth headset, and you stick them in your ear!). So typically around a kilowatt (60 dBm) of microwaves (2.45 GHz) with a wavelength of ~122 mm are inside during operation. To light the LED you will need to be much closer than 5cm, unless there is an issue with your microwave oven.

      …oooOOO 2.4 GHz WiFi (maximum power is typically 20 dBm ;100 mW ; 2.24V) OOOooo…
      2401 MHz has a wavelength of 124.86 mm (~4.916″) start of range
      2495 MHz has a wavelength of 120.16 mm (~4.731″) end of range

      For the WiFi antenna it appears to only work very close, inside the near field (one to two wavelengths) of the antenna.

      He always holds the 61mm schottky diode leads parallel to the WiFi antenna. Which would make sense since there would be nearly zero voltage difference across the 61 mm antenna if the polarisation were opposite between TX and RX.

      He holds the antenna by the plastic LED to minimise his body re-tuning the antenna to a slightly different frequency.

      He was running an online speedtest to generate maximum network traffic and therefore maximum RF signal during the demo.

      …oooOOO 8 watt (39dBm ; 19.9V) transmitter OOOooo…
      You might want to check the reverse voltage on the small signal diode you used was a lot higher than 20V because you may have pumped about 0.4A through it and fried the diode.

  3. I was rather dismayed by this article. Not because of the content. Jenny does a fantastic job of presentation.
    It was her perception that it was a useful article worthy of her time to write. Sadly, I think she is correct and should like to thank her for taking the time to teach the basics that the current crop of EEs tend to not know.

    I find the all too common ignorance of fundamentals distressing.

    1. Not everyone here is an EE. That’s not at all a requirement for enjoying this site. If you want a site where everyone knows all the fundamentals that you do, this is not that site.

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