This small E-field probe was developed for use on a car mag-mount antenna base, so I could drive around the local area (up to 25km) to measure the signal strength of my 137kHz QRSS3 beacon feeding a 20m spaced earth-electrode antenna in the garden. To my delight it worked very well and I was able to copy the signal, despite it being only around 6-7uW ERP to at least 24km, which was as far as I went.

The drain of the FET uses a small 3C90 toroid as part of a parallel tuned circuit tuned to 137kHz by chosing the parallel capacitance (actually a couple of values in combination) to bring it to the correct resonance. Although the schematic shows a decoupled source resistor, this was not fitted in the prototype as the sensitivity was adequate without.

The sensitivity was such that with just the unextended whip antenna on the E-field probe, perfectly reasonable QRSS3 copy of my 6uW signal was obtained at 24km range.

The E-field probe was run from a small 9V battery within the unit, which was housed in a small plastic box. The attachment to the mag-mount was via a screw on PL259 plug.

The E-field probe feeds an FT817. Note that on 136kHz the FT817 gives better sensitivity with the IPO engaged. On 472kHz whether IPO on or off makes little difference.

The EFP was build on a small piece of copper clad board with MeSquares and dead bug techniques. Around the very high impedance input to the FET the connections were made "in the air" to minimise losses.

Although J310 FETs would be better, the low cost MPF102 and 2N3904 emitter follower worked well in my intended application.