Simple RF field detector

[jujun]

Hello,

I want to build a simple RF field detector that will light a led when detecting an RF field.
I found this : http://rookieelectronics.com/sensor-rf-radiation-detector
But I don't understand why this kind of diode are used, I was thinking that something like an IN34A was more appropriate.
Maybe it's because it's cheaper ?

I found that it would be a great little DIY project for kids.

First I was thinking about using an analog uA meter (because the one I have work very well with it), but it's not very cheap, and a bit old fashion.

But near 2V are needed for a red led, it's quite a lot !

Any advices on how to have something simple, sensitive and cheap ? (but still toy-grade)

Thank you

[ChristofferB]

I suspect the 1n4148 is used cause it's THE most common small signal diode. You could probably use any diode, but I seem to recall germanium diodes being used in older field strength meters.

You could for the annoying solution of adding a small telephone transformer and piezo speaker. That would make it sortof a crystal receiver, but it would be very sensitive.

[David Hess]

1N34As are not all that common and the improvement in performance using a "zero bias" diode like a germanium or low barrier schottky when directly driving an LED would be small.  If a 50 or 100 microamp meter movement or an amplifier was used, then a more suitable diode or a different circuit configuration would improve the performance considerably even without amplification.

So they used the 1N4148 simply because it is ubiquitous and inexpensive.

[jujun]

Thank you all for you answer

I built it and it works quite well with an UHF signal (quite strong about 5W)

Is it true that it's a diode voltage doubler?
Maybe I can add another two diodes to have a higher voltage, and then be able to reach the 2v of the led easilly ?
It would be nice to have it more sensitive!

[David Hess]

The biggest limitation is the LED efficiency at low currents.  They make special high efficiency low current LEDs.

The 1N4148 diodes could be replaced with germanium diodes like the IN34A or 1N270 or small signal schottky diodes like the 1N5711, BAT41, or BAT83.

If I wanted to be clever, I would use a pair of the above diodes with an energy harvesting IC to act as a DC impedance transformer to drive the LED.

[wolferl1210]

A more sophisticated way to detect is by using a
https://www.aliexpress.com/item/AD8302-Amplitude-Phase-RF-Detector-Module-IF-2-7GHz-Phase-Detection-for-Ham-Radio-Amplifiers-Board/32852790850.html?spm=a2g0s.9042311.0.0.27e94c4dHPsGDy
module.

Connect to each input a short antenna (10 to 20 cm) copper or steel. They should be fixed in
distance and direct mounted on the SMA connectors.

Any radiating device nearby, on a very wide frequency range, will immediately result in amplitude
difference on that two antennas that can be meassured or indicated with a LED row.
That little board has an output signal, up to 2 volt, for amplitude- and another one for phase-difference.

The board is cheap ;-) just ~ US$ 5,- but you have to wait several weeks for delivery.

Commercial devices, using the same principle, used for counter surveillance, are on the market.   
Unfortunately beyond any private budget. But it is 'the swiss army knife in counter surveillance' ;-)

Look at the 'Delta V Advanced' and 'Product details' (in red) It does the same!
https://www.audiotel-international.com/countersurveillance/peaceofmind/

details in here: (attached on this post is a pdf).
https://www.eevblog.com/forum/blog/eevblog-956-countersurveillance-monitor-teardown/msg1095891/#msg1095891