ANTENTOP- 01- 2018 # 022

RF Sniffer

RF Sniffer

Igor Grigorov, VA3ZNW

FSM (Field Strength Meter) is the device that should be present on any amateur radio station. As for me, I have been using MFJ- 801 FSM for a long time. I use this device on all HF bands as well successfully tried it on the 145 and 430- MHz band. I used to the MFJ- 801 in the field under rain and snow and, of course, at home without problem from this one. However I've decided add to my radio station a simple RF Sniffer that may sniff the radio waves up to 3000 MHz i.e. that device should react on to cell phones and internet routers.

Experimental RF Sniffer

In the past times I made lots of different FSM so it was no problem to draw a schematic for my experimental Sniffer. The RF Sniffer was intended to sniff 2.4- GHz Band. Figure 1 shows the schematic. It is a usual radio detector. L1 is a loop of wire with perimeter in 12- cm length. Diameter of the wire is 1 mm. Actually L1 is a resonant antenna that could catch waves around of 12- cm band or near frequencies 2.4- GHz. I took the length because WiFi routers, microwave ovens, wireless devices like mouse and keyboard work in the 2.4 GHz band. The loop is not optimal for Canadian cell phones that are working in 850/1900 MHz band but due high power radiated by the cell phones the RF Sniffer (I believe) should sniff the devices. D1 is a UHF diode, R1, D2 and D3 protect M1 from overload by power RF signal.

Figure 1 RF Sniffer

At first deal I decided make the RF Sniffer on a breadboard and try different version of the schematic. Some time ago I have bought at ham flea market real 1N21 (microwave mixer S- X Band diode) and 1N35 (matched pair of 1N34), both of the parts made by SYLVANIA in USA, so I impatiently would like to try those old legendary diodes.

MFJ - 801 and Curious Cat

However, real test of 1N35 was disappointed for me. The diode worked fine up to 430 MHz but higher the sensitivity drop to zero. 1N21 was fine diode for my RF Sniffer. It worked fine on 2.4 GHz. The diode could catch radiation from cell phones, routers and wireless mouse. But... at one of my experiment with a cell phone the diode was burned out. I had just one such diode and next ham flea market where I could buy such stuff was months away... on E- Bay such diode costs pretty money... So, I decided give up old stuff and use to modern UHF diodes in the RF Sniffer.

1N21. Made in USA, SYLVANIA

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