E-field probe mag-mount now built

This morning I decided that it would be better to build the E-field probe for 137kHz RX at the base of the whip directly on the mag-mount base rather than inside the vehicle and having to tune out the cable capacitance. This way,  the cable capacitance is no longer an issue. I built the E-field probe into a small plastic box and arranged it so the whole unit attaches to the mag-mount using a PL259 plug in the bottom of the unit. Radio 4 on 198kHz is an S9+ signal with the mag-mount sitting on the bedroom window-sill with around 10 inches of whip. It appears to be working well.

The present design is a poor man's PA0RDT probe but using an MPF102 and a 2N3904 rather than the better devices used in Rodolph's high IP2/3 design. I may put a tuned circuit in the FET drain to increase the gain at 137kHz and reduce the gain outside the band. I will experiment with this before going out in the car.

Currently it is snowing continuously here, so road testing the EFP and checking coverage of my QRP 137kHz CW and QRSS3/30 beacon will now have to wait until later in the week.

UPDATE: I have modified the EFP so that the FET output comes from the drain with a drain tuned circuit tuned to 137kHz. I have the source resistor not decoupled initially but will consider adding this if I need a little more overall gain as the barefoot FT817 is very deaf on 137kHz.

A mag-mount as an E-field probe for 137kHz

Next week, assuming the snow has gone, I want to drive around the local area to plot the polar pattern of my earth-electrode antenna working on 137kHz.  My 7-8W beacon transmitter can send a range of QRSS and CW messages, so I want a way of driving to different places using my FT817 fed via a preamp and measuring the S/N using Spectran whilst remaining inside the car.

Portable RX loops are fine, but these are directional and need to be set up each time a measurement is taken outside of the car.  Ideally I need an omni-directional RX antenna like an E-field probe. So, I'm looking for a way to drive to a spot, take a quick measurement from inside the car, then move on; a loop works, but when it is -2 deg C outside and the ground is icy, it is not ideal.

Either I put the electronics for an E-field probe right at the base of the mag-mount (mechanically not ideal) or I have the electronics in the car and somehow tune out the coax cable capacitance.  At the moment I am still experimenting to find an arrangement that works with good sensitivity by checking the strong Greek commercial signal just outside the 137kHz band.

Based on 472kHz results, the earth-electrode antenna behaves like a loop with loop-like directionality. I 'd expect to see the same pattern on 137kHz.

VLF E-field probe update

Today I compared the PA0RDT E-field probe against my own VLF design (on the sub-9kHz website) that uses an 8-9kHz tuned circuit in the FET preamp drain. To do the test I had my wife hold each E-field probe at arms length above her head in the bottom of the garden whilst I made screenshots on the PC. The S/N on Alpha beacons and 18-22kHz MSK signals is similar with the PA0RDT and mine but the PA0RDT design has a lower noise floor. So, in future I will use Roelof's design. Some measurements made by G3ZJO today compared the PA0RDT design against the narrower band VLF E-field probe by DK7FC and Eddie's results suggest comparable performance within the limits of experimental error.

E-field probe tests

This afternoon I built a version of the PA0RDT E-field probe. In the shack the interference was horrendous, but in the garden with the laptop PC just below it the reception of the VLF Alpha beacons and MSK signals around 18-22kHz was excellent. I need to do more noisefloor measurements and elevate the probe higher, but I think this is going to work well when mounted at the top of my neighbours tree that overhangs into my garden. This is a sycamore which I trimmed back a few months ago. The E-field probe can be mounted "in the clear" some 20m from the nearest house at a height of around 5m off the ground. I now need to rebuild it into a watertight box and think how I am going to get the output and power feed to the shack that is at the wrong side of the house some 30m away. Some have used CAT5 twisted wires for this purpose and I guess this would be a good, simple solution, although less suitable if I want to use the probe at 137 and 500kHz where a coax feed would be better.

VLF E-field probes compared?

Both the PA0RDT and DK7FC E-field probes antennas are suitable for detecting weak VLF amateur signals around 8-9kHz when used with the right PC software. See https://sites.google.com/site/sub9khz/antennas for the schematics of these and others.

What I'm wondering is has anyone done a "like-for-like" test to compare their performance? With a near identical test set-up (same height above ground, same ground and PSU noise conditions) it should be possible to compare noise floor, sensitivity (S/N of Alpha beacons, VLF MSK signals), and by looking for the intensity of the intermod line at 9kHz from broadcast signals in Europe get an indication of dynamic range. Has anyone done this test at VLF? If not, it looks like I shall have to do it and "suck it and see".

Incidentally, the PA0RDT voltage probe antenna makes an ideal antenna for shortwave, medium wave and long wave DX SWLs. It is essentially a high dynamic range wideband receive antenna which is tiny, yet capable of excellent results if mounted in the clear in a low noise environment ideally away from house electrical interference.

VLF activity this last week

Whilst I was away on holiday there was considerable activity around 8.970kHz from Europe with Henny PA3CPM now putting a consistant signal from his small home station antenna on the G3ZJO grabber. Activity is continuing this weekend.

One of my priorities in the next 2 weeks is to get my VLF receive set-up back in full working order as this has been off-air for a couple of months. I will try a new E-field probe this time around and see how results compare with my 80m square wire loop. The PA0RDT or DK7FC E-field probes are well proven designs that work well if mounted high and in a low noise environment. See https://sites.google.com/site/sub9khz/antennas for details. Antenna size is less important than S/N and dynamic range on VLF RX. This is why tiny voltage probe antennas can work so well.

VLF E-field probe design from DK7FC

DK7FC's VLF E-field probe antenna

Stefan DK7FC is well known for his experiments at VLF. Here is his circuit for the E-field probe he designed which is in use at many VLF grabbers around Europe. It is highly recommended.

Improved E-field probe for VLF

Today I made an improved version of my VLF E-field probe. The first hi-Z FET input stage now an amplifier with an 8-9kHz tuned circuit in the drain. Sensitivity is now much better than before with strong Alpha beacon reception. It works well with the 80cm loop too and with the large TX loop in the garden via a step-up transformer. I'm looking forward to further tests by DK7FC and G3XIZ on 8.97kHz as my sensitivity is now better.  I hope to test this out in the field with my 8.76kHz transmitter over the weekend.