Near field RF PROBE

[BillC]

I constructed this RF PROBE this afternoon. The idea is to be able to "sniff" for rf around circuit boards ,amplifier stages, oscillators etc, This probe has high spatial resolution which means that it will respond to close in signals. An amplifier might be amplifying on one frequency and oscillating on another higher frequency without the operators knowledge. With the probe connected to a scope it may be able to display the spurious signal as well as the fundamental. Sometimes a tunable rf amplifier will burst into oscillation at some particular setting of the tuning capacitor, due to a change of impedance etc. With the probe applied just above a tuned element in the amplifier stage, then by adjusting the capacitor through the tuning range and with reference to the scope the amplifier may be checked for stability. There must be lots of uses for this kind of probe. If higher sensitivity is required an amplifier may be placed between the probe and the scope. I hope others will build this probe as it is very useful

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[VK3ZYZ]

That looks like it would be a very handy test device.
I must make one. Thanks

 

[BillC]

Here is a sniffer probe with a different type of pickup coil, this coil is wound in a figure eight pattern, and also there is no Faraday slit in the coil end of the brass tube body. This sniffer has a higher spatial resolution than the previous model. It is supposed to respond more to the electric field.The images show the general idea for layout and construction. The teflon spacers are slipped over the center conductor and into the tube so as to maintain co axial concentricity, after the ground end of the pickup coil is soldered to the bottom of brass body lastly the plug is soldered to the top end of the tube and the center conductor is soldered. The tube is 3/8" hobby tube and must be made slightly oval at the bottom end so as to accept the figure eight pickup coil.When assembled heat shrink tube is applied over the body tube. DO NOT use black heat shrink tube.

 

[BillC]

Here is a VSWR sweep of the probe from 2 to 200.MEG. It will go out to1.5 GIG. with minor variations in response.

 

[BillC]

Probe pickup response from 2 meg to 150 meg . Marker 1 hidden in the dip at 2 megs.